CHAPTER VII.

THE CONSTRUCTION OF THE HEAVENS.

The construction of the visible universe is one of great interest, but of considerable difficulty. If we reflect that in viewing the starry heavens we are placed at the centre of a hollow sphere of indefinite extent, and that the distance of only a few of the stars from the earth has hitherto been ascertained with any approach to accuracy, the great difficulty of framing a satisfactory theory of the construction of the heavens will be easily understood.

In considering the subject, let us first inquire as to the probable number of stars visible in our largest telescopes. Are the visible stars infinite or limited in number? The reply to this question is easy. As the number of stars visible to the naked eye is limited, so the number of stars visible in the largest telescopes is limited also. Those who do not give the subject sufficient consideration seem to think that the number of the stars is practically infinite, or at least that the number is so great that it cannot be estimated. But this idea is totally incorrect, and due to complete ignorance of telescopic revelations. It is certainly true that, to a certain extent, the larger the telescope used in the examination of the heavens, the more the number of the stars seems to increase; but we now know that there is a limit to this increase of telescopic vision. And the evidence clearly shows that we are rapidly approaching this limit. Although the number of stars visible in the Pleiades rapidly increases at first with increase in the size of the telescope used, and although photography has still further increased the number of stars in this remarkable cluster, it has recently been found that an increased length of exposure—beyond three hours—adds very few stars to the number visible on the photograph taken at the Paris Observatory in 1885, on which over 2,000 stars can be counted. Even with this great number on so small an area of the heavens, comparatively large vacant spaces are visible between the stars, and a glance at the original photograph is sufficient to show that there would be ample room for many times the number actually visible. I find that, if the whole heavens were as rich in stars as the Pleiades, there would be only 33 millions in both hemispheres.

On a photograph of the region surrounding Gamma Cassiopeiæ, taken by Dr. Roberts in December, 1895, with a reflecting telescope of 20 inches aperture, and an exposure of two hours and twelve minutes, he finds 17,100 stars on an area of four square degrees. This would give for the whole area of the heavens—if equally rich in stars—a total of about 176 millions; but Gamma Cassiopeiæ lies in a rich region of the Milky Way, and probably the great majority of the stars shown on Dr. Roberts’ photograph belong to the Galaxy, which we know to be especially rich in stars. One thing is certain, that the heavens as a whole are not nearly so rich as this particular spot. There may, perhaps, be richer spots elsewhere in the Milky Way, but in other parts of the sky there are many regions considerably poorer.

Let us consider a still more extreme case of stellar richness. On a photograph of the great globular cluster, Omega Centauri, recently taken in Peru, a count of the stars has been carefully made by Professor and Mrs. Bailey, and, as stated in the last chapter, the number of stars contained in the cluster may be taken as 10,000. Now, if the whole sky were as thickly studded with stars as in this cluster, the total number visible in the whole heavens would be 1,650 millions, a very large number, of course, but not much in excess of the present population of the earth, and I am not aware that the number of the earth’s inhabitants has ever been described as “infinite.”

Clusters, such as the Pleiades and Omega Centauri, are, of course, remarkable, and rare exceptions to the general rule of stellar distribution, and the heavens in general are not—even in the richest portions of the Milky Way—nearly so rich in stars as the globular clusters. The fact of these clusters being remarkable objects, proves that they are unusually rich in stars, and there is strong evidence—evidence amounting to absolute proof in the case of the globular clusters—that these collections of stars are really, and not apparently, close, and that they are actually systems of suns, and occupy a comparatively limited volume in space. We cannot, then, estimate the probable number of the visible stars by counting those visible in one of the globular clusters.

That the number of the visible stars will not probably be largely increased by any increase in telescopic power, is indicated by the fact that Celoria, using a small telescope, of power barely sufficient to show stars to the eleventh magnitude, found that he could see almost exactly the same number of stars near the north pole of the Milky Way as were visible in Sir William Herschel’s great telescope! thus indicating that, here at least, no increase of optical power will materially increase the number of stars visible in that direction; for Herschel’s large telescope certainly showed far fainter stars than those of the eleventh magnitude in other portions of the heavens. It should therefore have shown fainter stars at the pole of the Milky Way also, if such stars existed in that region of space. Their absence, therefore, seems certain proof that very faint stars do _not_ exist in that direction, and that, here at least, our sidereal universe is limited in extent A photograph, taken by Dr. Roberts not very far from the spot in question, shows only 178 stars to the square degree. This rate of distribution would give a total of only 7,343,000 stars for both hemispheres!

An examination by Miss Clerke of Professor Pickering’s catalogue of stars surrounding the north pole of the heavens shows that “the small stars are overwhelmingly too few for the space they must occupy, if of average brightness; and they are too few in a constantly increasing ratio.”[143] Here again, a “thinning out” of the stellar hosts seems clearly indicated, and suggests that a limit will soon be reached, beyond which our most powerful telescopes and photographic plates will fail to reveal any further stars.

Let us now consider the number of stars actually visible. Maps of the northern portion of the heavens have been published by Argelander and Heis, and charts of the southern sky by Behrmann and Gould. Heis shows stars to about magnitude 6⅓, and Behrmann to about the same brightness. I find that the total number shown by both observers, as visible to the naked eye, is 7,249. The total number, to the sixth magnitude inclusive, shown by both observers, is 4,181. Argelander gives 5,000 stars to the sixth magnitude inclusive, and for stars to the ninth magnitude, the following numbers in each magnitude:—First magnitude, 20; second magnitude, 65; third magnitude, 190; fourth magnitude, 425; fifth magnitude, 1,100; sixth magnitude, 3,200; seventh magnitude, 13,000; eighth magnitude, 40,000; and ninth magnitude, 142,000, or a total of “200,000 for the entire number of stars from the first to the ninth magnitude inclusive.”[144] This result agrees closely with an estimate previously made by Struve. From a formula given by Dr. Gould, deduced from observations in the Southern Hemisphere, I find the number of stars to the ninth magnitude inclusive would be 215,674, so that Argelanders estimate of 200,000 stars to the ninth magnitude inclusive cannot be far from the truth. It will be seen from Argelanders figures that the number of stars in each class of magnitude is roughly three times that in the class one magnitude brighter. Supposing this progressive increase continued to the seventeenth magnitude—the faintest visible in the great Lick telescope—I find that the total number of stars would be nearly 1,400 millions, or less than the number found from a consideration of the cluster Omega Centauri. But it is evident from Celoria’s observation, referred to above, and from Professor Pickering’s photographs of stars near the North Pole, that the fainter stars do _not_ increase in the ratio assumed above. We must therefore conclude that there is a “thinning out” of the fainter stars at some point below the ninth magnitude. Taking into consideration the rich regions of the Milky Way, and the comparatively poor portions of the sky, it is now generally admitted by astronomers, who have studied this particular question, that the probable number of stars visible in our largest telescopes does not exceed 100 millions, a number which, large as it absolutely is, may be considered as relatively very small, and even utterly insignificant, when compared with an “infinite number.”

Let us see what richness of stellar distribution is implied by this number of 100 millions of visible stars. It may be easily shown that the area of the whole sky, in both hemispheres, is 41,253 square degrees, or about 200,000 times the area of the full moon. This gives 2,424 stars to the square degree. The moon’s apparent diameter being slightly over half a degree (31′ 5″), the area of its disc is about one-fifth of a square degree. Hence, for 100 millions of stars in the whole star sphere, we have 485 stars to each space of sky, equal in area to the full moon. This seems a large number, but stars scattered even as thickly as this would appear at a considerable distance apart when viewed with a large telescope and a high power. As the area of the moon’s disc contains about 760 square minutes of arc, there would not be an average of even one star to each square minute. A pair of stars half a minute, or 30 seconds, apart, would form a very wide double star, and with stars placed at even this distance, the moon’s disc would cover about 3,000, or over six times the actual number visible in the largest telescopes. In Dr. Roberts’ photograph of the region surrounding Gamma Cassiopeiæ, which shows over 17,000 stars, on four square degrees, or over 4,000 stars to the square degree, the stars do not seem very crowded, and there is a good deal of black sky visible between them.

But, in addition to the conclusive evidence as to the limited number of the visible stars derived from actual observation and the results of photography, we have indisputable evidence from mathematical considerations that the number of the visible stars _must necessarily_ be limited. For were the stars infinite in number, and scattered through infinite space with any approach to uniformity, it may be proved that the whole heavens would shine with the brightness of the sun. As the surface of a sphere varies as the square of its radius, and light inversely as the square of the distance (or radius of the star sphere at any point), we have the diminished light of the stars exactly counterbalanced by the increased number at any given distance. For a distance of say ten times the distance of the nearest fixed star, the light of each star would be diminished by the square of 10 or 100 times, but the total number of stars would be 100 times greater, so that the total star light would be the same. This would be true for _all_ distances. The total light would therefore—by addition—be proportional to the distance, and hence, for an infinite distance we should have an infinite amount of light For an infinite number of stars, therefore, we should have a continuous blaze of light over the whole surface of the visible heavens. Far from this being the case, the amount of light afforded by the stars on the clearest nights is, on the contrary, comparatively small, and the blackness of the background, “the darkness behind the stars,” is very obvious. According to Miss Clerke (“System of the Stars,” p. 7), the total light of all the stars, to magnitude 9½, is about one-eightieth of full moonlight. M. G. l’Hermite found for the total amount of starlight one-tenth of moonlight; but this estimate is evidently too high. Assuming the sun’s brightness as 28 magnitudes brighter than a star of the first magnitude,[145] and Zöllner’s estimate that sunlight is 618,000 times that of moonlight, I find that the total light of the stars to magnitude 9½, as stated by Miss Clerke, would be equivalent to the combined light of about 320,000 stars of the sixth magnitude, or 3,200 stars of the first magnitude. Even taking M. l’Hermite’s high estimate of one-tenth of moonlight, the total starlight would be represented by 25,600 stars of the first magnitude.

To explain the limited number of the visible stars, several hypothesis have been advanced. If space be really infinite, as we seem compelled to suppose, it would be reasonable to expect that the number of the stars would be practically infinite also. But, as I have shown above, the number of the _visible_ stars is certainly finite, and the number visible and invisible must be finite also, for otherwise the amount of starlight would be much greater than it is. To account for the limited number of visible stars, it has been suggested that beyond a certain distance in space, there may be an “extinction of light,” caused by absorption in the luminiferous ether. In a recent paper on this subject, Schiaparelli, the famous Italian astronomer, suggests that if any extinction of light really takes place, it may probably be due, not to absorption in the ether, but to fine particles of matter scattered through interstellar space. In support of this hypothesis, he refers to the supposed constitution of comets’ tails, of falling stars, and meteorites, and he shows that the quantity of matter necessary to produce the required extinction would be very small—so small, indeed, that a quantity of this matter scattered through a volume equal to that of the earth, if collected into one mass, would only form a ball of less than one inch in diameter. We can readily admit the existence of such a minute quantity of matter in a fine state of subdivision scattered through space, but it seems to me much more probable that the limited number of the visible stars is due, not to any extinction of their light by absorption in the ether, or by fine particles scattered through space, but to a real thinning out of the stars as we approach the limits of our sidereal universe. Celoria’s observation, mentioned above, seems to prove that near the pole of the Milky Way very few stars fainter than the eleventh magnitude are visible, even in a large telescope, and Dr. Roberts’ photographs, taken in the vicinity of the celestial pole, confirm this conclusion. Now, this paucity of stars of the fainter magnitudes cannot be due to any absorption of light in the ether, for numerous stars of the sixteenth magnitude, or perhaps fainter, are visible in other parts of the heavens, and if in one place, why not in another? Sir John Herschel’s observations of the Milky Way in the Southern Hemisphere appear to render the hypothesis of any extinction of light very improbable. He says that the hypothesis, “if applicable to any, is equally so to every part of the Galaxy. We are not at liberty to argue that at one part of its circumference our view is limited by this sort of cosmical veil, which extinguishes the smaller magnitudes, cuts off the nebulous light of distant masses, and closes our view in impenetrable darkness; while at another we are compelled, by the clearest evidence telescopes can afford, to believe that star-strewn vistas _lie open_, exhausting their powers, and stretching out beyond their utmost reach, as is proved by that very phænomenon which the existence of such a veil would render impossible, _viz._, infinite increase of number and diminution of magnitude, terminating in complete irresolvable nebulosity.”

How then are we to explain the limited number of the visible stars? If space be infinite, as we seem compelled to suppose, the number of the stars would probably be infinite also, or at least vastly greater than the number actually visible. It has been suggested that, owing to the progressive motion of light, the light of very distant stars may probably not yet have reached the earth, although travelling through space for thousands of years. But considering the vast periods of time during which the stellar universe has probably been in existence, this hypothesis seems very unsatisfactory. The most probable hypothesis seems to be that all the stars, clusters and nebulæ, visible in our largest telescopes, form together one vast system, which constitutes our visible universe, and that this system is isolated by a starless void from other similar systems which probably exist in infinite space. The distance between these separate systems—or “island universes,” as they have been called—may be very great, compared with the diameter of each system, in the same way that the diameter of our visible universe is very great compared with the diameter of the solar system. As the sun is a star, and the stars are suns, and as our sun is separated from his neighbour suns in space by a sunless void, so may our universe be separated from other universes by a vast and starless abyss. On this hypothesis, the supposed extinction of light—which may have little or no perceptible effect within the limits of our visible universe—may possibly come into play across the vast and immeasurable distances which probably separate the different universes from each other, and may perhaps extinguish their light altogether.

Another hypothesis which also seems possible is that the luminiferous ether which extends throughout our visible universe may perhaps be confined to this universe itself, and that beyond its confines, the ether may thin out, as our atmosphere does at a certain distance from the earth, and finally cease to exist altogether, ending in an _absolute_ vacuum, which would, of course, arrest the passage of all light from outer space, and thus produce “the darkness behind the stars.”

Let us now consider the apparent distribution of the stars and nebulæ on the celestial vault, and their probable relation to each other in space. As already stated, Argelander considered the number of stars of the first magnitude to be about twenty, but modern photometric measures have reduced this number to thirteen or fourteen. According to the Harvard measures, the fourteen brightest stars in the heavens, in order of magnitude, are: Sirius, Canopus, Arcturus, Capella, Vega, Alpha Centauri, Rigel, Procyon, Achernar, Beta Centauri, Betelgeuse, Altair, Aldebaran and Alpha Crucis. Seven of these are in the Northern Hemisphere, namely: Arcturus, Capella, Vega, Procyon, Betelgeuse, Altair, and Aldebaran; and seven in the Southern Hemisphere: Sirius, Canopus, Alpha Centauri, Rigel, Achernar, Beta Centauri, and Alpha Crucis, so that the brightest stars are pretty evenly distributed between the two hemispheres. Of these bright stars, no less than twelve lie in or near the Milky Way, Arcturus and Achernar being the only two at any considerable distance from the Galaxy. This is very remarkable and suggestive, as the area covered by the Milky Way is probably not more than one-fourth of the whole star sphere.

Of the stars fainter than the first magnitude, but brighter than magnitude 2·0, there are about 10 in the Northern Hemisphere, of which 4 lie in or near the Milky Way, and about 19 in the Southern Hemisphere, of which no less than 14 are situated in or near the Galaxy.

Of those brighter than magnitude 3·0, I find 33 stars in or near the Milky Way out of a total of about 95 in both hemispheres. To extend this investigation to all stars visible to the naked eye, I made, some years since, an examination of all the stars in Heis’ atlas that lie in the Milky Way, and found that number to be 1,186 out of a total of 5,356, or a percentage of about 22. At my request, Col. Markwick, F.R.A.S., made a similar count for the stars in Dr. Gould’s charts of the Southern Hemisphere (_Uranometria Argentina_), and found that, down to the fourth magnitude, there are 121 stars on the Milky Way out of 228, or a percentage of 53, and for all stars to the seventh magnitude inclusive, there are 3,072 on the Milky Way out of a total of 6,694, or a percentage of nearly 46. Col. Markwick finds that the Milky Way in the Southern Hemisphere, as shown on Gould’s charts, covers about one-third of the whole hemisphere. As will be seen by the above figures, the percentage of stars, even to the fourth magnitude, lying on the Milky Way is considerably greater than this proportion.

The above results show that the brighter stars which are apparently projected on the Milky Way probably belong to that zone, and are not merely fortuitously scattered over the surface of the heavens.

To extend the investigation still further, and include stars to the eighth magnitude, I made an examination of the stars shown on Harding’s charts to that magnitude, in a zone of 30° in width—15° degrees on each side of the Equator—and found a marked increase in the number of stars where the zone crossed the Milky Way. The numbers per hour of Right Ascension varied from a minimum of 275 (hours I. and II.) to maxima of 601 in the Milky Way in Monoceros, and 611 in the Galaxy in Serpens and Aquila. A valuable investigation by the late Mr. Proctor went further still. He plotted all the stars shown in the charts of Argelander’s _Durchmusterung_, which contains stars to 9½ or 10th magnitude. In this remarkable chart the course of the Milky Way is clearly defined by a marked increase of stellar density. Proctor says: “In the very regions where the Herschelian gauges showed the minutest telescopic stars to be most crowded, my chart of 324,198 stars shows the stars of the higher orders (down to the eleventh magnitude) to be so crowded that, by their mere aggregation within the mass, they show the Milky Way with all its streams and clusterings. This evidence, I venture to affirm, is altogether decisive as to the main question, whether large and small stars are really intermixed in many regions of space, or whether the small stars are excessively remote. It is utterly impossible that excessively remote stars could seem to be clustered exactly where relatively near stars are richly spread. This might happen, no doubt, in a single instance; but that it could be repeated over and over again, so as to account for all the complicated features seen in my chart of 324,198 stars, I maintain to be utterly incredible.”[146]

From a careful examination of the Milky Way in Aquila and Cygnus, Mr. Easton finds that “(1) In the zones considered, the distribution of stars down to 9·5 magnitude corresponds to the greater or less intensity of galactic light. (2) There is a real correspondence of the general outlines of the galactic forms with the distribution of 11 magnitude stars, and with those of stars between 10 and 15 magnitude. (3) Thus, in general, for the zones considered, the faint stars which form the Milky Way are thickly or sparsely scattered in respectively the same regions as the stars in Argelander’s last class; it follows, therefore, with a great degree of probability, that there is a real connexion between the distribution of 9 and 10 magnitude stars and that of the very faint stars of the Milky Way. Consequently, the very faint stars are at a distance which does not greatly exceed that of 9–10 magnitude stars. If stars of 13–15 magnitude were at their theoretical distance, there would be no reason why they should have the same apparent distribution in galactic latitude and longitude as 9–10 magnitude stars separated from them by enormous intervals.”[147]

There are some regions in both hemispheres especially rich in naked eye stars. Of these the following may be mentioned in the Northern Hemisphere:—the region including the Pleiades, and Hyades in Taurus, the Northern portion of Orion, and the adjoining part of Gemini, the constellation Lyra, the northern portion of Cygnus, Cassiopeia’s Chair, and Coma Berenices. In the Southern Hemisphere there are several rich spots. A rich region extends from Canis Major to the Southern Cross, and nearly coincides with the course of the Milky Way. The richest spot of all, and perhaps the richest in the whole heavens in naked eye stars—with exception of the Pleiades—is that including the Southern Cross. This spot has an average of three stars to five square degrees, and if the whole heavens were as richly studded with stars there would be about 24,000 visible to the naked eye! The poverty of the adjoining “coal sack” is very remarkable. Another rich spot surrounds the variable star Eta Argûs, and the great nebula in Argo. There is another rich spot in the constellation Hydrus, not far from the greater Magellanic Cloud, and another will be found in Centaurus and Lupus, with its centre about Alpha of the latter constellation. According to Gould’s maps of the Southern Hemisphere, the richest region in stars down to the seventh magnitude is the southern portion of that part of the constellation Argo, known as Puppis.

In contrast to these rich regions, and in many cases closely adjoining them, are some barren regions, very poor in naked eye stars. For example, closely following the rich spot in Cassiopeia and between Iota Cassiopeiæ and Eta Persei is a remarkably poor spot, where a space of some sixty square degrees does not contain a single star brighter than the sixth magnitude! There is another poor region south of Alpha Hydræ, and another in the southern portion of the constellation Cetus.

A region of considerable extent, remarkably deficient in bright stars, will be noticed in the Northern Hemisphere. This comparatively barren region, which contains no star brighter than the fourth magnitude, is bounded by Cepheus, Cassiopeia, Perseus, Auriga, Gemini, Ursa Major, Draco, and Ursa Minor, and forms a conspicuous feature in the north-eastern portion of the sky in the early winter evenings. It will be noticed that the surrounding constellations all contain bright stars.

Whether the apparent crowding of stars in certain regions of the heavens is caused by a real proximity in space, or whether it is merely due to their being placed accidentally in the line of sight, is a question difficult to determine. In the case of star clusters, and especially the globular clusters, there is a high mathematical probability, amounting almost to absolute certainty, that they are comparatively close together, but in groups scattered over a considerable area, like those referred to above, the probability in favour of proximity is not so great. As we know the distance of so few stars from the earth, it is impossible to say whether the crowding is real or only apparent, but the probability seems to be that it is to some extent real.

A tendency to an arrangement of stars in streams was pointed out by Proctor in his “Universe and the Coming Transits.” This tendency to stream formation may be noticed on a large scale among the naked eye stars, for example, in Pisces, Scorpio, the River Eridanus, Aquarius, and the festoon of stars in Perseus. In some of these cases, of course, the stars are so far apart that the formation may be more apparent than real, but the tendency can also be clearly recognised among the fainter stars, and even among those only visible in telescopes and stellar photographs. This tendency to run in streams is well marked on the photographs taken at the Paris Observatory, and on those taken by Professor Barnard, Dr. Max Wolf, and others. It is a suggestive fact that these star streams are also very noticeable in star clusters, where there can be little or no doubt of a physical connexion between the component stars. With reference to a photograph of the southern portion of Aquila taken by Dr. Max Wolf in July, 1892, the late Mr. Ranyard, remarked: “Some of the streams of fainter stars in this region are very striking, and must convince the most sceptical of their reality. It is possible to draw an arc of a circle through any three stars, and a conic section through any five; but where we find ten or twenty stars falling into line, not once, but in many cases, and that there is a curious similarity between the strange curves and branching streams which these phalanges of stars mark out on the heavens, there is no room left for doubt that the mind is not being led away by a tendency of the imagination similar to that which finds faces in the fire, or sees a man carrying sticks on the face of the moon. If it is proved that a group of stars is arranged in line or marshalled in any order, it would follow that the individuals of the group must be actually as well as apparently close to one another, and that they form some kind of system, having all of them had a common origin, or been subject to some common influence.”[148]

The great majority of the star clusters are found along the course of the Milky Way, while the irresolvable nebulæ seem to congregate towards the poles of the galactic zone.

Dr. Gould is of opinion that “a belt or stream of bright stars appears to girdle the heavens very nearly in a great circle, which intersects the Milky Way at about the points of its highest declination, and forms with it an angle not far from 20°; the southern node being near the margin of the Cross, and the northern in Cassiopeia.” According to Gould, this belt covers Orion, Canis Major, Columba, Puppis, Carina, the Southern Cross, Centaurus, Lupus, and the head of Scorpion in the Southern Hemisphere, its northern course being indicated by the brightest stars in Taurus, Perseus, Cassiopeia, Cepheus, Cygnus, and Lyra. Dr. Gould considers that our sun may possibly be a member of this belt of stars, which perhaps numbers less than 500, and which constitute “a small cluster, distinct from the vast organisation of that which forms the Milky Way, and of a flattened and somewhat bifid form. The southern portion of this supposed stream of bright stars had been previously recognised by Sir John Herschel, who says in his ‘Cape Observations,’ (p. 385), ‘It is about this region, or, perhaps, somewhat earlier, in the interval between η Argus and α Crucis, that the galactic circle, or medial line of the Milky Way may be considered as crossed by that zone of large stars, which is marked out by the brilliant constellation of _Orion_, the bright stars of Canis Major, and almost all the more conspicuous stars of _Argo_, the Cross, the Centaur, Lupus, and _Scorpion_. A great circle passing through ε Orionis and α Crucis will mark out the axis of the zone in question, whose inclination to the galactic circle is, therefore, about 20°, and whose appearance would lead us to suspect that our nearest neighbours in the sidereal system (if really such) form part of a subordinate sheet or stratum deviating to that extent from parallelism to the general mass which, seen projected on the heavens, forms the Milky Way.’”

These conclusions might seem probable enough when we compare the supposed zone of bright stars with the very diagrammatic drawings of the Milky Way as shown in many star maps; but when we consider the stars referred to with reference to the more artistic and accurate delineations of the Milky Way as drawn by Boeddicker, and even by Gould himself, we see that most of them are involved in the milky light of the Galaxy, and their connexion with the Milky Way itself seems quite as probable as that they form a belt distinct from the galactic zone. The apparent connexion of the stars in question with the Milky Way does not, however, disprove the existence of Dr. Gould’s belt or zone of bright stars. If the plane of the supposed belt nearly coincided with that of the Milky Way, the apparent connexion might not be real.

Mr. J. R. Sutton advances the theory[149] that the Milky Way consists of “a great ring of large stars”—Dr. Gould’s solar cluster above referred to—“intersecting an equal ring of small ones (the Milky Way) at the extremities of a common diameter.” He considers that “the great star belt is a genuine girdle of stars in space, in which also the foundations of the sidereal system are laid, the Milky Way being an appendant to it of lesser rank.”

That the Milky Way really forms a ring of stars in space there is strong evidence to show. Sir William Herschel’s original theory that the galactic gleam is due to our sun being situated near the centre of an indefinite stratum of stars—the “disc theory,” as it is termed—was abandoned by its illustrious author in his later writings, and is now considered to be wholly untenable by nearly all astronomers who have studied the subject. Sir John Herschel remarks that the general aspect of the galaxy near the Southern Cross indicates “that the Milky Way, in this neighbourhood, at any rate, is really what it appears to be, a belt or zone of stars separated from us by a starless interval.” It certainly seems utterly improbable that the nearly circular blank space near the Southern Cross, known as “the coal sack,” should represent a tunnel through a disc, of which the thickness is comparatively small, while its diameter, on the “disc theory,” stretches out almost to infinity. A straight, tunnel-shaped opening of great length, pointing directly towards the earth, would form an extraordinary phenomenon even in a solitary instance; yet there are several somewhat similar openings to be found in the Milky Way, as viewed both with the naked eye and with a telescope. That _all_ these openings should represent tunnels radiating from a common centre is quite beyond the bounds of probability, and, indeed, such an hypothesis does not deserve serious consideration. With reference to a photograph of the Milky Way in the constellation Cepheus, Professor Barnard says, “the sky (or Milky Way) is broken up into numerous black cracks or crevices. Looking at these peculiar features, I cannot well see how one can avoid the conclusion that they are necessarily real vacancies in the Milky Way, through which we look out into the blackness of space.”[150] Using a telescope with a low power, Mr. S. M. Baird Gemmill says, “December 1, 1886. In sweeping over the constellation of Monoceros, I was much struck with the reticulated character of the arrangement of the brighter stars upon the glimmering background, and the way in which this background seemed to follow the reticulation. By ‘brighter stars’ are meant stars of from 8 to 10 magnitude, for it was among these that I noticed this peculiarity of arrangement. It put me in mind of M. M. Henry’s photographs of Cygnus. The region seemed, in fact, a vast network of stars, the reticulations of which were separated by desert, or comparatively desert spaces.”[151] I have noticed the same thing myself while examining the Milky Way with a binocular field-glass. On October 26, 1889, I noted as follows: “North of Alpha Cygni, and near Xi and Nu Cygni, the nebulous light of the Milky Way seems to cling round and follow streams of small stars in a very remarkable way; numerous small ‘coal sacks’ and rifts are visible, in which comparatively few stars are to be seen with the binocular.” This observation has been fully confirmed by photographs of this region, taken by Dr. Max Wolf in 1891.

[Illustration:

FIG. 19.—_Photograph of Milky Way, Sagittarius._

(From “Visible Universe.”) ]

That the Milky Way is not indefinitely extended in the line of sight seems clearly shown by Sir John Herschel’s observations in the Southern Hemisphere. In his “Outlines of Astronomy” (p. 578), he says: “When examined with powerful telescopes, the constitution of this wonderful zone is found to be no less various than its aspect to the eye is irregular. In some regions, the stars of which it is wholly composed are scattered with remarkable uniformity over immense tracts, while in others the irregularity of their distribution is quite as striking, exhibiting a rapid succession of closely clustering rich patches, separated by comparatively poor intervals, and indeed, in some instances, by spaces absolutely dark _and completely void of any star_,[152] even of the smallest telescopic magnitude.... In some, for instance, extremely minute stars, though never altogether wanting, occur in numbers so moderate, as to lead us irresistibly to the conclusion that, in those regions, we see _fairly through_ the starry stratum, since it is impossible otherwise (supposing their light not intercepted), that the members of the smaller magnitude should not go on increasing _ad infinitum_. In such cases, moreover, the ground of the heavens, as seen between the stars, is for the most part perfectly dark, which again would not be the case if innumerable multitudes of stars, too minute to be individually discernible, existed beyond. In other regions we are presented with the phænomenon of an almost uniform degree of brightness of the individual stars, accompanied with a very even distribution of them over the ground of the heavens, both the larger and smaller magnitudes being strikingly deficient. In such cases it is equally impossible not to perceive that we are looking _through_ a sheet of stars nearly of a size and of no great thickness compared with the distance which separates them from us. Were it otherwise, we should be driven to suppose the more distant stars uniformly the larger, so as to compensate by their greater intrinsic brightness for their greater distance, a supposition contrary to all probability. In others again, and that not unfrequently, we are presented with a double phænomenon of the same kind, _viz._, a tissue, as it were, of large stars spread over another of very small ones, the intermediate magnitude being wanting. The conclusion here seems equally evident that in such cases we look through two sidereal sheets separated by a starless interval.”

An examination of the evidence at present available, with reference to the distribution of the visible stars in space, has recently been undertaken by Professor Kapteyn of Groningen, and an account of the conclusions he has arrived at may prove of interest to the reader.

[Illustration:

FIG. 20.—_The Milky Way._

(From _Knowledge_, Nov., 1894.) ]

We must first explain that in order to obtain a clear view of the construction of the visible universe, it would be necessary to know the relative distances of a large number of stars; but as the distances of only a few stars from the earth have yet been determined by actual measurement, and the results hitherto obtained are open to much uncertainty, we must have recourse to some other method of estimating the distances. While travelling in a railway carriage, if we fix our attention on trees, buildings, and other objects we pass on our journey, it will be noticed that all objects apparently move past us in the opposite direction to that in which we are travelling, and that the nearer the object is the faster it seems to move with reference to distant objects near the horizon. So it is with the stars. As we showed in Chapter III., the sun is moving through space, carrying along with the earth all the planets, satellites, and comets, forming the solar system. The effect of this motion is to cause an apparent small motion of the stars in the opposite direction, and the nearer the star is to the earth, the greater will this apparent motion seem to be as in the case of the railway train. In addition to this apparent motion, the stars are themselves—like the sun—moving through space, and this _real_ motion is also visible. If this real motion takes place in the _opposite_ direction to that in which the sun and earth are moving, it will add to the apparent motion, and will increase the star’s “proper motion,” as it is termed. If, on the other hand, the real motion is in the _same_ direction as the earth’s motion, the proper motion will be diminished. In either case, the nearer the star is to the earth, the greater will be its apparent annual displacement on the background of the heavens. The amount of the “proper motion” is, therefore, considered by astronomers to form a reliable criterion of the star’s distance from the earth, and the actual measures of distance which have been made show that this assumption is approximately true. Of fourteen stars which have proper motion of over three seconds of arc per annum, eleven have yielded a measurable parallax, or displacement, due to the earth’s annual motion round the sun; that is to say, eleven out of fourteen fast-moving stars are within a measurable distance of the earth, and are, therefore, near us, when compared with the great majority of stars which are not within measurable distance, or, at least, are beyond the reach of our present methods of measurement.

In the case of small groups of stars, we may assume that the real motions of the individual stars take place indifferently in all directions, and that consequently, taking an average of all the motions of the stars composing the group, the effects due to the real motions will destroy each other, and there will remain, as the most reliable criterion, the effect due to the sun’s motion in space. If, however, we compare the proper motions of groups situated in _different parts_ of the sky, there is a consideration which, to a great extent, vitiates this conclusion. For, near the point of the heavens, towards which the sun and earth are moving, known as the “apex of the solar way,” and probably situated not far from the bright star Vega, as indicated by recent researches, and near the point away _from_ which the sun is moving known as the _ant-apex_, about 15° south of Sirius, there will be no apparent displacement due to the solar motion through space, as this motion takes place in the line of sight with reference to these points of the sky. The observed proper motion at these points will, therefore, be solely due to the real motions of the stars themselves in those regions. In other parts of the heavens, however, the total proper motion will be a combination of the apparent and real motions of the stars, and for stars in different parts of the sky, it will not follow that stars having equal proper motions are necessarily at the same distance from the earth. To make this point clearer, let us suppose that there are two stars at absolutely the same distance from the earth, one situated at or near the solar “apex,” and the other at a point 90° from the apex, and let us suppose that both stars are moving through space with exactly the same velocity and in the same direction, say at right angles to the direction of the solar motion. Then in the case of the star near the apex, the observed “proper motion” will be solely due to the star’s real motion, and in the star 90° distant from the apex, the proper motion will be solely due to the solar motion, as the star’s _real motion_, being in the line of sight, will not be visible. Now, unless the stellar motion and the solar motion happen to be equal, the observed “proper motions” will not be equal, although both stars are at the same distance from the earth. If both the stars are really at rest, the star at the apex will have no proper motion, while the star 90° distant will have an apparent proper motion due to the sun’s motion. To overcome this source of error in estimating the distance of a star from its proper motion, Professor Kapteyn made use of another measure, which is independent of the solar motion. This is the component of the proper motion measured at right angles to a great circle of the sphere passing through a star and the solar apex. The amount of motion in this direction will evidently not be affected by the sun’s motion, and from a discussion of the stars, contained in the Draper “Catalogue of Stellar Spectra,” which were observed by Bradley (and of which the proper motions are now known with accuracy), Professor Kapteyn finds that this motion is “nearly inversely proportional to the distance,” that is, the greater the motion, the less the distance of the stars, and the smaller the motion, the greater the distance. Excluding stars with proper motions greater than half a second of arc per annum, Professor Kapteyn found that for stars at various distances from the Milky Way this component of the “proper motion” forms a good measure of distance.

As the result of his investigations on the subject, Professor Kapteyn arrives at the following conclusions. Neglecting stars with small or imperceptible proper motions, we have a group of stars which no longer show any condensation in a plane. Stars with very small or no proper motions show a condensation towards the plane of the Milky Way. This applies to stars of the second or solar type, as well as to those of the first or Sirian type of spectrum, and evidently indicates that the stars composing the Milky Way lie at a great distance from the earth. The extreme faintness of the majority of the stars composing the Galaxy seems in favour of this conclusion. The condensation of stars of the first type is more marked than those of the second, and this agrees with the fact which has been noticed by Professor Pickering, that the majority of the brighter stars of the Milky Way have spectra of the Sirian type.

Professor Kapteyn finds that this condensation of stars with small proper motions is very perceptible even for stars visible to the naked eye, and is as well marked in those stars which have spectra of the second type as for all the stars of the ninth magnitude; but for stars of the first type the condensation is still more marked. He considers that this condensation is either partly real, or that there is a real thinning out of stars near the pole of the Milky Way. As already mentioned (in the beginning of this chapter), Celoria’s observations with a small telescope, compared with Sir William Herschel’s observations with a large telescope, indicate clearly that there _is a real thinning out_ of stars near the poles of the Galaxy.

Professor Kapteyn concludes that the arrangement of the stars suggested by Struve—a modification of the “disc theory”—has no real existence.[153] He attributes the fallacy in Struve’s hypothesis to the fact that the mean distance of stars of a given magnitude in the Milky Way, and outside it, is not the same.

Professor Kapteyn finds that the vicinity of the sun is almost exclusively occupied by stars of the second or solar type, a conclusion which evidently tends to strengthen Dr. Gould’s theory of a “solar cluster.” He finds that the number of Sirian type stars increases gradually with the distance, and that beyond a distance corresponding to a proper motion of about ¹⁄₁₄th of a second of arc per annum, the Sirian stars largely predominate. In the group of stars known as the Hyades, however, the components of which have a common proper motion both in amount and direction, stars of the first and second types appear to be mixed, and Professor Kapteyn assumes that the two types represent different phases of evolution, and that as the brightest stars of the group are chiefly of the solar type, these stars must be the largest of the group. From this fact he concludes the solar type stars are in a less advanced stage of evolution than those of the Sirian type. This does not agree with the generally accepted view. Professor Vogel considers the Sirian stars to represent an earlier stage of stellar evolution. Mr. Proctor held the same opinion, and in Professor Lockyer’s hypothesis of increasing and decreasing temperatures in stars of various types, he places the Sirian stars at the summit of the evolution curve, and the sun and solar stars just below them on the descending branch of the curve.[154] These hypotheses are in conformity also with the current opinion that the sun is a cooling body. The discrepancy may perhaps be explained by supposing that the _brighter_ stars of the Hyades form a connected group, and that some, at least, of the fainter stars do not belong to the group, but lie at a great distance behind it. In the case of the Pleiades, which form a more evident cluster, I find from the Draper “Catalogue of Stellar Spectra” that the great majority of the brighter stars have spectra of the Sirian type. Most of the stars in the Pleiades have a very similar proper motion, both in amount and in direction, and there can be no doubt that most of the brighter stars, at least, form a connected system. As already stated, it seems highly probable that the fainter stars in the Pleiades lie far beyond the brighter components, and have merely an optical connexion with them, and the same may be the case in the Hyades. The superior brilliancy of the stars composing the Hyades would suggest that they are nearer to the earth than the Pleiades group, and they may possibly form members of Gould’s “solar cluster.”

Assuming that the distances are inversely proportional to the proper motions, Professor Kapteyn computes the relative volumes of the spherical shells which contain the stars with different proper motions (from one-tenth of a second to one second of arc and more). Comparing these volumes with the corresponding number of stars, we arrive at an estimate of the density of star distribution at various distances. The result of this calculation shows that the distribution of stars of the Sirian type approaches uniformity when a large number of the faint stars (ninth magnitude) are considered. With reference to the stars of the second type, however, the larger the proper motion the greater the number of the stars; or, in other words, the second type, or solar stars, are crowded together in the sun’s vicinity. Evidence in favour of this conclusion is afforded by the fact that, of eight stars having the largest measured parallax (and whose spectrum has been determined), I find that seven have spectra of the solar type. The exception is Sirius, which is evidently an exceptional star with reference to its brightness and comparative proximity to the earth, no other star of the first magnitude having nearly so large a parallax. Indeed, the average distance of all the first magnitude stars is about forty times the distance of Sirius.

Professor Kapteyn finds that the centre of greatest condensation of the solar type stars lies near a point situated about ten degrees to the west of the great nebula in Andromeda, and that this centre nearly coincides with the point which, according to Struve and Herschel, represents the apparent centre of the Milky Way considered as a ring. This would indicate that the sun and solar system lie a little to the north of the Milky Way, and towards a point situated in the northern portion of the constellation of the Centaur. The fact is worth noting, that the nearest fixed star to the earth, Alpha Centauri, lies not very far from this point. Possibly there may be other stars in this direction having a measured parallax, as the southern portion of the heavens has not yet been thoroughly explored.

Professor Kapteyn finds that for stars of equal brightness, those of the Sirian type are, on an average, about two and three-quarter times farther from the earth than those of the solar type. Now, as light varies inversely as the square of the distance, this would imply that the Sirian stars are intrinsically brighter than those of the solar type. This conclusion is confirmed by the great brilliancy of Sirius and other stars of the same type in proportion to their mass. I have shown in Chapter IV. that Sirius is about ten times brighter than the sun would be if placed at the same distance, although its mass is only twice the sun’s mass, as computed from the orbit of its satellite.

The general conclusions to be derived from the above results seems to be that the sun is a member of a cluster of stars, possibly distributed in the form of a ring, and that outside this ring, at a much greater distance from us than the stars of the solar cluster, lies a considerably richer ring-shaped cluster, the light of which, reduced to nebulosity by immensity of distance, produces the Milky Way gleam of our midnight skies.

INDEX

A

Aberration of light, discovered, 18; a proof of the earth’s revolution, 57; of meteor-radiants, 396

Aboul Wefa, the moon’s variation, 5

Acceleration, 152

Achromatic lens, 177

Adams, 449; discovery of Neptune, 32, 349; orbit of November meteors, 393

Aerolites. _See_ Meteorites

Airy, reduction of Greenwich observations, 19; search for Neptune, 32

Albategnius, movement of the sun’s apogee, 5

Albedo of Mercury, 274; of Venus, 278; of the earth, 289; of the moon, 290; of Mars, 298, 334; of asteroids, 312; of Jupiter, 320; of Jupiter’s satellites, 330, 332; of Saturn, 334; of rings, 338; of Titan, 342; of Uranus, 345; of Neptune, 349

Alcor, 402

Alcyone, 499–502

Aldebaran, 403, 404, 407, 415, 421, 423, 427

Algol, 407, 415, 453, 457, 469–474

Almagest, 4, 6

Al-Mamûm’s school of astronomy at Baghdad, 5

Alphard, 409, 415

Alphonsine tables, 6

Al-Sûfi, description of the stars, 5; Alphard, red, 415; Algol, red, 472

Altair, 404, 427

Altazimuth, 184, 202

Altitude, 65

Amplitude, 66

Anderson, Dr., discovery of new star, 489

Andromeda nebula, 409, 529–532

Andromedæ, Gamma, 412, 417

— Nova, 489, 491

Andromede meteor-showers, 393, 394

Angelot, lunar volcanic action, 293

Annular eclipse, 113

— nebulæ, 526, 527

Antares, 404, 415

Anthelmus, new star, 484

Antlia, 468

Aphelion, 75

Apogee, 89

Apse Line, 75

Aquilæ, Eta, 467

Arago, nature of meteorites, 392; parallax of 61 Cygni, 422

Arc of meridian, 130

Arcturus, 403, 405, 406, 415, 423, 427

Argelander, solar translation, 28; survey of the heavens, 38; comet of 1811, 357; estimate of stars of ninth magnitude, 541

Argo Nebula, 522, 523, 549

— Eta, 462–464

Argon, not a solar element, 250; peculiar qualities, 255; found in meteorites, 389

Aries, first point of, 67

Aristarchus, heliocentric system, 4

Aristotle, description of a comet, 358

Asteroids, position in solar system, 229, 230, 310; discoveries, 311, 314; diameters, 312, 315; computation of orbits, 314; numbers and joint mass, 315; distribution, 316; groups, 317; origin, 318

Asterope, 498, 499

Astronomy, Greek, 3, 4; Arab, 4–6; Tartar, 5; of the Invisible, 31; gravitational, 11, 33; spectroscopic, 33–36; photographic, 36–38

Astrophysics, foundation of, 36

Atmosphere, of the sun, 240, 271; of Mercury, 277; of Venus, 278, 279; of the earth, 286, 313; of the moon, 294, 313; of Mars, 299, 307; of Vesta, 312–313; presence dependant upon mass, 313; of Jupiter, 326; of Uranus, 345

Atmospheric refraction, 52

Augmentation of moon’s diameter, 144

Aurigæ Beta, 404, 454, 456, 457

— New Star, 489

Auroræ, magnetic relations, 17, 288

Auwers’ reduction of Bradley’s observations, 19; proper motion of Sirius, 437

Azimuth, 65

B

Babinet, rarity of cometary matter, 366

Baden-Powell, Sir George, eclipse-expedition, 259; coronal photographs, 271

Bailey, Prof., 441, 464, 511, 513, 539

Ball, Sir Robert, 422, 433

Barnard, Prof., photograph of corona of January 1, 1889, 268–9; effect of totality, 270; zodiacal counterglow, 272; photograph of eclipsed moon, 296; drawing of Mars, 302; seas of Mars, 306; measurements of asteroids, 312; markings on Jupiter’s satellites, 330; discovery of fifth satellite, 331; measures of Saturn, 335; of ring-system, 330; disappearance of rings, 337; eclipse of Japetus, 338; compression of Uranus, 343, 344; Encke’s comet, 366; comet-photographs, 378–381; Swift’s comet, 383; Nova in Auriga, 494; Alcyone, 500; curved nebulosity stretching over constellation of Orion, 520; annular nebulæ, 526; stars in streams, 551; vacancies in the Milky Way, 554

Base line, 131

Baxendell, 484

Bayer, 404, 529

Behrmann, 400, 433, 541

Bellatrix, 408

Bélopolsky, spectrographic determination of Jupiter’s rotation, 325; absolute velocity of 61 Cygni, 427; spectroscopic examination of Castor, 451; observation of Delta Cephei, 456; Beta Lyræ, 466–467

Berberich, variability of Encke’s comet, 360

Berson, aeronautic ascent, 286

Bessel, _Fundamenta Astronomiæ_, 19; astronomy of the invisible, 31, 32; measurement of the Pleiades, 37; Halley’s comet, 355; comet of 1807, 362; Epsilon Lyræ, 411

Betelgeuse, 404, 408, 415, 427

Bianchi, 459

Bianchini, rotation of Venus, 280

Biela, discovery of a comet, 365

Bigelow, theory of Zodiacal Light, 272

Binary stars, 431

Biot, meteoric fall, 387

Bird, quadrants, 19, 20

“Bird, Red,” 415

Birmingham, 417, 485, 486

“Blaze Star,” 485, 487

Bliss, astronomer-royal, 19

Bode’s law, 145, 232, 311, 317, 349

Boeddicker, Dr., heat-phases of eclipsed moon, 295

Bolometer, 226, 239

Bompas, 430

Bond, W. C., discoveries of Hyperion and of Saturn’s dusky ring, 25, 336, 341; celestial photography, 36, 37; the great nebula, 530

Bradley, discoveries of aberration and nutation, 18, 20; reduction of his observations, 19; Saturn’s rings, 337; the distance of stars, 419–420; Gamma Virginis, 445–446

Brahé, Tycho, the moon’s variation, 5; career, 8; scheme of the celestial movements, 9

Bredichin, theory of comets’ tails, 369, 370. (_See also_ Tycho.)

Brenner, ashen light of Venus, 279; rotation of Venus, 280

Brightest stars, 403, 404, 546

Brinkley, 422

British catalogue, 15, 16

Brooks’ cometary discoveries, 365, 371, 380

Bunsen, foundation of spectrum analysis, 33

Burnham, 433, 437, 441, 448, 509

C

Calcium, represented in Fraunhofer spectrum, 230; in chromospheric and prominence-spectra, 258, 261, 262

Calendar, 86

Callandreau, capture of comets, 372

Campbell, Prof., spectrum of Mars, 306; mountains on, 307

Canals of Mars, 301–305

Cancri, S., 474

— Zeta, 439, 440

Canis Majoris, R, 473

Canopus, 403

Capella, 403, 406, 415, 427

Capricornus, 411

Capture-theory of comets, 372

Carbon in sun, 242, 250; in comets, 368; in meteorites, 389

Cardinal points, 51

Carrington, sun-spot zones, 247; sun’s rotation, 248, 249

Casey, 433

Cassegrain telescope, 180

Cassini, rotation of Venus, 280; red spot on Jupiter, 323; division of Saturn’s rings, 336; discoveries of Saturnian satellites, 341

Cassiopeia, Chair of, 405, 481, 549

Cassiopeiæ, Eta, 413, 450

Castor, 404, 406, 413, 450, 451

Catalogues of stars, 70

Celoria, 433, 442, 540

Centauri, Alpha, 410, 413, 422, 440, 441

— Omega, 512, 513, 516, 539

— R, 478

Cephii Delta, 417, 456, 466

— U, 474

Ceraski, luminous night-clouds, 286; discovery of U Cephei, 474

Ceres, discovery, 311; diameter, 312

Cerulli, rotation of Venus, 280

Ceti, Mira, 458

Challis, search for Neptune, 32

Chandler, 462, 472, 476

Charlois, asteroidal discoveries, 314

Chemistry, universal, 35, 36; solar, 250, 255; of prominences, 256; of chromosphere, 258; of comets, 368, 370, 384; of meteorites, 389

Chromosphere, 253, 258

Chronograph, 175

Chronometer, 175

Circle, meridian, 198; transit, 198; position, 208

Circumpolar stars, 46

Clairaut, verification of Newton’s law, 11; calculation of Halley’s comet, 16

Clark, Alvan, great refractors, 26

— — G., detection of the companion of Sirius, 26, 437

Clarke, dimensions of earth, 134

Clausen, groups of comets, 361

Clerke, Agnes, appearance of R Sculptoris, 416; examination of Pickering’s catalogue of stars, 541; estimate of total light of stars to magnitude 9½, 543

Clock, astronomical, 174; driving, 186; sidereal, 68

Clock stars, 82

Clusters, globular, 507–517; irregular, 497–507

“Coal sacks” in Milky Way, 554

Coelostat, 194

Collimation of transit instrument, 200

Collimator of spectroscope, 215

Colours of double stars, 417

Comæ Berenices, 434, 502, 549

Comet, Aristotle’s, 352, 353; of 1743, 354; Newton’s, 355; of 1843, 358, 359; Tebbutt’s, 362, 368; Donati’s, 362, 369; Lexell’s, 365, 370, 371; Brooks’, of 1889, 365; of 1893, 380; Winnecke’s, 368, 371, 372; Brorsen’s, 370; Tuttle’s, 372, 393; Wolf’s, 377; Rordame’s, 383; Gale’s, 383; Leonid, 393, 395

— Halley’s, return in 1759, 16, 17; status in solar system, 230, 232; return in 1835, 335, 336; type of tail, 369; a client of Neptune, 371, 372

— Encke’s, disturbed by Mercury, 273; rarefaction, 366; acceleration, 307; exempt from Jupiter’s influence, 371

— of 1811, structure, 356, 357; type of tail, 369; bulk, 383

— of 1843, surprising appearance, 358; conditions of movement, 359

— of 1882, photographs, 38, 361; transit, 359, 361; period, 300; spectrum, 369

Comet, Biela’s, discovery, 365; duplication, 366; related meteor-swarm, 393, 394

— Wells, spectrum, 368

— photographically detected, 377

Comets, orbits of, 108; periodic, 109; domiciled in solar system, 230, 371, 372; granular nuclei, 353, 379, 384; tenuity, 354, 384; classification by Olbers, 358, 383; groups, 359, 361, 362; disruption, 360, 366, 379; photographs, 361, 377, 380; chemistry, 368, 370, 385; luminous by electricity, 309, 384; lost, 370; short-period, 370, 371; capture by planets, 371, 372, 384; share sun’s translation, 372; meteoric relationships, 384, 393, 394

— tails, multiple, 354, 355, 361, 377; electrical theory, 357, 369, 383; passage of the earth through, 302, 365; three types, 369; structure shown in photographs, 377, 383

Common, Dr., 25, 510, 520, 532, 534

Conjunctions, 99, 103

Constant of aberration, 59

Constellations, 45

Contacts in eclipse, 114

Copeland, Dr., cometary spectra, 368; Nova in Auriga, 490; helium, 519

Copernicus, residence in Italy, 7; theory of planetary revolutions, 8, 9, 418

Cornelius, Gamma, 479

Corona Borealis, Eta, 437

— — Gamma, 442

— solar, 253; compound nature of light, 202; daylight photography, 267; periodicity of type, 208, 270, 272; photographs, 268–271; rarefaction, 271, 361; connexion with Zodiacal Light, 272, 273

Coronium, 238, 262

Co-tidal lines, 165

Coudé telescope, 27, 296

Crateris, R, 478

Craters, lunar, 292, 307

Crema meteorite, 386

Cross, Southern, 410, 416, 549

Crosswires, 195, 199, 206

Crucis, Kappa, 506

Cygni Beta, 417

— Chi, 460

— (_34_), 482

— (_61_), 422, 427

— Rho, new star near, 486

— Y, 474

Cygnus, 407

D

D’Alembert, verification of Newton’s Law, 11

D’Arrest, asteroidal orbits, 316; comet, 371, 372

Darwin, G. H., tidal friction, 236; origin of the moon, 236, 237; density of Saturn, 333

Day and night, 52

— apparent solar, 79; mean solar, 79

Declination, 66

De la Rue, celestial photography, 36, 295

Delphini, Beta, 435

— Gamma, 412

Deneb, 407

Denning, rotation of Saturn, 334; discovery of a comet, 370; August meteors, 391; meteor-radiants, 395, 396

Density of earth, 160

Deslandres, prominence-photography, 261; photographs of the sun as a bright-line star, 262; daylight coronal photography, 267; eclipse of 1893, 270; rotation of Jupiter, 325

Dewar, atmospheric resistance to meteorites, 388

Dhurmsala meteorite, 389

Diameters, determination of, 141

Diamonds in meteorites, 390

Diffraction grating, 216

Direct movement, 89

— vision spectroscope, 216

Distance of the stars, 417

Doberck, Dr., 441, 442, 447, 450, 451

Dollond, invention of achromatic lenses, 21

Donati, discovery of a comet, 362; cometary spectrum, 368

Double-slit method of photography, 261

Draconis, Gamma, 419, 420

Draper, Henry, photograph of the moon, 36

Dubjago, 435

Dunér, spectroscopic measurement of the sun’s rotation, 249; R Hydræ, 462; Y Cygni, 474; Z Herculis, 475

E

Earth, shape of, 41, 134; size of, 42, 134; rotation of, 47, 48, 283, 284; revolution of, 57; orbit of, 59, 72; varying speed of, 75; real path of, 77; shadow of, 110; mass of, 159; internal heat, 284, 285; age, 285; atmosphere, 286, 289; magnetic relations, 287, 288

Easton, 532, 548

Eccentricity of ellipse, 74

Eclipse, solar, of 1842, 253; of 1860, 254; of 1868, 254; of 1870, 258; of 1896, 259, 271; of 1882, 260, 268; of 1878, 268; of 1889, 268, 270; of 1893, 270

Eclipses, lunar, 111; partial, 111, 114; annular, 113; magnitude of, 113; total of sun, 113; duration of solar, 115; number of in a year, 118; recurrence of, 119; of satellites, 121; varieties of lunar, 295; of Jupiter’s satellites, 329; of Saturn’s, 342

Ecliptic, 56

— obliquity of, 61

Electrical theory of photospheric radiance, 242; of corona, 271, 272; of comets’ tails, 357, 358, 383; of cometary luminosity, 369, 384

Electra, 498, 499

Elements of an orbit, 106

Elevating floor, 193

Elger, lunar _maria_, 290

Elkin, Dr., transit of great comet, 359; meteorograph, 396; measurements, 421–423, 425, 433, 438

Ellipse, properties of, 73; eccentricity of, 74; foci of, 74; to draw an, 74

Elliptical nebulæ, 529–533

Elongations, 99

Encke, discovery of a comet, 366; resisting medium, 367

Enoch, Book of, 404

Equation of time, 79

Equator, terrestrial, 50; celestial, 66

Equatorial coudé, 193

Equatorial telescope, 185

Equinoxes, 55; precession of, 69, 167, 170

Equulei, Delta, 433

Eridani, (_40_), 444

Espin, 460, 482, 494

Establishment of a port, 165

Ether of space, 546

Euler, lunar theory, 11

Evening star, 100

Evolution, of solar system, 235, 310; of terrestrial, 236, 237, 283

Eye-pieces, 182

F

Fabricius, 458, 483

Fabry, cometary orbits, 372

Faculæ, associated with sun-spots, 244; rotation, 249; photographed, 262

Faye, planetary origin, 235, 350; water on Mars, 299

Fényi, solar eruptions, 259, 260

Finder of telescope, 187

First Point of Aries, 67

Fixed stars, 45, 423

Flammarion, rotation of Venus, 280; canals of Mars, 304; condition of Mars, 309

Flamsteed, first astronomer-royal, 15; stellar parallax, 18; Flamsteed’s star, 461

Fleming, Mrs., 460, 465, 489, 494–496

Fletcher, 447

Fomalhaut, 404

Fontana, pseudo-satellite of Venus, 282

Forbes, ultra-Neptunian planets, 231

Foucault’s pendulum, 48–50

Fraunhofer, improvement of telescopes, 21; solar spectrum mapped by, 34

Fraunhofer lines, 34, 249, 259, 271; interpreted, 35, 250; reflected in spectrum of Uranus, 346; in spectra of comets, 368

Fritsche, 433

Frost, spectrograph of Uranus, 346

Froley, 434

G

Galaxy. _See_ Milky Way

Galileo, telescopic observations, 9; double-star method of parallaxes, 28, 418

Gaseous nebula, 517

Gemini, star cluster in, 504

Geminorum, Zeta, 468

Gemma, Cornelius, 479

Gemmill, 554

Geocentric positions, 70

Geodesy, 129

Gill, Dr., photographs of comet of 1882, 38, 361; parallax of Sirius, 421; parallax and velocity, Lacaille, 424; Omega Centauri, 513

Glasenapp, 433–434

Gledhill, red spot on Jupiter, 323

Globular clusters, 507

Gnomon, 125

Goodricke, 465, 466, 470

Gould, Dr., photographic measurement of the Pleiades, 37; planetary photography, 327; Pi Gruis and R Sculptoris, 416; Kappa Crucis, 506; stars in Southern Hemisphere, 541; belt of stars intersecting the Milky Way, 551

Graduated circles, 171

Grating spectroscope, 216

Gravity, surface, on Mercury, 274; on Venus, 278; on the moon, 293; on Mars, 298; on Saturn, 335; on Uranus, 345; on Neptune, 349

Gravitation, laws of, 153; universal, 156

Greenwich observations, 15, 19, 20

Groombridge, 424

Grosch, corona of 1867, 268

Grubb, Sir Howard, great refractors, 26

— Thomas, Melbourne reflecting telescope, 24

Guinand, optical glass, 21

Gully, Ludovic, 488

Gylden, 423

Gyroscope, 50

H

Hadley, improvement of reflecting telescopes, 21

Hale, spectrographs of prominences, 261; calcium light pictures of sun and surroundings, 262; double-slit method of coronal photography, 267

Hall, Prof. Asaph, discovery of the moons of Mars, 26, 309; rotation of Saturn, 334

— Chester More, invention of achromatic lenses, 20

— Maxwell, 472

Halley, law of gravitation, 10; acceleration of the moon, 12; astronomer-royal, 16; comet calculated by, 16; transits of Venus, 17; discovery of proper motion in stars, 423; discovery of the star cluster in Hercules, 507

Harding, 548

Hartwig, 488

Harvest moon, 95

Heavens, diurnal motion of, 45

Heis, 400, 401, 541

Heliocentric positions, 70

Heliometer, 209

Helium, a chromospheric element, 255, 258; extracted from clevite, 255

Helmholtz, maintenance of sun’s heat, 234; past duration of sunlight, 285

Hencke, asteroidal discoveries, 314

Henderson, 422, 447

Henry’s belts of Uranus, 343

Hepidannus, 479

Herculis, Alpha, 413, 416

— Zeta, 435

— Z, 475

Herschel, Sir John, mathematical analysis at Cambridge, 15; observations of nebulæ, 23, 31; Magellanic clouds, 30, 31; survey of the heavens, 31; photography of sun-spots, 36; telescope, 180; great spot-group in 1837, 244; cyclonic theory of sun-spots, 252; Halley’s comet, 355; comet of 1843, 358; Biela’s comet, 365; red stars, 416; orbit of Gamma Virginis, 446; Kappa Crucis, 506; 2 Messier, 511–512; 22 Messier, 514; nebula round Eta Argus, 522–523; 30 Doradus, 524; the trifid nebula, Sagittarius, 525; planetary nebula, 528–529; the Nubecula Major, 534–536; Milky Way, crossed by zone of large stars, 552; observations in the Southern Hemisphere, 554

— Sir William, the sun’s translation, 19, 28; reflecting telescopes, 21–23; discovery of Uranus, 21, 22; of binary stars, 28; comprehensive designs, 27, 29; nebular theory, 30, 35; rotation of Jupiter’s satellites, 331; variability of Japetus, 341; discovery of Uranian moons, 347; binary stars, 419, 431; motion real and apparent, 428; Zeta Herculis, 435; Xi Ursæ Majoris, 440; 70 Ophiuchi, 441; 5 Messier, 510

Hevelius, 459, 462, 484

Hind, 433, 474, 482, 484

Hipparchus, construction of a star catalogue, 3; mathematical standpoint, 4

Holden, Prof., solar rotation, 249; names of asteroids, 315; helical nebulæ, 528

Holmes, discovery of a comet, 379

Holwarda, Phocylides, 458

Hooke, law of gravitation, 10; observations of Greek letter Draconis, 18; Gamma Arietis, 412; parallax of Gamma Draconis, 419–420

Horizon, visible, 41; sensible, 44; celestial, 44; rational, 44

Horrebow, satellite of Venus, 282

Hour circle, 186

Howlett, depression of sun-spot umbræ, 251

Huggins, Dr., stellar and nebular spectra, 35; photographed, 37; observations of prominences, 255; daylight coronal photography, 267; prismatic occultation of a star, 294; spectrum of Mars, 306; of Jupiter, 326; of Uranus, 345; of Winnecke’s comet, 368; spectrograph of Tebbutt’s comet, 368; measurement of motion in the line of sight, 426; spectroscopic examination of new star, 493; spectroscopic examination of the “fish-mouth” nebula, 518; discovery of gaseous spectrum, 528

Humboldt, meteoric shower of 1799, 392; temporary star of 1572, 479–481

Hussey, cometary forms, 380; photograph of Rordame’s comet, 383

Huygens, 417, 517

Hyades, 407, 549

Hydræ, R, 462

Hydrogen, ultra-violet spectrum in stars, 37; a gaseous metal, 250; a constituent of prominences and chromosphere, 255, 258; velocity of molecules, 313; free in atmospheres of Uranus and Neptune, 346, 349; assumed constituent of comets’ tails, 369, 370

Hypothesis of external galaxies, 546

I

Infinity of Space, 546

J

Jacob, 433, 447

Jacoby, measures of photographs, 37

Janssen, photograph of the sun, 243; spectroscopic method of prominence-observation, 254; double-slit method, 261

Japetus, remarkable eclipse, 338; variability, 341; plane of orbit, 342

Jesse, luminous night-clouds, 286

Job, Book of, 404

Johnson, 450

Juno, discovery, 311; diameter and albedo, 312, 316; a twin of Clotho, 317

Jupiter, long inequality, 12, 17; disturbance of Halley’s comet, 16; influence upon asteroidal distribution, 316–318; mass and figure, 318; rotation, 318, 325, 326; density, 319, 326; reflective power, 320; belts and streamers, 321, 322, 326; spots, 323, 325; photographs, 327; disturbance of comets, 371

Jupiter’s satellites, Galilean quartette, 9, 327, 328; transits, 329; constitution, 330; fifth satellite, 331, 332

K

Kapteyn, 422, 556, 561–563

Keeler, drawings of Jupiter, 321; description of markings, 322; spectroscopic test of the meteoric constitution of Saturn’s rings, 339; measuring velocities of nebula in line of sight, 428; spectra of the Orion nebula, 519–520

Kelvin, Lord, subterranean temperature, 285

Kepler’s Laws, 10, 155, 339, 417

Kirch, 460, 470, 510

Kirchhoff, spectrum analysis, 33; Fraunhofer’s lines, 34

Kirkwood, distribution of asteroids, 316, 317; divisions in Saturn’s rings, 338

Kleiber, number of shooting stars, 390

Koch, 461

Kreutz, relations of great southern comets, 360

Krüger, 442

L

Lacaille, southern nebulæ, 30

Lagrange, verified principle of gravitation, 11; stability of solar system, 13

Lajoye, 488

Lamp, fate of Brorsen’s comet, 370

Lane’s law, 242

Langley, solar radiation, 238, 239; spectroscopic effects of sun’s rotation, 249; temperature of the moon, 294; fireball, 386

Laplace, verified Newton’s law, 11; lunar acceleration, 12; _Mécanique Céleste_, 13, 14; nebular hypothesis, 235

Lassell, large reflectors, 24; discoveries of Hyperion, Ariel, and Umbriel, 24, 341, 347; Saturn’s dark ring, 336

Latitude, terrestrial, 50, 125; celestial, 68; of sun, 77; geocentric, 135; geographical, 135; astronomical, 136; variation of, 136

Leland, Miss, 511

Leonid meteors, 391–395

Leonis, Gamma, 413

— R, 461

Lepaute, Madame, computation of Halley’s comet, 16

Leverrier, discovery of Neptune, 32; intra-Mercurian planet, 232; mass of asteroids, 315; orbit of November meteors, 395

Lewis, 426

Libræ, Delta, 473

Librations, of Mercury, 277; of Venus, 281; of the moon, 93, 289

Lick observatory, 25, 26

Light-equation, 329

“Light journey,” 420

Limited number of visible stars, 538, 545

Limiting apertures, 212

Lippershey, inventor of the telescope, 9

Lockyer, spectroscopic observations at the sun’s limb, 254; classification of prominences, 250; solar tornadoes, 259

Loewy, Coudé telescope, 27; lunar photography, 296

Longitude, terrestrial, 50, 125; celestial, 68

Lowell, rotation of Mercury, 277; observations of Venus, 279, 281; lakes of Mars, 301, 302; relation to canals, 302–304

Luminous night-clouds, 286

Lunar distances, 129

— ecliptic limit, 112

Lyncis (_12_), 450

Lyra, annular nebula in, 526

Lyræ, Beta, 465

Lyraid meteors, 393, 395

M

Maclear, 464

Mädler, search for Martian moons, 309; compression of Uranus, 343

Madrid meteorite, 385

Magellanic clouds, 30, 534–537

Magnetism, terrestrial, 287, 288

Magnitude, of eclipses, 113; of stars, 212

Magnitudes, star, 403, 404

Mann, 433

Maps, 133

Maraldi, 462, 470, 511

Marchand, observations of the Zodiacal Light, 273

Markwick, Col., 547

Mars, phases of, 104; parallax of, 147; a superior planet, 297; seasons, 298, 301, 302; snow-caps, 299, 303, 306; land and water, 299–301, 305, 306; continents, 300, 301; canals, 301, 304; duplication, 301, 305; spectrum, 306; atmosphere, 307, 313; mountains, 307; climate, 308; moons, 309, 310

Marth, Neptune’s satellite, 350

Mascari, rotation of Venus, 280

Maskelyne, astronomer-royal, 19; founded _Nautical Almanac_, 20; star-motions, 28

Mass, defined, 151; sun, 156; planets, 157; moon, 158; of asteroids, 158; earth, 159; satellites, 159

Maunder, 460, 488, 493

Maxwell, Clerk, constitution of Saturn’s rings, 337, 340

Mayer, Tobias, lunar tables, 11; star-motions, 28

Mazapil meteorite, 396

Measurement, of earth, 42, 129; of sun’s distance, 146; of binary stars, 208; of planets, 208

_Mécanique Céleste_, character, 13, 14

Megrez, 402

Mercury, Copernican theory of movements, 8; transit of, 101; phases of, 101; orbit, 273, 274; atmosphere, 274, 275; rotation, 275–277; as an abode of life, 277; capture of Encke’s comet, 372

Meridian, 50; line, 51; arc of, 130; circle, 198; photometer, 214

Merope, 498, 499

Messier (_3_), 509

— (_5_), 510

— (_11_), 506

— (_22_), 514

— (_37_), 505

— (_51_), 533

— (_57_), 526

— (_80_), new star in, 485

— (_92_), 509

— (_99_), 534

— discoveries of nebulæ, 30

Metonic cycle, 92

Meteoric systems, 231, 390, 391; radiants, 392, 395, 396

Meteorites, falls, 385–387; legal status, 387; velocities, 387, 388, 390; thumb-marks, 388; chemical composition, 389; enclosed diamonds, 390

Meteors, Perseid, 391, 393; Leonid, 391–393; Andromede, 393–394, 396; relations to comets, 393, 395

Micrometer, wire, or pillar, 205; evolution of, 207

Michell, prevision of binary stars, 28

Midnight sun, 63

Milky Way, 402, 430, 555, 557

— — star streams, 9; disc theory, 29

Minimum deviation, 215

Mira Ceti, 458, 459

Mitchell, 421, 431

Mizar, 402, 411, 455, 457

Molyneux, 419, 420

Montanari, 471

Month, 91

Moon, acceleration, 12; _contumax sidus_, 16; observations, 19; apparent motion of, 87; orbit of, 88, 94; phases of, 89; sidereal period of, 89; synodic period of, 91; rotation of, 92; librations of, 93; harvest, 95; high and low, 97; shadow of, 115; distance of, 143; size of, 144; mass of, 158; possible disintegration, 233; origin, 236, 237; rotation, 289; cones and craters, 290, 292, 293; rays and rills, 293; absence of air and water, 294, 313; temperature, 294, 295; eclipses, 295; photography, 295–297

Morning star, 100

Müller, surface of Mercury, 275; photometry of asteroids, 312; albedo of Jupiter, 320; of Saturn, 334; of Neptune, 349

Muscæ, R, 468

N

Nadir, 45

Nasir Eddin, planetary tables, 5

Nasmyth, conjunction of Mercury and Venus, 278

Nearest fixed stars, 417

Nebula, Orion, 23, 25, 30

Nebulæ, structure, 23; spiral, 24; photographs, 23, 25; first discoveries, 29, 30; status, 30, 31; gaseous nature, 30, 35; annular, 526, 527; elliptical, 529, 533; gaseous, 517–524; planetary, 527–529; spiral, 533, 534

Nebular hypothesis, 30, 35, 235, 530

Nebulous stars, 529

Neptune, discovery, 32, 229; distance from the sun, 232; dimensions, 349; compression, 351; retrograde rotation, 351; planets as viewed from, 351, 352; family of comets, 371, 372

Neptune’s satellite, discovery, 24; plane of revolution, 350; precessional disturbance, 351

Newall, 25-inch refractor, 26

Newcomb, Prof., past duration of sunlight, 285; light changes of Ariel, 347; satellite of Neptune, 351; the runaway star, 424; proper motion of Alcyone, 501

New stars, 477–497

Newton, H. A., capture of comets, 372; meteoric cult, 387; daily number of shooting stars, 390

— Sir Isaac, law of gravitation, 10, 11; invention of reflecting telescope, 21; comet of 1680, 355; decay of comets, 366

Newtonian telescope, 179

Nichol, Dr., 508

Niesten, rotation of Venus, 280; mass of asteroids, 315

Nodes, 94

North polar distance, 66

Nova Andromedæ, 488

— Aurigæ, 489

— Cassiopeiæ, 479

— Cygni, 486

— Ophiuchi, 484

— Serpentarii, 483

— Vulpeculæ, 484

Nubecula Major, 534

— Minor, 535

Number of visible stars, 538–544

Nutation, 169

O

Oases of Mars, 302–305

Object-glass, achromatic, 177; photographic, 195; photo-telescope, 196

Objective prism, 223

Obliquity of ecliptic, 61

Observatories, 191

— Lick, 189, 190, 202

— Nice, 192

— Yerkes, 189

Occultations, 121

— of stars, by the moon, 294; by comets, 366

Olbers, discovery of Pallas and Vesta, 311; origin of asteroids, 311, 316; electrical theory of comets, 357; classification, 358, 383; comet discovered by, 371

Ophiuchi, Nova, 483–485

— (_70_), 441

— U, 473, 476

Opposition, 103, 105

Orbit, of earth, 72, 76; of moon, 88; elements of a planetary orbit, 106; of binary stars, 432

Orion, 406, 408, 417

— great nebula in, 517–521

Orionis, Alpha (Betelgeuse), 404, 408, 415, 427

— Iota, 414

— Sigma, 414

— Theta, 414

“Owl,” nebula, 528

P

P (_34_) Cygni, 482

Palisa, discoveries of asteroids, 314

Palitzsch, 470

Pallas, discovery, 311; diameter, 312

Parallax of stars, 419, 420

— diurnal, 140; equatorial horizontal, 140; horizontal, 140; of sun, 146; of Mars, 147

Parmentier, distribution of asteroids, 316

Pegasus, Square of, 409

Pegasi, Kappa, 433

— (_85_), 434

— U, 469

Pendulum observations, 135; compensated, 174

Penumbra, of earth’s shadow, 111

Percentage of stars in Milky Way, 547, 548

Perigee, 89

Perihelion, 75

Perrotin, rotation of Venus, 280; of Uranus, 343; markings on Uranus, 344

Persei, Beta (Algol). _See_ Algol

Perseid meteors, 391; associated with Tuttle’s comet, 393

Perseus, 407

— star clusters in, 503

Perturbations, 158

Peters, 428

Phases of moon, 89; of Venus, 101; of Mars, 104

Phocylides Holwarda, 458

Photographic telescopes, 194

Photography of nebulæ, 23, 25, 38; of sun-spots, 36, 243, 244; of the moon, 36, 295–297; of stellar spectra, 37; of comets, 38, 354, 377–383; celestial, 194; of spectra, 219, 223; of the eclipsed sun, 254; of the reversing layer, 259; of prominence-spectra, 260; of prominences and faculæ, 261, 262; of the corona, 267, 269–271; planetary, 327; meteoric, 396

Photoheliograph, 197

Photometers, wedge, 213; meridian, 214

Photosphere, visible structure, 242

Piazzi, five-foot circle, 20; discovery of Ceres, 311

Pickering, Prof. E. C., photometric measures of asteroids, 312; photograph of Jupiter, 327; the spectrum of Alpha Centauri, 441; the spectrum of Pleione, 498

Pickering, W. H., lunar photographs, 296; mounting of telescopes, 297; lakes and canals of Mars, 301, 304; water area on Mars, 305; star collisions, 495; nebula surrounding Zeta Orionis, 520

Pigott, 467

“Pilgrim star,” 479–482

Planetary nebulæ, 527–529

Planets, apparent movements of, 98; interior and exterior, 98; conjunctions of, 99, 103; phases of, 101, 104; oppositions of, 103; synodic periods of, 107; times of revolution, 107; relative distances of, 144; distances of, 150; terrestrial, 229; giant, 229, 319, 343; trans-Neptunian, 231; intra-Mercurian, 232; decay, 233; comets captured by, 371, 372

— minor. _See_ Asteroids

Pleiades, 404, 407, 497–502, 539, 549

Pleione, 498

“Plough,” 400–402, 405

Plummer, short-period comets, 371; Encke’s, 372

Podmaniczky, Baroness, 488

Pogson, 485

Polar axis, 185

Polaris. _See_ Pole Star

Pole, celestial, 46; terrestrial, 50; movements of, 138

Pole Star, 46, 405, 412, 421, 427

Pollux, 404, 406, 415, 427

Pond, defects of Greenwich quadrant, 19; astronomer-royal, 20

Position, angle, 208; circle, 208

Poynting’s experiment, 160

Præsepe, 502

Precession, of equinoxes, 69; effects of, 170; luni-solar, 169

Prime vertical, 210

Principia, publication, 10, 13; character, 14

Prism, action of, 215; objective, 223

Prismatic camera, 223

— spectroscope, 215

Pritchard, Prof., 422, 424

Proctor, Saturn’s rings, 341; distance of Uranus, 344; Proctor’s chart, 548; stars in streams, 550

Procyon, supposed satellite, 31, 32; order of magnitude, 404; parallax of Procyon, 421; Procyon approaching the Earth, 427

Prominences, solar appendages, 253, 254; spectrum, 254, 256, 258, 260; daylight observations, 254, 255; quiescent and eruptive, 256; periodicity, 257; rapid development, 259; spectral photography, 260, 261

“Proper motions” of stars, 423–431

Ptolemaic system, 3, 4, 6

Q

Quadrature, 105

R

Rambaud, absorption in solar atmosphere, 240; fireball, 380

Ramsay, terrestrial discovery of helium, 255

Ramsden, astronomical circles, 20

Raynard, the sun a nebulous body, 253; future of Saturn’s ring-system, 340; outflows from comets, 380; star streams, 551

Ravené, gravitational disturbance by asteroids, 315

R Centauri, 478

Reading microscope, 172

Recurrence of eclipses, 119

“Red Bird,” 415

Red spot on Jupiter, 323, 324

Red stars, 416

Reduction of observations, 18, 19

Refracting telescope, 176

Refraction, 52

— in Venus, 278

Reflecting telescope, 178

Regression of moon’s nodes, 94

Regulus, 406, 410, 427

Retrogradation, 89, 103

Reversing layer, 249, 258, 271; photographed, 259

Rich and poor regions, 549

Richaud, 440

Rigel, 404, 414, 427

Right ascension, 66

Roberts, Dr., 23, 107, 502, 503, 508, 511, 520, 525–528, 530, 533, 534, 539, 540

Roberts, A. W., 440, 441, 469

Roche, minimum distance of satellites, 340

Römer, velocity of light, 329

Rosse, Earl of, giant reflector, 24

Roszl, mass of 311 asteroids, 315

Rotation of earth, 47, 48; of moon, 92

Rowland grating, 217

— solar elements, 250

Russell, photograph of Swift’s comet, 377; Kappa Crucis, 506; the “key-hole” nebula, 522; the Magellanic clouds, 536

Rutherfurd, photographs of the moon, 295

S

Sacrobosco, treatise on the sphere, 6

Sagittarii, Zeta, 434

Saros, 120

Satellites, movements of Satellites, 108; masses of Satellites, 159

— discoveries, 9, 23, 25, 26, 309, 347; apportionment, 230; formation checked by tidal friction, 277, 282; planes of revolution, 328, 347, 348, 350; transits, 329, 330, 342; eclipses, 329, 342; variability, 330, 341, 347; rotation, 331, 341, 342, 347

Saturn, density, 333; spectrum, 334; rotation, 334, 339; dimensions, 535

Saturn’s ring-system, dusky member, 25, 336, 338; dimensions, 336; constitution, 337, 339, 340; albedo, 338

Sawyer, U Ophiuchi discovered, 473; variability of R Canis Majoris detected, 473

Schaeberle, photographs of corona of 1893, 270; land and water on Mars, 306

Scheiner, spectra of sun-spots, 251

Schiaparelli, rotation of Mercury, 275; map of Mercury, 277; rotation of Venus, 280, 281; canals of Mars, 301; duplication, 305; climate of Mars, 308; compression of Uranus, 343; comets and meteors, 393; theory of extinction of light, 544

Schiehallion experiment, 161

Schmidt, map of the moon, 290

Schönfeld, 461, 464, 466, 467, 473, 483

Schorr, 442

Schur, 433, 442

Schuster, photograph of eclipsed sun, 268

Schwabe, discovery of sun-spot periodicity, 245

Seasons, 61

Secchi, observations of prominences, 256; spectrum of Uranus, 345

See, Dr., 413, 433–435, 440, 442, 447, 448

Seeliger, photometric measures of Saturn’s rings, 339

Serpentarii, Nova, 483

Sextant, 211

Shackleton, photograph of the reversing layer, 259

Ship, position of, 128

“Sickle” in Leo, 406

Siderostat, 194

Sidgreaves, elevations of chromosphere, 258

Sirius, proper motion, 17, 31; companion, 32; spectrum, 37; size, 403; position, 409; colour, 414; distance, 418, 421; discovery of proper motion, 423; a binary star, 437; comparative magnitude, 438

Smyth, 447, 448, 502, 505, 508

Solar, constant, 239

— diagonal, 183

— eclipses, 113

— ecliptic limit, 118

— System, dominated by gravity, 29; constitution, 229, 232; dimensions, 231; stability, 232; origin, 235, 236

Southern Cross, 410, 416, 549

Shouting, 68, 198

Spectroheliograph, 225

Spectroscope, prismatic, 215; direct vision, 216; grating, 216; Lick star-, 219; Rowland, 217; tele-, 219

Spectroscopic measurements of rotation; the sun, 248; Venus, 281; Saturn, 339

Spectrum, solar, 34, 250; of stars and nebulæ, 35, 37; measurement of, 218; sun-spot, 250, 251; prominence, 254, 256, 261; chromospheric, 258; of Mercury, 275; of Venus, 279; auroral, 288; of Jupiter, 326; of Saturn’s rings, 338; of Uranus, 345, 346; of Neptune, 351; of comets, 368

Spherical excess, 133

Spica, 404, 410

Spiral nebulæ, 533, 534

Spoerer, solar rotation, 249

Star of Bethlehem, 101

Star-charting, photographic, 38

— cluster, 17

— spectroscope, 219

— time, 68

Stars, temporary, 3, 8, 477; proper motions of, 17, 19, 28, 425, 427; fixed, 45; circumpolar, 46; diurnal motion of, 46; aberration of, 58; catalogues of, 71; clock, 82; morning and evening, 100; magnitudes, 403, 404; Pole, 405, 412; double, 410; coloured, 416; red, 416; nearest, 417; binary, 431; variable, 458

Stationary points, 103

Stone, mass of Titan, 342

Stoney, G. Johnstone, atmospheres of planets, 313

Stratonoff, sun’s rotation from faculæ, 249

Suess, theory of lunar formations, 292

Sun, translation, 28, 229; apparent movements of, 55, 77; midnight, 63; apparent diameter of, 72; mean, 79; eclipses of, 113; distance of, 146; mass of, 156; maintenance of heat, 234; radiative power, 237–239, 241, 242; temperature, 239, 240; magnitude, 240, 241; luminous surface, 242; spots, 243–249, 251, 252; periodicity, 246; rotation, 247–249; chemistry, 250; theories, 252

Sun-dial, 78

Sun’s motion in space, 428

Sun-spots, observed by Galileo, 9; construction, 243, 251; zones, 245, 247; periodicity, 245, 247; irregular movements, 247–249; spectra, 250–252

Sutton, 553

Swift, Lewis, comet discovered by, 377, 378, 383

Sykora, elevation of spotted areas on the sun, 252

Synodic period, of moon, 91; of planets, 107

T

Tacchini, spectrum of Venus, 279; rotation, 280

Talcott’s latitude method, 124

Tauri, Alpha. _See_ Aldebaran

— Lambda, 473

Tebbutt’s comet, 362, 368

Telescope, invention of, 9; achromatic, 20, 21; reflecting, 21, 24, 25, 178; refracting, 20, 25–27, 176; future improvement, 26, 27, 297; Newtonian, 179; Cassegrain, 180, 181; Herschellian, 180; Skew Cassegrain, 181; magnifying power of, 184; illuminating power of, 184; altazimuth, 184; equatorial, 185; Rosse, 187; Common, 5-foot, 188; Lick, 190; fixed, 194; photographic, 194

Telespectroscope, 219

Tempel, 501

“Temporary stars,” 477–497

Theodolite, 205

Thiele, 435, 447, 451

Thome, comet of 1887, 360

Tidal evolution, 167

Tidal friction, 166; in earth-moon system, 236, 283, 284; on Mercury, 277; effect on satellite-formation, 278, 282; on Venus, 282; on Phobos, 310; on Saturnian satellites, 342

Tides, 162; spring and neap, 164; priming and lagging, 164

Time, apparent, 78; equation of, 79; mean solar, 79; determination of, 82; at different places, 83; Greenwich mean, 83; local, 83; telegraphy, 84; zone, 84; balls, 85

Tisserand, revolutions of Jupiter’s fifth satellite, 331; disturbance of Neptune’s satellite, 351; capture of comets, 372

Todd, Miss M. L., drawing of corona, 268

— Prof., trans-Neptunian planet, 231

Toucani (_41_), 513

Transit circle, 198–202

— instrument, 202

— of Venus, 101, 148

Triangulation, 53

Troughton, instrumental improvements, 19, 20

Trouvelot, mountains of Venus, 279; rotation, 280

Twilight, 53

Tycho Brahé, 5, 8, 9, 405, 418, 479, 481, 529

U

Ulugh Beigh, observations at Samarcand, 5

Umbra of earth’s shadow, 111

Uranus, discovery, 22, 229; perturbations, 32, 231; dimensions and markings, 343–345; analogy with Neptune, 343, 351; rotation, 344, 348; spectrum, 345, 346; satellites, 347, 348; comets captured by, 371, 395

Ursa Major, stars in, 400, 401

Ursæ Majoris, Xi, 440

V

Variation of latitude, 136

Variable stars, 458

Vega, 403, 406, 414, 422, 427

Venus, phases observed by Galileo, 9; transits, 17; phases of, 101; transit of, 101, 148; atmosphere, 278, 281, 282; ashen light, 279; spectrum, 279; rotation, 280, 281; imaginary satellite, 282

Vernier, 172

Very, distribution of lunar heat, 295

Vesta, discovery, 311; diameter and brightness, 312; mass, 313

Villarceau, 433

Virginis, Alpha (Spica), 404, 410

— Gamma, 413, 444–450

— Tau, 456

— W, 469

Visible stars, number of, 538–546

Vogel, spectrum of Jupiter, 326; of Uranus, 345; binary or multiple system of Beta Lyræ, 466; diameter of Algol, 472

Volcanic action, terrestrial, 284; lunar, 290, 292

Von Gothard, 465

Vulpeculæ, Nova, 484

— S, 484

W

Ward, 488

Way, Milky, 402, 430, 549, 557

Webb, 528

Wedge, photometer, 213

Weight, defined, 151; of the earth, 160

Wells’ comet, 368

Williams, A. Stanley, rotation of Venus, 280; of Jupiter, 325; photographs of Jupiter, 327; spots on Saturn, 334

Wilson, Alexander, depression of sun-spots, 251

— W. E., temperature of the sun, 240

Winnecke, 422

Winnecke’s comet, 368, 371, 372

Wire micrometer, 205

Wolf, Max, photographic discovery of asteroids, 314; comet discovered by, 377; chart of the Pleiades, 499

Wrublewsky, 434

Y

Year, 85; sidereal, 85; tropical, 85; leap, 86

Yendell, 474

Yerkes, 40-inch refractor, 26, 27

Young, solar eruption, 256; spectrum of chromosphere, 258; reversing layer, 258; spectrum of Venus, 279; brightness of Phobos, 309; belts of Uranus, 343; size of Uranus, 345; the sun and planets, seen from Neptune, 349, 352; Andromede meteors, 394

Z

Zenith, 45

— telescope, 210

Zodiac, 60

Zodiacal Light, 272, 273

Zöllner, albedo of Mars, 298, 334; of Jupiter, 320; of Neptune, 349; estimate of sunlight, 543

Zone time, 84

THE END.

-----

Footnote 1:

There is a very complete paper on “How to find Easter,” by Dr. Downing, in the _Journal_ of the British Astronomical Association, vol. ii., p. 264.

Footnote 2:

The application of Kepler’s third law gives us P = _a_^{³⁄₂} years, but as this is not strictly true, both P and _a_ must be given where the greatest possible accuracy is desired.

Footnote 3:

The diagram is based upon one given by Prof. Albrech in the _Astronomische Nachrichten_, No. 3333. The dotted part of the curve could not be directly derived on account of insufficient observations.

Footnote 4:

The focal length of a lens is the distance from its centre at which an image of a very distant object, such as the sun, is formed.

Footnote 5:

In a British inch there are 25·4 millimetres.

Footnote 6:

Proctor: “Old and New Astronomy,” p. 327.

Footnote 7:

Langley: “The New Astronomy,” p. 108.

Footnote 8:

The “bolometer,” invented by Langley, measures heat with exquisite refinement by means of its electrical effects.

Footnote 9:

W. E. Wilson: _Monthly Notices_, vol. lv., p. 457.

Footnote 10:

_Observatory_, vol. xviii., p. 344.

Footnote 11:

Frost-Scheiner: “Astronomical Spectroscopy,” p. 177.

Footnote 12:

_Astronomische Nachrichten_, No. 3330.

Footnote 13:

_Knowledge_, vol. vi., p. 13.

Footnote 14:

“The Sun,” p. 206, first edition.

Footnote 15:

“Memoirs of the Royal Astronomical Society,” vol. xli., p. 435.

Footnote 16:

_Astronomy and Astro-Physics_, vol. xiii., p. 122.

Footnote 17:

_Comptes Rendus_, December 26, 1893.

Footnote 18:

_Knowledge_, vol. iv., p. 105.

Footnote 19:

“Rapport de la Mission envoyée an Sénégal,” p. 31.

Footnote 20:

“Harvard Annals,” vol. xix., part ii.; 1893.

Footnote 21:

“The Solar Corona discussed by Spherical Harmonics;” Washington, 1889.

Footnote 22:

_Bulletin Astronomique_, April, 1896.

Footnote 23:

According to G. Müller, _Potsdam Publicationen_, No. 30, p. 369, Zöllner fixed the albedo of Mercury at 0·13.

Footnote 24:

_Astr. Nach._, No. 3171.

Footnote 25:

_Astr. Nach._, No. 3406.

Footnote 26:

_Ibid._, No. 2944.

Footnote 27:

_Astr. Nach._, No. 3332.

Footnote 28:

This was in principle suggested by Proctor in “The Old and New Astronomy.”

Footnote 29:

_Nature_, vol. li., p. 227.

Footnote 30:

Kelvin, _Nature_, p. 440; Clarence King, _American Journal of Science_, January, 1893.

Footnote 31:

_Ciel et Terre_, 16th March, 1895.

Footnote 32:

_Himmel und Erde_, Feb., 1889; _Astr. Nach._, No. 3347; A. Battandier, _L’Astronomie_, 1894.

Footnote 33:

Balfour Stewart: “Ency. Brit.,” vol. xvi. pp. 164, 165.

Footnote 34:

A. Paulsen: _Ciel et Terre_, 1 Juillet, 1895, p. 202.

Footnote 35:

Elger: “The Moon,” p. 73.

Footnote 36:

“Publications, Astronomical Society of the Pacific,” vol. vii., p. 144.

Footnote 37:

“Harvard Annals,” vol. xxxii., part i., p. 109.

Footnote 38:

_Astronomy and Astro-Physics_, Nov., 1894, p. 718.

Footnote 39:

“Popular Astronomy,” 1895, p. 347.

Footnote 40:

_Astr. Nach._, No. 3271 (Schiaparelli).

Footnote 41:

“Popular Astronomy,” vol. i., p. 348.

Footnote 42:

_Scientific American_, Feb. 29, 1896.

Footnote 43:

Schiaparelli: _Astronomy and Astro-Physics_, Nov., 1894, p. 720.

Footnote 44:

_Astronomy and Astro-Physics_, August, 1894, p. 554.

Footnote 45:

“Publ. Astro. Soc. of the Pacific,” vol. iv., p. 196.

Footnote 46:

_Monthly Notices_, vol. lvi., p. 166.

Footnote 47:

Campbell: “Publ. A. S. P.,” vol. vi., p. 273.

Footnote 48:

_Ibid._, vol. ii., p. 248.

Footnote 49:

_Ibid._, vol. vi., p. 110.

Footnote 50:

_Astronomy and Astro-Physics_, October, 1894, p. 640.

Footnote 51:

_Potsdam Publicationen_, No. 30, 1893.

Footnote 52:

Barnard: _Monthly Notices_, vol. lvi., p. 55.

Footnote 53:

John Hopkins’ _University Circular_, Jan., 1895.

Footnote 54:

_Astr. Nach._, No. 3359.

Footnote 55:

_Monthly Notices_, vol. lvi., p. 250.

Footnote 56:

Barnard: _Astr. Journal_, No. 325, 1894.

Footnote 57:

“Publ. A. S. P.,” vol. ii., p. 286.

Footnote 58:

Maunder: _Knowledge_, vol. xix., p. 5.

Footnote 59:

_Monthly Notices_, vol. lvi., p. 143.

Footnote 60:

“Scientific Proceedings, R. Dublin Society,” vol. viii., p. 398.

Footnote 61:

_Astro.-Phys. Journal_, May, 1896, p. 394; “Rapport de l’Observatoire de Paris,” 1895, p. 22.

Footnote 62:

Proctor: “Old and New Astronomy,” p. 584.

Footnote 63:

“The subject of slant-markings,” Mr. Stanley Williams remarks (_loc. cit._), “has only just begun to be investigated.”

Footnote 64:

“Jupiter and his System,” by Ellen M. Clerke, p. 43.

Footnote 65:

_Comptes Rendus_, t. cxix., p. 581.

Footnote 66:

G. H. Darwin: _Harper’s Magazine_, June, 1889.

Footnote 67:

Barnard, _Monthly Notices_, vol. lvi., p. 163.

Footnote 68:

Lewis: _Observatory_, vol. xviii., p. 379.

Footnote 69:

_Monthly Notices_, vol. lii., p. 419.

Footnote 70:

“Abhandlungen Akad. der Wissensch.” München, Bl. xvi., p. 403.

Footnote 71:

_Astro-Physical Journal_, May, June, 1895.

Footnote 72:

“Old and New Astronomy,” p. 640.

Footnote 73:

“Phil. Trans.,” vol. lxxxii., p. 17.

Footnote 74:

“Publications Astr. Soc. of the Pacific,” vol. iii., p. 284.

Footnote 75:

_Astr. Journal_, No. 370.

Footnote 76:

Perrotin: “Vierteljahrsschrift Astr. Ges.,” Jahrg. xxiv., p. 267.

Footnote 77:

“Annales de l’Observatoire de Nice,” t. ii., 1887.

Footnote 78:

Keeler: _Astr. Nach._, No. 2927.

Footnote 79:

Gregory: _Nature_, vol. xl., p. 236.

Footnote 80:

“General Astronomy,” p. 372.

Footnote 81:

_Astronomical Journal_, No. 342.

Footnote 82:

Tisserand: _Astronomy and Astro-Physics_, vol. xiii., p. 291 (1894).

Footnote 83:

_Comptes Rendus_, t. cvii., p. 804.

Footnote 84:

_Astronomical Journal_, No. 186.

Footnote 85:

“General Astronomy,” p. 372.

Footnote 86:

“Observations at the Cape of Good Hope,” p. 396.

Footnote 87:

“Monat. Correspondenz,” Bd. xxv., pp. 3–22, 1812.

Footnote 88:

Fessenden: _Astro-Physical Journal_, vol. iii., p. 40.

Footnote 89:

_Astr. Nach._, No. 2837.

Footnote 90:

Guillemin: “The World of Comets,” p. 282.

Footnote 91:

_Astr. Nach._, No. 2437.

Footnote 92:

_Knowledge_, Feb., 1896, p. 41.

Footnote 93:

For an account of its spectral changes, see Campbell in _Astr. and Astr.-Physics_, vol. xi., p. 698.

Footnote 94:

Barnard, _Knowledge_, vol. viii., p. 229.

Footnote 95:

Denning: _Astronomy and Astro-Physics_, vol. xii., p. 371.

Footnote 96:

_Astroph. Journal_, Jan., 1896, p. 42.

Footnote 97:

Ranyard: _Knowledge_, vol. ix., p. 159.

Footnote 98:

“Publications Astr. Pac. Society,” vol. vii., p. 166.

Footnote 99:

Hussey: _loc. cit._, p. 171.

Footnote 100:

Holden: “Publ. Astr. Pac. Society,” vol. ii., p. 19. H. A. Newton: _Ibid._, vol. iii., p. 91.

Footnote 101:

“Report Bri. Ass.,” 1891, p. 805.

Footnote 102:

S. Meunier: “Encycl. Chimique,” t. ii., p. 461.

Footnote 103:

Young: “Gen. Astr.,” p. 435.

Footnote 104:

Cornish: _Knowledge_, vol. vi., p. 163.

Footnote 105:

_Journal Brit. Astr. Ass._, vol. vi., p. 432.

Footnote 106:

H. A. Newton: “Proc. Amer. Phil. Society,” vol. xxxii.

Footnote 107:

Quoted by Sir F. Palgrave: “Phil. Trans.,” vol. cxxx., p. 175.

Footnote 108:

_Observatory_, April, 1895.

Footnote 109:

_Observatory_, Jan., 1896.

Footnote 110:

It has been recently seen again in America.

Footnote 111:

_Journal of the British Astronomical Association_, March, 1891.

Footnote 112:

_Nature_, Feb. 13, 1896.

Footnote 113:

“Planetary and Stellar Studies,” p. 257.

Footnote 114:

See Chapter V.

Footnote 115:

_Comptes Rendus_, March 30, 1896.

Footnote 116:

_Nature_, April 30, 1896.

Footnote 117:

“Cape Observations,” p. 34.

Footnote 118:

_Journal of the British Astronomical Association_, vol. iv., No. 11, p. 21.

Footnote 119:

_Journal of the British Astronomical Association_, vol. vi., No. 6, p. 312.

Footnote 120:

Recent observations show that the total variation is 2·71 magnitudes—the largest variation known for an Algol star.

Footnote 121:

“Cosmos,” Bohn’s edition, vol. iii., p. 205.

Footnote 122:

It was, however, asserted by Herlicius that he had seen it on Sept. 27.

Footnote 123:

The spectrum, however, seems to have since become continuous.

Footnote 124:

_Astronomical Journal_, No. 100.

Footnote 125:

_Journal of the British Astronomical Association_, March, 1892.

Footnote 126:

_Journal of the British Astronomical Association_, February, 1895, vol. v. No. 4.

Footnote 127:

_Ibid._, April, 1895, p. 328.

Footnote 128:

_Journal of the British Astronomical Association_, February, 1892.

Footnote 129:

_The Observatory_, December, 1895.

Footnote 130:

“Planetary and Stellar Studies,” p. 188.

Footnote 131:

_Nature_, September 6, 1894.

Footnote 132:

_Nature_, June 4, 1896.

Footnote 133:

“Cosmos,” vol. iii., Bohn’s edition, p. 192.

Footnote 134:

Humboldt’s “Cosmos,” Bohn’s edition, vol. iv., pp. 327, 328.

Footnote 135:

_Monthly Notices_, Royal Astronomical Society, June, 1888.

Footnote 136:

“Old and New Astronomy,” p. 794.

Footnote 137:

_Nature_, June 4, 1896.

Footnote 138:

_Nature_, September, 1894.

Footnote 139:

_Ibid._, October 4, 1894.

Footnote 140:

“Outlines of Astronomy,” tenth edition, p. 657.

Footnote 141:

_Nature_, November, 21, 1895.

Footnote 142:

_Nature_, January 16, 1896.

Footnote 143:

_Nature_, August 9, 1888.

Footnote 144:

Humboldt’s “Cosmos,” Bohn’s edition, vol, iii., p. 143.

Footnote 145:

See _Knowledge_, June, 1895.

Footnote 146:

“The Universe and the Coming Transits,” p. 200.

Footnote 147:

_Journal of the British Astronomical Association_, May, 1895, p. 383.

Footnote 148:

_Knowledge_, May, 1896.

Footnote 149:

_Knowledge_, July, 1891.

Footnote 150:

_Knowledge_, January, 1894, p. 17.

Footnote 151:

_Journal of the British Astronomical Association_, April, 1895, p. 304.

Footnote 152:

The Italics are Herschel’s.

Footnote 153:

A full discussion of Struve’s views will be found in Chapter XVI. of “The Visible Universe,” by the present writer.

Footnote 154:

“The Meteoritic Theory,” pp. 380, 381.

------------------------------------------------------------------------

D. APPLETON AND COMPANY’S PUBLICATIONS.

_THE SUN._ By C. A. YOUNG, Ph. D., LL. D., Professor of Astronomy in Princeton University. New and revised edition, with numerous Illustrations. 12mo. Cloth, $2.00.

“In this book we see a master’s hand. Professor Young has no superiors, if he has rivals, among astronomers in this country.... ‘The Sun’ is a book of facts and achievements, and not a discussion of theories, and it will be read and appreciated by all scientific students, and not by them alone. Being written in untechnical language, it is equally adapted to a large class of educated readers not engaged in scientific pursuits.”—_Journal of Education, Boston._

“Professor Young’s work is essentially a record of facts and achievements, rather than of theories and attempts at the interpretation of mysteries; yet the great questions still remaining to be answered are of course discussed, and in a masterly manner.”—_Philadelphia Evening Bulletin._

“It is one of the best books of popular science ever written, and deserves a host of readers.”—_The Dial, Chicago._

“You feel throughout that a master is leading you amid the intricacies and mazes of one of the most absorbing of studies.... Many a one whose views are hazy and dim will find here just that enlightenment, without an overburdened technicality, that will prove most useful.”—_The Interior._

_THE STORY OF THE SUN._ By Sir ROBERT S. BALL, F. R. S., author of “An Atlas of Astronomy,” “The Cause of an Ice Age,” etc. 8vo. Cloth, $5.00.

“Sir Robert Ball has the happy gift of making abstruse problems intelligible to the ‘wayfaring man’ by the aid of simple language and a few diagrams. Science moves so fast that there was room for a volume which should enlighten the general reader on the present state of knowledge about solar phenomena, and that place the present treatise admirably fills.”—_London Chronicle._

“As a specimen of the publisher’s art it is superb. It is printed on paper which entices the reader to make marginal notes of reference to other books in his library, the type is large, the binding is excellent, and the volume is neither too large nor too small to handle without fatigue.”—_New York Herald._

_AN ATLAS OF ASTRONOMY._ By Sir ROBERT S. BALL, F. R. S., Professor of Astronomy and Geometry at the University of Cambridge; Author of “Starland,” “The Cause of an Ice Age,” etc. With 72 Plates, Explanatory Text, and Complete Index. Small 4to. Cloth, $4.00.

“The high reputation of Sir Robert Ball as a writer on astronomy at once popular and scientific is in itself more than sufficient recommendation of his newly published ‘Atlas of Astronomy.’ The plates are clear and well arranged, and those of them which represent the more striking aspects of the more important heavenly bodies are very beautifully executed. The introduction is written with Sir Robert Ball’s well-known lucidity and simplicity of exposition, and altogether the Atlas is admirably adapted to meet the needs and smooth the difficulties of young and inexperienced students of astronomy, as well as materially to assist the researches of those are more advanced.”—_London Times._

_OUTLINES OF ASTRONOMY._ By Sir J. J. W. HERSCHEL. With Plates and Woodcuts. Eleventh edition. 8vo. Cloth, $4.00.

_ORIGIN OF THE STARS, AND THE CAUSES OF THEIR MOTIONS AND THEIR LIGHT._ By JACOB ENNIS. 12mo. Cloth, $2.00.

_SPECTRUM ANALYSIS, IN ITS APPLICATION TO TERRESTRIAL SUBSTANCES, and the Physical Constitution of the Heavenly Bodies._ Familiarly explained by Dr. H. Schellen, Director der Realschule I. O. Cologne. Translated from the second enlarged and revised German edition by Jane and Caroline Lassell. Edited, with Notes, by William Huggins, LL. D. With numerous Woodcuts, Colored Plates, and Portraits; also, Angström’s and Kirchhoff’s Maps. 8vo. Cloth, $6.00.

“Certainly, as regards mere knowledge, the ‘Spectrum Analysis’ has let us into many secrets of the physical universe which Newton and Laplace would have declared impossible for man’s intellect to attain. The science is still in its infancy, but it is prosecuted by some of the ablest, most patient, and most enthusiastic observers, and some of the keenest thinkers at present existing on our little, insignificant physical globe.”—_Boston Globe._

_STUDIES IN SPECTRUM ANALYSIS._ By J. NORMAN LOCKYER, F.R.S., Correspondent of the Institute of France, etc. With 60 Illustrations. 12mo. Cloth, $2.50.

“The study of spectrum analysis is one fraught with a peculiar fascination, and some of the author’s experiments are exceedingly picturesque in their results. They are so lucidly described, too, that the reader keeps on, from page to page, never flagging in interest in the matter before him, nor putting down the book until the last page is reached.”—_New York Evening Express._

_ASTRONOMY AND GEOLOGY COMPARED._ By Lord ORMATHWAITE. 18mo. Tinted paper. Cloth, $1.00.

_THE STORY OF THE STARS._ By G. F. CHAMBERS, F. R. A. S., author of “Handbook of Descriptive and Practical Astronomy,” etc. With 24 Illustrations. Library of Useful Stories. 16mo. Cloth, 40 cents.

“One can here get a clear conception of the relative condition of the stars and constellations, and of the existent universe so far as it is disclosed to view. The author presents his wonderful and at times bewildering facts in a bright and cheery spirit that makes the book doubly attractive.”—_Boston Home Journal._

RICHARD A. PROCTOR’S WORKS.

_OTHER WORLDS THAN OURS. The Plurality of Worlds, Studied under the Light of Recent Scientific Researches._ By RICHARD ANTHONY PROCTOR. With Illustrations, some colored. 12mo. Cloth, $1.75.

CONTENTS.—Introduction. What the Earth teaches us. What we learn from the Sun. The Inferior Planets. Mars, the Miniature of our Earth. Jupiter, the Giant of the Solar System. Saturn, the Ringed World. Uranus and Neptune, the Arctic Planets. The Moon and other Satellites. Meteors and Comets: Their Office in the Solar System. Other Suns than Ours. Of Minor Stars, and of the Distribution of Stars in Space. The Nebulæ: Are they External Galaxies? Supervision and Control.

_OUR PLACE AMONG INFINITIES._ A Series of Essays contrasting our Little Abode in Space and Time with the Infinities around us. To which are added Essays on the Jewish Sabbath and Astrology. 12mo. Cloth, $1.75.

CONTENTS.—Past and Future of the Earth. Seeming Wastes in Nature. New Theory of Life in other Worlds. A Missing Comet. The Lost Comet and its Meteor Train. Jupiter. Saturn and its System. A Giant Sun. The Star Depths. Star Gauging. Saturn and the Sabbath of the Jews. Thoughts on Astrology.

_THE EXPANSE OF HEAVEN._ A Series of Essays on the Wonders of the Firmament. 12mo. Cloth, $2.00.

CONTENTS.—A Dream that was not all a Dream. The Sun. The Queen of Night. The Evening Star. The Ruddy Planet. Life in the Ruddy Planet. The Prince of Planets. Jupiter’s Family of Moons. The Ring-Girdled Planet. Newton and the Law of the Universe. The Discovery of Two Giant Planets. The Lost Comet. Visitants from the Star Depths. Whence come the Comets? The Comet Families of the Giant Planets. The Earth’s Journey through Showers. How the Planets Grew. Our Daily Light. The Flight of Light. A Cluster of Suns. Worlds ruled by Colored Suns. The King of Suns. Four Orders of Suns. The Depths of Space. Charting the Star Depths. The Star Depths Astir with Life. The Drifting Stars. The Milky Way.

_THE MOON: Her Motions, Aspect, Scenery, and Physical Conditions._ With Three Lunar Photographs, Map, and many Plates, Charts, etc. 12mo. Cloth, $2.00.

CONTENTS.—The Moon’s Distance, Size, and Mass. The Moon’s Motions. The Moon’s Changes of Aspect, Rotation, Libration, etc. Study of the Moon’s Surface. Lunar Celestial Phenomena. Condition of the Moon’s Surface. Index to the Map of the Moon.

_LIGHT SCIENCE FOR LEISURE HOURS._ A Series of Familiar Essays on Scientific Subjects, Natural Phenomena, etc. 12mo. Cloth, $1.75.

_POPULAR ASTRONOMY._ A General Description of the Heavens. By CAMILLE FLAMMARION. Translated from the French by J. Ellard Gore. With 3 Plates and 288 Illustrations. 8vo. Cloth, $4.50.

“The fullest and most elaborate compendium of popular knowledge of astronomy.... The book might reasonably be pronounced the most desirable of its kind.”—_New York Sun._

“M. Flammarion has produced a work that charms while it interests. He has classified astronomy so perfectly that any person of ordinary intelligence may learn from his book practically all the men in the observatories know.”—_New York Times._

“Flammarion talks, and his conversation is free from those technical expressions which make the obscure style more obscure. He treats the most abstruse problems in such a fashion that you see through them more clearly than you ever thought it possible to do without years of study.”—_New York Herald._

“While the translator has done excellent work, he has also added largely to the value of the book by his carefully prepared notes, in which he brings every astronomical theme down to date.”—_Chicago Inter-Ocean._

“The book is one of extreme interest, and to our mind far surpasses in fascination any novel that was ever written.”—_London Literary World._

_ASTRONOMY WITH AN OPERA-GLASS._ A Popular Introduction to the Study of the Starry Heavens with the Simplest of Optical Instruments. By GARRETT P. SERVISS. 8vo. Cloth, $1.50.

“The glimpses he allows to be seen of far-stretching vistas opening out on every side of his modest course of observation help to fix the attention of the negligent, and lighten the toil of the painstaking student.... Mr. Serviss writes with freshness and vivacity.”—_London Saturday Review._

“By its aid thousands of people who have resigned themselves to the ignorance in which they were left at school, by our wretched system of teaching by the book only, will thank Mr. Serviss for the suggestions he has so well carried out”—_New York Times._

“We are glad to welcome this popular introduction to the study of the heavens.... There could hardly be a more pleasant road to astronomical knowledge than it affords.... A child may understand the text, which reads more like a collection of anecdotes than anything else, but this does not mar its scientific value.”—_Nature._

“Mr. Garrett P. Serviss’s book, ‘Astronomy with an Opera-Glass,’ offers us an admirable handbook and guide in the cultivation of this noble æsthetic discipline (the study of the stars).”—_New York Home Journal._

“The book should belong to every family library.”—_Boston Home Journal._

WORKS BY ARABELLA B. BUCKLEY (MRS. FISHER).

“Mrs. Fisher has a positive genius for presenting the science of living things in shapes that boys and girls can understand.”—_Philadelphia Ledger._

“Mrs. Fisher’s apprenticeship as secretary to Sir Charles Lyell has resulted in a life-long devotion to the natural sciences, the fruits of which have been a series of charming books valuable to the young, and indeed to grown-ups.”—_The Critic._

_THE FAIRY-LAND OF SCIENCE._ With 74 Illustrations. Cloth, $1.50.

CONTENTS.—The Fairy-Land of Science: How to Enter it; How to Use it; How to Enjoy it.—Sunbeams, and the Work they do.—The Aërial Ocean in which we Live.—A Drop of Water on its Travels.—The Two Great Sculptors, Water and Ice.—The Voices of Nature, and How we Hear them.—The Life of a Primrose.—The History of a Piece of Coal.—Bees in the Hive.—Bees and Flowers.

_THROUGH MAGIC GLASSES, and other Lectures._ A Sequel to “The Fairy-Land of Science.” Cloth, $1.50.

CONTENTS.—The Magician’s Chamber by Moonlight.—Magic Glasses and How to Use Them.—Fairy Rings and How They are Made.—The Life History of Lichens and Mosses.—The History of a Lava-Stream.—An Hour with the Sun.—An Evening with the Stars.—Little Beings from a Miniature Ocean.—The Dartmoor Ponies.—The Magician’s Dream of Ancient Days.

_LIFE AND HER CHILDREN: Glimpses of Animal Life, from the Amœba to the Insects._ With over 100 Illustrations. Cloth, $1.50.

CONTENTS.—Life’s Simplest Children; How they Live, and Move, and Build.—How Sponges Live.—The Lasso-Throwers of the Ponds and Oceans.—How Star-Fish Walk and Sea-Urchins Grow.—The Mantle-covered Animals, and How they Live with Heads and without them.—The Outcasts of Animal Life, and the Elastic-ringed Animals by Sea and by Land.—The Mailed Warriors of the Sea, with Ringed Bodies and Jointed Feet.—The Snare-Weavers and their Hunting Relations.—Insect Suckers and Biters which Change their Coats but not their Bodies.—Insect Sippers and Gnawers which Remodel their Bodies within their Coats.—Intelligent Insects with Helpless Children, as illustrated by the Ants.

_WINNERS IN LIFE’S RACE; or, The Great Backboned Family._ With numerous Illustrations. Cloth, $1.50.

CONTENTS.—The Threshold of Backboned Life.—How the Quaint Old Fishes of Ancient Times have Lived on into Our Day.—The Bony Fish, and How they have Spread over Sea, and Lake, and River.—How the Backboned Animals pass from Water-breathing to Air-breathing, and find their Way out upon the Land.—The Cold-blooded Air-breathers of the Globe in Times both Past and Present.—The Feathered Conquerors of the Air.—Their Wanderings over Sea and Marsh, Desert and Plain.—From Running to Flying, from Mound-laying to Nest-building, from Cry to Song.—The Mammalia or Milk-Givers, the simplest Suckling Mother, the active Pouchbearers, and the Imperfect-toothed Animals.—From the Lower and Small Milk-Givers which find Safety in Concealment, to the Intelligent Apes and Monkeys.—The Large Milk-givers which have conquered the World by Strength and Intelligence.—How the Backboned Animals have returned to the Water, and Large Milk-givers Imitate the Fish.—A Bird’s-eye View of the Rise and Progress of Backboned Life.

_A SHORT HISTORY OF NATURAL SCIENCE; and of the Progress of Discovery from the Time of the Greeks to the Present Time._ New edition, revised and rearranged. With 77 Illustrations. Cloth, $2.00.

“The work, though mainly intended for children and young persons, may be most advantageously read by many persons of riper age, and may serve to implant in their minds a fuller and clearer conception of ‘the promises, the achievements, and claims of science.’”—_Journal of Science._

_PRINCIPLES OF GEOLOGY; or, The Modern Changes of the Earth and its Inhabitants, considered as illustrative of Geology._ By Sir CHARLES LYELL, Bart. Illustrated with Maps, Plates, and Woodcuts. Two vols., royal 8vo. Cloth, $8.00.

The “Principles of Geology” may be looked upon with pride not only as a representative of English science, but as without a rival of its kind anywhere. Growing in fullness and accuracy with the growth of experience and observation in every region of the world, the work has incorporated with itself each established discovery, and has been modified by every hypothesis of value which has been brought to bear upon, or been evolved from, the most recent body of facts.

_THE AUTOBIOGRAPHY OF THE EARTH._ A Popular Account of Geological History. By Rev. H. N. HUTCHINSON, B. A., F. G. S. With 27 Illustrations. Crown 8vo. Cloth, $1.50.

“A scientific work, divested of technicalities, and put into a bright, narrative form which can not but attract even the most general reader.”—_Boston Transcript._

“It is hardly conceivable that the rich results of the science of geology can be so treated as to prove uninteresting to thinking people, be they young or old. When, therefore, we say that Mr. Hutchinson’s book is extremely interesting, no more might be implied than that the author has skillfully used the vast materials at his hand. But Mr. Hutchinson has successfully carried out a difficult design on an admirable plan, and has adhered to that plan throughout. His sketch of historical geology has a genuine continuity.”—_Saturday Review._

_A REPRINT OF ANNUAL REPORTS AND OTHER PAPERS OF THE GEOLOGY OF THE VIRGINIAS._ By the late WILLIAM BARTON ROGERS, LL. D., etc., Director of the Geological Survey of Virginia from 1835 to 1841. With numerous Maps, Colored Charts, etc. 12mo. Cloth, $5.00.

_NATURAL RESOURCES OF THE UNITED STATES._ By JACOB HARRIS PATTON, M. A., Ph. D. 8vo, 523 pages. Cloth, $3.00.

“This portly octavo of over 500 pages is an encyclopedic directory to its subject, and a work of comprehensive scope, exhaustive research, scientific spirit, and good literary form.... Capitalists, investors, railroad projectors, land speculators, and all who need to know the distribution of land values, will find a vast amount of information in this well-arranged work, the contents of which, it is safe to say, could not be found assembled in similar compass elsewhere.”—_Boston Literary World._

“As interesting to read as it is valuable to consult. By the employment of fine white paper and large, clear type, the publishers have made it an elegant specimen of the printer’s art.”—_New York Sun._

BOOKS BY PROF. G. FREDERICK WRIGHT.

_GREENLAND ICEFIELDS, AND LIFE IN THE NORTH ATLANTIC_. With a New Discussion of the Causes of the Ice Age. By G. FREDERICK WRIGHT, D. D., LL. D., F. G. S. A., author of “The Ice Age in North America,” “Man and the Glacial Period,” etc., and WARREN UPHAM, A. M., F. G. S. A., late of the Geological Surveys of New Hampshire, Minnesota, and the United States. With numerous Maps and Illustrations. 12mo. Cloth, $2.00.

The immediate impulse to the preparation of this volume arose in connection with a trip to Greenland by Professor Wright in the summer of 1894 on the steamer Miranda. The work aims to give within moderate limits a comprehensive view of the scenery, the glacial phenomena, the natural history, the people, and the explorations of Greenland. The photographs are all original, and the maps have been prepared to show the latest state of knowledge concerning the region. The volume treats of the ice of the Labrador current, the coast of Labrador, Spitzbergen ice in Davis Strait, the Greenland Eskimos, Europeans in Greenland, explorations of the inland ice, the plants and animals of Greenland, changes of level since the advent of the Glacial period, and includes a summary of the bearing of the facts upon glacial theories. The work is of both popular and scientific interest.

_THE ICE AGE IN NORTH AMERICA, and its Bearings upon the Antiquity of Man._ With an Appendix on “The Probable Cause of Glaciation,” by WARREN UPHAM, F. G. S. A., Assistant on the Geological Surveys of New Hampshire, Minnesota, and the United States. New and enlarged edition. With 150 Maps and Illustrations. 8vo, 625 pages, and Index. Cloth, $5.00.

“The author has seen with his own eyes the most important phenomena of the Ice age on this continent from Maine to Alaska. In the work itself, elementary description is combined with a broad, scientific, and philosophic method, without abandoning for a moment the purely scientific character. Professor Wright has contrived to give the whole a philosophical direction which lends interest and inspiration to it, and which in the chapters on Man and the Glacial Period rises to something like dramatic intensity.”—_The Independent._

_MAN AND THE GLACIAL PERIOD._ International Scientific Series. With numerous Illustrations. 12mo. Cloth, $1.75.

“The earlier chapters describing glacial action, and the traces of it in North America—especially the defining of its limits, such as the terminal moraine of the great movement itself—are of great interest and value. The maps and diagrams are of much assistance in enabling the reader to grasp the vast extent of the movement.”—_London Spectator._

_PIONEERS OF SCIENCE IN AMERICA._ Sketches of their Lives and Scientific Work. Edited and revised by WILLIAM JAY YOUMANS, M. D. With Portraits. 8vo. Cloth, $4.00.

Impelled solely by an enthusiastic love of Nature, and neither asking nor receiving outside aid, these early workers opened the way and initiated the movement through which American science has reached its present commanding position. This book gives some account of these men, their early struggles, their scientific labors, and, whenever possible, something of their personal characteristics. This information, often very difficult to obtain, has been collected from a great variety of sources, with the utmost care to secure accuracy. It is presented in a series of sketches, some fifty in all, each with a single exception accompanied with a well-authenticated portrait.

“Fills a place that needed filling, and is likely to be widely read.”—_New York Sun._

“It is certainly a useful and convenient volume, and readable too, if we judge correctly of the degree of accuracy of the whole by critical examination of those cases in which our own knowledge enables us to form an opinion.... In general, it seems to us that the handy volume is specially to be commended for setting in just historical perspective many of the earlier scientists who are neither very generally nor very well known.”—_New York Evening Post._

“A wonderfully interesting volume. Many a young man will find it fascinating. The compilation of the book is a work well done, well worth the doing.”—_Philadelphia Press._

“One of the most valuable books which we have received.”—_Boston Advertiser._

“A book of no little educational value.... An extremely valuable work of reference.”—_Boston Beacon._

“A valuable handbook for those whose work runs on these same lines, and is likely to prove of lasting interest to those for whom ‘_les documents humain_’ are second only to history in importance—nay, are a vital part of history.”—_Boston Transcript._

“A biographical history of science in America, noteworthy for its completeness and scope.... All of the sketches are excellently prepared and unusually interesting.”—_Chicago Record._

“One of the most valuable contributions to American literature recently made.... The pleasing style in which these sketches are written, the plans taken to secure accuracy, and the information conveyed, combine to give them great value and interest. No better or more inspiring reading could be placed in the hands of an intelligent and aspiring young man.”—_New York Christian Work._

“A book whose interest and value are not for to-day or to-morrow, but for indefinite time.”—_Rochester Herald._

“It is difficult to imagine a reader of ordinary intelligence who would not be entertained by the book.... Conciseness, exactness, urbanity of tone, and interestingness are the four qualities which chiefly impress the reader of these sketches.”—_Buffalo Express._

“Full of interesting and valuable matter.”—_The Churchman._

THE ANTHROPOLOGICAL SERIES.

NOW READY.

_THE BEGINNINGS OF ART._ By ERNST GROSSE, Professor of Philosophy in the University of Freiburg. A new volume in the Anthropological Series, edited by Professor Frederick Starr. Illustrated. 12mo. Cloth, $1.75.

“This book can not fail to interest students of every branch of art, while the general reader who will dare to take hold of it will have his mind broadened and enriched beyond what he would conceive a work of many times its dimensions might effect.”—_Brooklyn Eagle._

“The volume is clearly written, and should prove a popular exposition of a deeply interesting theme.”—_Philadelphia Public Ledger._

_WOMAN’S SHARE IN PRIMITIVE CULTURE._ By OTIS TUFTON MASON, A. M., Curator of the Department of Ethnology in the United States National Museum. With numerous Illustrations. 12mo. Cloth, $1.75.

“A most interesting _résumé_ of the revelations which science has made concerning the habits of human beings in primitive times, and especially as to the place, the duties, and the customs of women.”—_Philadelphia Inquirer._

_THE PYGMIES._ By A. DE QUATREFAGES, late Professor of Anthropology at the Museum of Natural History, Paris. With numerous Illustrations. 12mo. Cloth, $1.75.

“Probably no one was better equipped to illustrate the general subject than Quatrefages. While constantly occupied upon the anatomical and osseous phases of his subject, he was none the less well acquainted with what literature and history had to say concerning the pygmies.... This book ought to be in every divinity school in which man as well as God is studied, and from which missionaries go out to convert the human being of reality and not the man of rhetoric and text-books.”—_Boston Literary World._

_THE BEGINNINGS OF WRITING._ By W. J. HOFFMAN, M. D. With numerous Illustrations. 12mo. Cloth, $1.75.

This interesting book gives a most attractive account of the rude methods employed by primitive man for recording his deeds. The earliest writing consists of pictographs which were traced on stone, wood, bone, skins, and various paperlike substances. Dr. Hoffman shows how the several classes of symbols used in these records are to be interpreted, and traces the growth of conventional signs up to syllabaries and alphabets—the two classes of signs employed by modern peoples.

IN PREPARATION.

_THE SOUTH SEA ISLANDERS._ By Dr. SCHMELTZ. _THE ZUÑI._ By FRANK HAMILTON CUSHING. _THE AZTECS._ By Mrs. ZELIA NUTTALL.

RECENT VOLUMES OF THE INTERNATIONAL SCIENTIFIC SERIES.

_THE AURORA BOREALIS._ By ALFRED ANGOT, Honorary Meteorologist to the Central Meteorological Office of France. With 18 Illustrations. $1.75.

While there have been many monographs in different languages upon various phases of this subject, there has been a want of a convenient and comprehensive survey of the whole field. Professor Angot has cited a few illustrations of each class of phenomena, and, without encumbering his book with a mass of minor details, he presents a picture of the actual state of present knowledge, with a summary both of definite results and of the points demanding additional investigation.

_THE EVOLUTION OF THE ART OF MUSIC._ By C. HUBERT H. PARRY, D. C. L., M. A., etc. $1.75.

Dr. Parry’s high rank among modern writers upon music assures to this book a cordial welcome. It was first published as “The Art of Music,” in octavo form. The title of this revised edition has been slightly amplified, with a view of suggesting the intention of the work more effectually.

_WHAT IS ELECTRICITY?_ By JOHN TROWBRIDGE, S. D., Rumford Professor and Lecturer on the Applications of Science to the Useful Arts, Harvard University. Illustrated. $1.50.

Professor Trowbridge’s long experience both as an original investigator and as a teacher imparts a peculiar value to this important work. Finding that no treatise could be recommended which answers the question, What is Electricity? satisfactorily, he has explained in a popular way the electro-magnetic theory of light and heat, and the subject of periodic currents and electric waves, seeking an answer for his titular question in the study of the transformation of energy and a consideration of the hypotheses of movements in the ether.

_ICE-WORK, PRESENT AND PAST._ By T. G. BONNEY, D. Sc., F. R. S., F. S. A., etc., Professor of Geology at University College, London. $1.50.

In his work Professor Bonney has endeavored to give greater prominence to those facts of glacial geology on which all inferences must be founded. After setting forth the facts shown in various regions, he has given the various interpretations which have been proposed, adding his comments and criticisms. He also explains a method by which he believes we can approximate to the temperature at various places during the Glacial epoch, and the different explanations of this general refrigeration are stated and briefly discussed.

_MOVEMENT._ By E. J. MAREY, Member of the Institute and of the Academy of Medicine; Professor at the College of France; Author of “Animal Mechanism.” Translated by Eric Pritchard, M. A. With 200 Illustrations. $1.75.

The present work describes the methods employed in the extended development of photography of moving objects attained in the last few years, and shows the importance of such researches in mechanics and other departments of physics, the fine arts, physiology, and zoölogy, and in regulating the walking or marching of men and the gait of horses.

D. APPLETON AND COMPANY, NEW YORK.

------------------------------------------------------------------------

TRANSCRIBER’S NOTES

Page Changed from Changed to

475 the diameter of our sun, I find, the diameter of our sun, I find, from the above date, that the from the above data, that the

● Typos fixed; non-standard spelling and dialect retained. ● Used numbers for footnotes, placing them all at the end of the last chapter. ● Enclosed italics font in _underscores_. ● Enclosed bold or blackletter font in =equals=. ● The caret (^) serves as a superscript indicator, applicable to individual characters (like 2^d) and even entire phrases (like 1^{st}). ● Subscripts are shown using an underscore (_) with curly braces { }, as in H_{2}O. ● HTML alt text was added for images that didn’t have captions.