VI.

THE ELECTRICITY OF THE SMOKE AND ASHES.

Our ancestors could judge that a great amount of electricity was occasionally evolved in the smoke, from their observation of the lightning flashes that darted through the Vesuvian pine tree; but they had no proper instruments for ascertaining whether this evolution of electricity was constant or accidental, or what laws regulated its manifestations. My _apparatus, with movable conductor_, by which comparative observations of electric meteorology can be made, and the errors arising from dispersion corrected, supplied me with an easy method of studying the electricity evolved during eruptions.

I must begin by describing the bifilar electrometer, in order to explain the apparatus which I have named as above, "_Apparechio a conduttore mobile_."

_A A_ (Plate VIa, Fig. 1) is a glass cylinder, the lower edge of which is ground, well varnished with gum lac, and let into a wooden base, B, furnished with three levelling screws. Through a sufficiently wide glass tube, _a a_, runs a copper rod covered with insulating mastic, having a little plate or cylindrical cavity of gilded brass at the top (Figs. 2 and 3), with two arms _d d_, _d' d_. In the plate a disc of aluminium, _m_, is suspended by means of two silk fibres, and to the disc a very fine aluminium wire is attached, _f f'_, bent a little at the ends, as are the arms, _d d_, _d' d_. The disc has about three millimetres less diameter than the plate. The diameter of the plate may vary within certain limits, but I have found it convenient to make it eighteen millimetres. The glass tube, _a a_ (Fig. 1), should descend below the base as much as it rises above it, that is three to four centimetres. The length of the index is about one decimetre.

The upper ends of the two silk fibres, by which the disc and index are suspended, are attached to the top of the glass tube, _C_, by a contrivance which permits a change in the distance between the two points of suspension, and a screw, _p_, is provided to raise and lower the disc with the index. At _n_, at the lower part of the tube, _C_, there is a kind of torsion micrometer, arranged so as to bring the index to the zero of the scale engraved on the graduated ring, _B_, which is formed of a strip of good paper pasted on the rim of a glass disc. The index must be placed at the zero of the scale, and must be some distance from the ends of the arms of the plate with which it is parallel. The plate is about three millimetres deep.

Having levelled the instrument, so as to render the disc concentric with the plate, and placed the index at zero, it is obvious that if an electric charge through the wire, _h_, reach the plate with the arms, it will electrify the disc and index: the disc will have the opposite electricity, and the extremities of the index will take the same electricity as the arms, and consequently the index will describe an arc more or less great. The motion of the index is sufficiently slow to allow the eye conveniently to follow it. Having traversed the first arc, which I call the _impulsive_ one, the index returns, and, after only two oscillations, comes to rest at what I shall call the _definite_ arc.

When the electric charges are of very brief duration, the impulsive arcs are within certain limits proportional to the tensions, and the ratio between the impulsive and definite arcs is expressed by the following equation:

A (B - A) / B = tang. (1/2) A

In which B is the impulsive arc and A the definite arc, showing that A comes out nearly equal to 1/2 B. In dry weather all goes perfectly within the limits of proportion, and I can tell whether, during the time in which the index traversed the impulsive arc, there were any _dispersions_ and of what nature; for if the definite arc is not close to the limit of the impulsive arc, it is a sign of _dispersions_ having taken place during the motions of the index. Every degree less in the definite arc denotes two degrees of loss for the impulsive arc; but as the index employs double the time traversing the definite as it does the impulsive arc, we may consider the loss of one equal to the loss of the other.

In excessively damp weather the index gives no definite arc, and it is necessary to resort to artificial heat in order to dry the insulators. The most simple means I know of is to hold the instrument over some hollow vessel, which, for the time, is converted into a stove by the introduction of a spirit lamp.

From Gauss's formula for the bifilar system of instruments of this class, we learn that the maximum sensitiveness of such instruments is given when the length of the suspending fibres is greatest, and the distance between them is smallest, with the weight of the movable or rotating member a minimum; and these elements being the same, the sensitiveness of the instruments is invariable.

To some electrometers, in order to avoid errors of parallax, a small telescope, with a micrometer wire, has been added; but, with a little practice, we can read accurately without this refinement. In order to obtain comparative measurements, it is necessary to select some given unit of tension. I have observed that by making a galvanic pile of copper, zinc and distilled water, and insulating it well, each pole has a tension which remains the same for many days, if the conditions of temperature and the moisture of the surrounding atmosphere are not very different. With thirty pairs of this pile, each element having twenty-five square centimetres of surface, I have on the electrometer a definite arc of 15°, with the temperature of the atmosphere at 20° C., and with the difference of 4° to 5° C. between the thermometers of the psychrometer of August's construction. The first observation was made twenty-four hours after mounting the pile. For unit of tension I took that which corresponded to a single pair, that is, the thirtieth part of the total tension. Other electrometers may be compared with one already properly adjusted, without always having recourse to the pile.

This done, let us see the arrangement of all the apparatus:

_H H_ (Plate VIIa, Fig. 1) is the ceiling of a well-situated lofty room, with an opening, _o o_, at the upper part.

_M M_, a bracket or table fastened against the wall, about a metre distant from the ceiling, _H H_.

_N N_, a wooden platform for the observer.

_A_, the bifilar electrometer.

_B_, Bohnenberger's electroscope.

_a a_, a movable conductor formed of a brass rod 15 to 18 millimetres in diameter, insulated below by means of a glass rod, well varnished with gum lac, having a suspending pulley, _c_, and a wooden guide-rod underneath it, _l_, within the guiding tube, _k_. At the upper part of this conductor, _a a_, there is a sliding roof, _b_, which can be adjusted so as to prevent rain entering at the opening, _o o_. The conductor terminates in a disc made of a sheet of thin brass, _d_, 24 centimetres in diameter. Upon this disc, or even in place of it, we may use metallic points.

As a support to the conductor at the upper part, I have made use of a triangular ring, _x_, drawn at its full size in Fig. 2. The conductor passes between three springs, and the triangular ring is held in place by three silk cords, _m m m_. Their material should not be mixed with any cotton, and it may be advisable to saturate them with an alcoholic solution of gum lac.

_f f f_ is a hempen cord, which is used to raise and lower the conductor.

_i_ is a copper wire covered with silk, by means of which the triangular ring, _x_, and through that and its springs the conductor communicates with either the electrometer or the electroscope.

Quickly raising the conductor by pulling the cord, _f_, the index of the electrometer will describe a more or less large impulsive arc, and, after two oscillations, will stop at the definite arc. Having thus measured the electric tension of the air, and having lowered the conductor, I next place the wire, _i_, in communication with the electroscope, _B_, and by again raising the conductor, I ascertain whether the electricity be positive or negative. It is scarcely necessary to say that the conductor, when raised, gives electricity of the same nature as that prevailing at the moment in the atmosphere; and when lowered, manifests the opposite. In some conjunctures we must keep the conductor raised and in communication with the electroscope, in order to observe certain phenomena which I shall presently describe: this method I call observation with a _fixed conductor_.

I have also constructed a similar but portable apparatus for use on eruptive cones, when required.

Having given this description of the apparatus, it remains for me to relate the results obtained, especially on the occasion of the last eruption of Vesuvius.

The Observatory is distant, in a direct line from the central crater of Vesuvius, 2,380 metres, so that, when the smoke is copious, it is properly situated for the study of electricity, particularly when the wind inclines the pine-tree cloud in the direction of the Observatory, as frequently happened on the last occasion.

With smoke alone, without ashes, we obtained strong tensions of positive electricity; with ashes only, which sometimes fell while the smoke turned in the other direction, we had strong negative electricity; when the smoke inclined towards the Observatory, accompanied with ashes and lapilli, we had sometimes one kind of electricity, and sometimes the other, just as the smoke or the ashes predominated; and often with a "fixed conductor" we obtained negative electricity, and with a "movable conductor" positive electricity. In Naples, too, at the Meteorological Observatory attached to the University, my colleague, Professor Eugenio Semmola, observed negative electricity of strong tension whilst ashes were falling there in abundance. The tensions on this occasion were so strong as to equal those obtained at changes of weather or during storms (_temporali_), and, being beyond measure with a delicate electrometer, we marked them with the symbol for infinity: the same phenomena were observed when lightnings flashed.

When there is but little smoke, it is necessary to approach the eruptive mouths with a portable apparatus, in order to observe those phenomena which, in great eruptions, may be studied from the Observatory itself.

The conditions under which (_folgori_) lightning flashes are seen from the cloud of smoke are, that it is conveying great abundance of ashes. In 1861, there were small flashes even from the line of eccentric mouths above Torre del Greco, although the smoke was not very great; and when these ceased to discharge, and the central crater became somewhat active, with a moderate amount of smoke but a great deal of ashes, small and frequent lightning flashes were observed in the twilight darting through the smoke, which was dark in colour. In 1850 the eruption was more vigorous, the smoke more abundant, and the ashes scarce, but the flashes were very rare. In 1855, 1858, and 1868, with a scanty supply of ashes and at intervals, no flashes were observed, and the electricity remained constantly positive. But having regard to the facts of antecedent eruptions, one sees that the flashes are always derived, from the midst of smoke accompanied with ashes and lapilli, which separate like rain from the rolling volumes of smoke, in the midst of which they were ejected.

But how can we account for the positive electricity of the smoke, and the negative electricity of the falling ashes? Without denying the probability that a part of the positive electricity depends upon the elevation of the smoke, as in the case of every other conductor we raise aloft, or with a jet of water sent from a vessel by compressed air, I think that the greater part of the electricity proceeds from the rapid condensation of vapours, which are changed from the gaseous condition into dense clouds; for even when the smoke issues tranquilly and does not rise, because carried away horizontally by the wind, it gives signs of positive electricity. From all my studies of atmospheric electricity, and from some experiments made specially, it follows that the condensation of vapours is the origin of this development of positive electricity.

The negative electricity of the falling ashes certainly arises from the fact itself of their fall; for if we place a metallic vessel full of ashes upon an elevated and well-situated terrace, while the atmospheric electricity is positive, and cause the ashes from the vessel to fall gradually into an insulated metallic cup, communicating with Bohnenberger's electroscope placed at three or four metres distance from the vessel, the electroscope will manifest negative electricity. If the upper vessel be insulated, and the ashes permitted to fall upon the ground, we shall obtain, from the vessel, positive electricity. The intensity of these electric manifestations depends (other things being equal) upon that predominant at the moment in the air; so that if the experiment be made while negative electricity prevails, the falling ashes will manifest positive electricity, the upper vessel then showing negative electricity. Now, as the ashes separate from the positively electrified smoke in order to approach the ground, which is negatively electrified, it follows that they must manifest negative electricity upon touching the ground, leaving the positive electricity in the smoke above. For this reason, the electric tension of the smoke is increased by the descent of the ashes and lapilli, so that discharges between the upper and lower part of the pine-tree cloud, or the surface of the crater, are rendered possible. Hence it follows that the flashes of lightning of Vesuvius play through the smoke, and with difficulty strike bodies upon the earth; and from this circumstance our ancestors believed the thunderbolts of Vesuvius to be harmless. However, if the smoke were very great, and driven by the force of the wind to some distance from the crater, with an abundant fall of ashes, it would be possible to have lightning flashes proceed from the smoke to the earth. I possess some documents which relate that, in 1631, thunderbolts fell upon the Church of Santa Maria del Arco, and other places on the coast of Sorrento.

After upwards of twenty years' study and observation of meteoric electricity, I am enabled to prove that atmospheric electricity is never manifested without rain, hail or snow, and that manifestations of light are always accompanied by thunder--manifestations of light (_lampi_), thunder and rain being most closely connected. We may have rain without manifestations of light, but never the latter without rain or hail. I cannot here repeat what I have demonstrated in other memoirs; I can only say that the lightnings of Vesuvius, erroneously believed to be not accompanied by thunder, are really not accompanied by rain, but are induced by the descent of ashes and lapilli.[6]

GENERAL CONCLUSIONS.

We may conclude from what I have stated:

1. That by the assiduous study of the central crater, and the indications afforded by the "Apparatus of Variations" and the "Electro-Magnetic Seismograph," we can obtain precursory signals of eruptions; and that the other premonitory signs pointed out by our ancestors, such as the drying up of wells, either only happen occasionally or are mere coincidences, such as those of the coincidence of a dry or a rainy season, the prevalence of certain winds, etc.[F]

2. That the fumaroles of the lavas are communications between the external surface of the lava, hardened and more or less cooled, and the interior lava still pasty, or at least incandescent.

3. That from the lava, while flowing, there is no escape of acid vapours, neither from the fumaroles at the first period of their existence, but these, if they last long enough, arrive at an acid period.

4. That hydrochloric is the first acid that appears, combined afterwards with sulphurous acid, and, still later, with sulphuretted hydrogen.

5. That vigorous lava streams may have eruptive fumaroles. (See Translator's Note 2 to p. 94.)

6. That the sublimations follow a certain order in their appearance. In the neutral period we get sea-salt mixed with some metallic oxides, the first of which is oxide of copper. But in the great lavas, chloride of iron appears simultaneously with the acid period. Hydrochloric acid transforms the oxides into chlorides, which, in their turn, change into sulphurets or sulphates on the appearance of sulphurous acid.

7. That the acids, by attacking the scoriæ, create new chlorides and sulphates, which are thus not products merely of sublimation.

8. That micaceous peroxide of iron--so common and abundant near the eruptive mouths--is very scarce and rare on the lavas, unless conveyed there from the craters.

9. That chloride of iron--so manifest on the fumaroles of the great lavas--is only found in small eruptions close to the discharging mouths.

10. That the frequency of chloride of iron in the lavas of great eruptions masks the order of transformation of the other products.

11. The fumaroles at the summit of Vesuvius present even greater gradations, for they often emit carbonic acid or pure watery vapour.

12. Lead, which I first discovered in the fumaroles of the lavas of 1855, is a constant product of fumaroles which have a certain duration. It is often obtained as a distinct and crystallized chloride, and often is found in combination with other products.

13. Oxide of copper is also a constant and primary (_primitivo_) product of fumaroles. The chloride and sulphate of copper are formed from the oxide, directly contrary to general belief.

14. I do not think that the chloride of calcium, which I found on this occasion in almost all the deliquescent sublimations, is a product peculiar to this eruption only, in which alone, however, I found it. I was, therefore, induced to look for it in other sublimates, in which I might possibly have overlooked it, as, without doubt, my predecessors have done, owing to the deliquescence of the chloride of iron with which it was constantly combined. I think that this chloride, in accordance with the general law, is transformed into a sulphate--a transformation which readily occurs on Vesuvius.

15. Copious and well-crystallized sal ammoniac is only found on the fumaroles of those lavas which have covered cultivated or wooded ground.

16. The scarcity of oxygen in the gases of fumaroles may possibly arise from the formation of the oxides which precede the chlorides.

17. Lavas give a continuous spectrum, although covered with smoke, when looked at with Hoffmann's spectroscope with direct vision.[G]

18. The smoke gives positive electricity, and the falling ashes negative electricity.

REFERENCE TO THE PLATES.

PLATE

Ia. The Cone of Vesuvius, in 1870, from a Photograph taken near the Observatory.

_a._ The Atria del Cavallo. _b b._ Fossa della Vetrana. _c._ Punta del Crocella. _d._ Lava of 1858 and 1867. _e._ Police Barrack near the Observatory. _f._ Part of Monte Somma.

IIa. Profile of Vesuvius, taken from a Photograph of the Observatory in the month of September, 1871.

1. The Cone, on the 13th January, 1871. 2, 2. Lava of 1871.

IIIa. Profile of Vesuvius on the 16th April, 1872, about ten days before the last Conflagration.

IVa. Vesuvius, on the 26th April, 1872, from a Photograph taken in the neighbourhood of Naples.

1. The Observatory. 2. Fossa della Vetrana. 3. Eruption of Smoke and Ashes, with Stones, from the surface of the Lava. 4. The Novelle, St. Sebastiano, and Massa. 5. Lava which took the direction of Resina. 6. Lava which, from the Crater, took the direction of the Camaldoli. 7. The Grain Stores, near Naples. 8. Resina. 9. Torre del Greco. 10. The Camaldoli.

Va. Profile of Vesuvius after the Eruption of the 26th April, 1872, from a Photograph taken near the Observatory.

1, 1. The Fissures of the 26th of April. 2, 3. Small Hill thrown up on the morning of the 26th of April, from below which issued the great current of Lava. 4, 4, 4. The Mouths out of which the Lava issued. 5, 5. The larger Lava Stream, which passed near the Observatory by the Fossa della Vetrana. 6, 6. The other Lava Stream, which, after dividing from the last, took the direction of Resina. 7, 7. The Lava which ran down towards the Camaldoli. 8 & 9. The two Craters on the summit of the Cone.

VIa. The Bifilar Electrometer of Signor Palmieri. (_Details._)

VIIa. The assemblage of the Electroscopic Apparatus of Signor Palmieri, as arranged at the Vesuvian Observatory.

VIII. Professor Palmieri's Seismographic Apparatus.

[A] This small cone, as it appeared on the 1st April, is described and drawn in a Memoir of Professor von Rath, of the University of Bonn, on "Vesuvius on the 1st and 17th of April, 1871."

[B] Eight young medical students perished beneath the lava, with others unknown by name. They were all youths of good promise; their names will be recorded on the marble monument to be erected near the Observatory. They are: Girolamo Pausini, Antonio and Maurizio Fraggiacomo, Francesco Binetti da Molfettu, Giuseppe Carbone da Bari, Francesco Spezzaferri da Trani, and Giovanni Busco da Casamassima and Vitangelo Poli.

[C] If this enormous height of projection really means, that above the brim of the crater, it involves an initial velocity of projection of above 600 feet (British) per second.

Observations of the height of ascent of volcanic blocks are always difficult and deceptive, and never free from error.--_Translator._

[D] Assuming these flashes to have emanated from somewhere within the cloudy volume of steam and dust called "the head of the pine-tree," this interval would indicate that the mean height of this cloudy volume itself was not more than about four thousand feet above the top of the cone; and, if so, that is not very far from the limit in height of projection of the dust and lapilli.--_Translator._

[E] COTUNUITE, chloride of lead, in white, lustrous, acicular crystals, of the trimetric system, easily scratched, Sp. gr., 5·238.

TENORITE, peroxide of copper, in thin, hexagonal plates or scales, translucent when very thin, dark steel gray, of the cubic system; hard and lustrous. Sp. gr. about 5·950.--_Translator._

[F] Earthquakes, though in distant regions, usually precede eruptions. The Earthquake of Melfi preceded the great Eruption of Etna in 1852; the Earthquake of Basilicata of December, 1857, terminated with the Eruption of 1858, which filled the Fossa Grande with lava; the Earthquakes of Calabria of 1867 and 1870 were the precursors of the Vesuvian conflagrations of 1868, 1871, 1872. A Volcano, also, in the Island of Java had a great eruption in the month of April, some days before the last conflagration of Vesuvius, as I learnt from a letter addressed to Signor Herzel, Swiss Consul at Palermo, communicated to me[7] by the astronomer, Signor Cacciatore.--_Palmieri._

[G] I have made a large collection of sublimates, which I purpose examining with the spectroscope, and I shall be able to place some at the disposal of experimentalists who may desire to pursue investigations of this kind.

NOTES

BY THE TRANSLATOR.

[1] (P. 82, text). Professor Palmieri has not given any description in this Memoir of his seismograph--the instruments described being those only which have relation to atmospheric electricity. The following brief account of his seismograph will, therefore, form a not unsuitable complement to his Memoir. The instrument, in general terms, is of that class in which the wave movements are indicated by the displacement, relative or absolute, of columns of mercury in glass tubes. It is a self-recording instrument, composed of two distinct portions--one for record of horizontal, or rather of what are called undulatory shocks; the other for vertical shocks. In point of general principle, therefore, it is very similar to that proposed by me ("Transactions, Royal Irish Academy," in 1846), and in certain respects appears to me less advantageous than the latter. Some account of the Palmieri instrument, together with some critical remarks as to its action, may be found in my "Fourth Report on Earthquakes" ("Reports, British Association, 1858," pp. 75-81). The following description of the instrument is derived from "The Engineer," of 7th June, 1872, and the publishers have to thank the proprietors of that journal for permission to use the illustration, Plate 8.

In Fig. 1, _E_ is a helix of brass wire (gauge about one millimetre); the helix consists of fourteen or fifteen turns, and has a diameter of from twenty to twenty-five millimetres; it hangs from a fine metal spring, and can be raised or lowered by a thumb screw. From the lower end of the helix hangs a copper cone with a platinum point; the latter is kept close to the surface of mercury in the iron basin, _f_, which rests on an insulating column of wood or marble, _G_. The distance of the point from the surface of the mercury remains constant, as the metal pillar, _T_, is of such a length that its expansion or contraction by change of temperature compensates that of the helix; the latter is in connection (by _T_) with one pole of a Daniell's battery of two cells, and the basin, _f_, is connected with the other pole. Any vertical movement, however slight, makes the platinum point dip into the mercury, and thus completes the circuit. In this circuit are included two electro-magnets, _C_ and _D_; these, during the circulation of a current, attract their armatures, which are connected with levers. The action of _C's_ lever is to stop the clock, _A_, which thus records, to a half-second, the time of the occurrence of the shock, at the same instant that the clock strikes an alarm bell, which attracts the attention of an observer. The lever, attached to the armature of _D_, at the first instant of the current frees the pendulum of the clock, _B_, which was before kept from swinging, in a position out of the vertical; the clock then acts as a time-piece, and its motion unrolls a band of paper, _k k k_, at a rate of three metres an hour. At the same time the armature of _D_, while attracted, presses a pencil point against the band of paper which passes over the roller, _m_, marking on it, while the earthquake lasts, a series of points or strokes which occupy a length of paper corresponding to its duration, and which record the work of the shock. After it is over the paper continues to unroll from the drum, _i_, and passing round the clock, rolls on to the drum, _l_. If a fresh shock occur the pencil indicates it, as before, on the paper, and the length of blank paper between the two sets of marks is a measure of the interval of time between the shocks. By way of additional check, several helices, _h h h_, are hung from a stand, with small permanent magnets suspended from their ends; below and close to these latter are small basins, holding iron filings; into these the points of the magnets dip, when their helices oscillate vertically, and some filings remain sticking to the magnets as a record of the shock. One of the magnets has a shoulder on it which moves an index hand along a graduated arc, as shown in Fig. 2, thus again registering the amount of the vertical movement. Such are the arrangements intended for the record of the undulatory or horizontal elements of the wave of shock.

The following are the arrangements proposed for recording the horizontal motions: On the stand, to the right of the clock, _A_, are set four U-shaped glass tubes, open at their ends. One of each pair of vertical branches must have a diameter at least double that of the other. These pairs, with their supporting columns, are shown in plan, where one pair lies N. and S., another E. and W., a third N.E. and S.W., and the other N.W. and S.E. It will be observed that metallic bars pass from the pillar, _P_, over the ends of all the long branches, and similar bars pass from _R_, over the ends of the short branches; the pillars themselves, as in the case of the other instruments, are each connected with one pole of a Daniell's battery, the connections including the electro-magnets, _C_ and _D_. The description of one U tube, _n_, will apply to all the others; _n_ is partly filled with mercury, and an iron or platinum wire, _o_, suspended from the bar above the short branch, dips into the mercury therein, while another platinum wire hung from the bar over the mouth of the longer branch, has its end very close to the surface of the mercury in that branch. Any shock which is not perpendicular in direction to the plane of the branches of the U will cause the mercury to oscillate in the tubes, and more sensibly in that with the smaller diameter; when it rises up in the latter, so as to touch the platinum point, the connection between _P_ and _R_ is made and the circuit completed, starting the action of the electro-magnets _C_ and _D_, which record the shock, as already described. By having the planes of the tubes set in the different azimuths, already mentioned, one or more of the pairs is sure to be acted upon, and by observing in which the oscillation takes place the direction of the shock is supposed to be ascertained. Besides this, each long branch of the U, viz., that of smaller diameter, has a small ivory pulley, _q_, fixed above it, over which passes a single fibre of silk, with an iron float at one end, resting on the surface of the mercury; at the other end of the fibre hangs a counterpoise; fixed to the pulley is a fine index hand, capable of moving along a graduated arc. When the shock takes place the mercury, rising in the long branch, raises the float on its surface, the silk fibre at the same time makes the pulley revolve with its index hand, which afterwards remains stationary, as the counterpoise prevents the float from sinking again with the mercury. The reading on the graduated arc is thus a measure of the movements produced in the instrument by the horizontal element of the shock, and is supposed to measure that shock. It is assumed that in all these instruments shocks, however small, can be recorded with certainty by adjusting the distance between the platinum points and the mercury.

The arrangement of Daniell's battery used for the seismograph is shown in Fig. 4, where, for convenience of cleaning, the copper element is made of wire (about No. 8 Birmingham wire gauge) coiled flat without the spirals touching. Crystals of sulphate of copper are placed at the bottom of the outer cell, into which water is poured; and the inner cell, into which the zinc plate goes, is filled with siliceous sand.

In addition to the above some instruments of a rougher description are employed as checks. Thus, at the foot of the pillar, _G_, there is a wooden trough with eight holes, facing as many equidistant points of the compass (two of them shown in section) round its inner circumference; mercury is poured into the basin until its level is nearly up to the lips of the holes. The effect of a shock is to throw some of the mercury into one or more of these holes, and the greater the oscillation the more mercury is thrown into the cells through the holes. The screws shown outside are for drawing off the mercury from the cells, when its quantity can be measured. The direction of the shock is shown by seeing which cells are filled with mercury. This is the old Cacciatore seismometer which has been long employed in Italy. (See 4 "Report of British Association, 1858," p. 73), and Daubeny's "Volcanoes," Appendix. The following is another contrivance. From the arm of the pillar, _G_, a fine metal wire hangs, with a metal ball at its end, which, by its oscillation, thrusts out one or more light glass tubes, set horizontally in a stand, as shown in Fig. 3. The two rings are of wood, and the glass tubes pass through holes in them; small leather washers are placed outside the outer rings; the displacement of one or more tubes is assumed to measure the horizontal element of the shock. By means of this apparatus the time of the first shock is recorded, as well as the interval between the shocks, and the duration of each; their direction, whether vertical or horizontal, is given, as also the maximum of intensity. Professor Palmieri has the instruments examined three times a day, and an assistant-observer is always at hand to attend to the bell, and put back the apparatus to its normal position for fresh observation.

It has been stated that this instrument is sensible to most of the shocks which occur in the Mediterranean basin.

It is not my intention here to offer any criticism as to the construction or performances of this instrument, the rather as I must confess I do not quite share the high opinion of its inventor as to the certainty or exactitude of its indications.

There can be no question as to the extreme importance to science of the establishment and continued use of a seismographic instrument of unexceptionable construction at the Observatory upon Vesuvius; and it would be a valuable gift to science, were the Italian Government to enable Signor Palmieri to establish such an one. Its great value and the very first problem to set the instrument to solve should be, by _a rigid determination of the direction of propagation of the wave of shock_, of those slight or stronger pulsations which precede or accompany the Vesuvian like all other eruptions, on arriving at the Observatory, _to fix the depth, and the position vertically beneath the cone, whence these pulses are derived_. This would be, in fact, to fix the depth and position beneath the mountain at which the volcanic focus is situated for the time, or, at least, where the volcanic activity is at the time greatest. And the assured knowledge, even within moderate limits of accuracy, of this depth, and even for this single mountain, would be an immense accession to our positive knowledge, and a really new stage gained for future advances. At present, we know but little as to the actual depth below our globe's surface at which volcanic activity occurs, or to which it is limited, either upwards or downwards. I have, myself, established some data upon the flanks of Etna, not yet published, which may enable me to afford some information on the subject hereafter. Meanwhile, Professor Palmieri possesses unrivalled opportunities for such observations; and I trust health, life and means may be afforded him, to become the first who shall have made this great addition to our positive knowledge of Vulcanology.

So far, popularly at least, the alleged chief uses and value of these seismographic instruments, at the Observatory of Vesuvius, have been made to depend upon their being presumed to afford means for foretelling eruptions, or affording precursory warnings of their probable progress and destructive course.

I feel compelled to express my own total disbelief in the possibility of any such predictions in the present state of science, by the help of any instruments whatsoever, of such a nature as to be of any _practical value_, or any certainty beyond that which a certain amount of _mere experience_ as to the _rôle commonly played_ by Vesuvius or other Volcanoes in pretty habitual activity affords to the observer for a lengthened period. And even this affords scarcely any guide as to what may happen next. Monte Nuovo was thrown up in a night; Vesuvius _might_ double its volume in a night, or might sink into a hollow like that of the Val del Bove in a not much longer time. A small _fusillade_ may go on for months, and yet, without an hour's notice, by any premonitory sign, may waken up to a roar and darken the air with ashes and lapilli such as those which overwhelmed Pompeii. One eruption may blow forth little but dust and ashes (so called), another may pour out rivers of lava and little else.

The _main_ mischief of all eruptions is effected in two ways: by the deposit of dust and ashes, lapilli, etc., to the injury or destruction of fertile land, and by the streams of lava which overwhelm it, as well as buildings, etc. But what information of any value can seismographic observation afford as to the course that either of these may take in any eruption? The volume of pulverulent material that may be ejected cannot be foreseen; its distribution depends mainly upon its nature and upon the direction and force of the wind at the time; or again, how shall these warn us as to the course that the lava, if it appear, shall take, when we cannot possibly foretell when, how, or by what mouth it may issue. Even in this late eruption of 1872, with Palmieri stoutly at his post upon the mountain, and the Observatory instruments in full activity, they gave no forewarning of the sudden and unexpected belch forth from the base of the cone, of that tremendous gush of liquid lava which in a few minutes cut off from life the unhappy visitors whose deaths he has recorded.

[2] (P. 94). It can scarcely be supposed that these small eruptive-looking belchings forth from the lava stream, _en route_, are truly of an eruptive nature at all, _i.e._, in any way connected with forces seated deeply beneath the bed of the lava stream, or in any way connected with the volcanic ducts of the cone or beneath it. They are most probably merely the bursting upwards of large bubbles; that is, of cavities formed in the mass of the more or less liquid lava by intestine movements, as its mass winds and rolls along, and by the aggregation of smaller cavities--all being filled with steam and gases--together with dust and volatile products which are ejected when the cavity opens up, and its contents escape at the upper surface of the lava stream in virtue of the continuation of the twistings and convolutions due to the stream motion itself, and to the unbalanced hydrostatic pressures acting upon the parietes of the bubble. Very large single bubbles of like character rise in the fluid lava within craters in vigorous action, and often so regularly that their recurrence causes a sort of rhythmical rush and roar in the column of steam, etc., issuing above the mouth. This was evident in the discharges issuing in 1857 from the highly instructive minor _bocca_, then existing, examined by me, and referred to ("Report, Naples, Earthquakes," etc. Vol. II., pp. 313, 314), as presenting at the time great facilities for determining pyrometrically the temperature of the lava within, and of the dry superheated steam issuing with a rhythmic roar from it. M. Le Coq ("Époques Géologiques d'Auvergne," Tome IV.) has recorded some examples of the formation and opening-out of large bubble-like cavities in lava already ejected. Perhaps that able and laborious vulcanologist, whose death a few months ago science still deplores, attributes too much importance as well as magnitude to them, when attributing the formation of what he has denominated "craters of explosion," to the mechanism of the rise and bursting of such bubbles upon a gigantic scale. Such blowings forth, sudden or prolonged, from particular spots of lava streams, _en route_, undoubtedly may also have their origin in damp places, or water or air-filled cavities in or beneath the bed over which the lava rolls, which, getting gradually heated, generate steam, or air or gases under tension by expansion, etc., which thus at length blow through the liquid or pasty lava flowing above, and which in bursting through delivers much dust also, and so simulates a little eruptive crater. Examples of this, upon a great and convincing scale, can be pointed to in the Val di Calanna and elsewhere on Etna.

[3] (P. 96). There are strong grounds for the gravest doubts that there exists any real connection of a physical character between Volcanic Eruptions, and Earthquakes more or less _approximately_ coincident only, in time of occurrence; the respective sites being widely apart, and the less the probability as the intervening distance is greater. The discussions of the large number of records that are to be found of such coincidences--mostly but partial, and in but _very_ few instances complete coincidences--by Perrey, von Hoff, and others, as well as by myself, do not tend to sustain the view that such imperfect contemporaneity is based upon any causative connection. The seismic region of Greece appears to have no _direct_ connection with that of Southern Italy: the band of connection, if any, seems to lie between Northern Italy, across the Northern Adriatic, by Ragusa, and thence spreading into Asia Minor.

[4] (P. 97). The abundance of coleoptera and of various other forms of insect life about lava beds, both recent and old, is a very singular fact, and one worthy of the careful observation of entomologists. In the autumn of 1864, at mid-day, when sitting sketching upon the lava about the middle of the Val del Bove (Etna), I found it almost impossible to work, or even to remain for an instant still, in consequence of the continual cloud of insects, large and small, that struck against me in flight, endangered the eyes, and swarmed upon my clothes. It is quite possible that this local superabundance of insect life may arise merely from the general dryness and warmth of such places, and the plentiful _nidus_ that the innumerable cavities in lava afford for the eggs and earlier stages of insect life; still, this apparition of one form of life may also be connected with other circumstances not unimportant to discover.

[5] (P. 120). The _Crocella_ is a small wooden cross, erected several years ago, and which one passes to the right hand at the upper end of the path along the ridge of tufa and volcanic conglomerate upon which the Observatory stands, in ascending thence to the Atria del Cavallo.

[6] (P. 134). That the causes assigned by Professor Palmieri for the potent developments of electricity (positive or negative) which characterise the ascent of the issuing columns of (chiefly if not always) _dry_ steam, with a relatively small volume of various gases, and throwing up, in their blast, volumes of small solid particles in ashes and lapilli, etc., and the subsequent fall as a mineral or stony hail-shower of the latter, through the partially condensing vapours and the circumambient air, are the main causes of electrical development evidencing itself in lightning flashes, is no doubt true. We must not, however, lose sight of the many other and very effective agencies at work here to produce electric excitement. The actual _bocca_ of the volcanic vent whence the steam roars off constitute the cone a veritable hydro-electric machine. Mechanical energy in various forms is transformed into electric energy. Chemical action is going on both in the solid and in the vapourous and gaseous emanations as they rush into and remain in the air or descend from it, and chemical action is transformed in part into electric energy. Percussion between ascending and descending particles and fragments, fractures and breaking up of more or less of these, thus and by sudden changes of temperature in cooling, are likewise operative. In addition, great and violent movements in the atmosphere itself result from the large local accessions of temperature by the heated volume driven up into it, and which in turn give rise to electric disturbance of the same character as those produced in wind storms and whirlwinds, brought about by the natural causes which every day effect disturbances in our atmosphere all over the globe.

[7] (P. 135). The views stated in note 3 (to page 96) may here again be referred to as in point. How is it possible, in the present state of science at least, to establish any physical connection between an eruption in Java and one of Vesuvius, "with half the world between," when not even having the solitary connecting link of complete contemporaneity, and which, if it existed, yet might be nothing but accidental? A list of shocks upon record, which have occurred more or less nearly simultaneously at distant parts of the world, may be found in my fourth Report, ("Facts of Earthquakes," "British Association Reports, 1858") and the reasons are there given for rejecting the notion of any direct physical connection between the origins of the respective shocks.

Shocks, emanating from the close neighbourhood of volcanic vents, or simultaneity of eruption, in vents not far distant from each other, stand upon a different footing.

Transcriber's Notes.

Preserved the unusual, but consistent, spelling of "develope."

Preserved the unusual, and inconsistent, references to the Plates. Sometimes Arabic numerals are used, but usually Roman numerals. Most have "A" attached to the name (which was changed to "a" to avoid confusion in the Roman numerals), but not VIII, and sometimes not in the references to them.

Equations were converted to linear text, adding spacing and parentheses as necessary. Greek characters were also replaced by Latin letters in the Latin-1 version.

Replaced the oe ligature with the two characters in "Goethe" in the front material.

Changed "fumarolles" to "fumaroles" on page 3: "fumaroles and salfatares."

Changed "Lyall" to "Lyell" in footnote originally on page 9: "Daubeny, Lyell, Phillips and others."

Changed "throught" to "through" on page 35: "passing through it."

Removed duplicated word "the" on page 40: "the great Calabrian Earthquake."

Page 89 refers to the "Fossa del Vetrano;" elsewhere there are references to "Fossa della Vetrana," which may be what was intended. However, I did not change this.

Changed "hydrochloride" to "hydrochloric" on page 109: "the hydrochloric acid which it discharged."

Changed "disk" to "disc" on page 126: "the disc and index."

Replaced the infinity symbol on page 131 with "for infinity," in the Latin-1 text version.

Changed "azismuths" to "azimuths" on page 143: "the different azimuths."

Changed "silicious" to "siliceous" on page 143: "filled with siliceous sand."

End of Project Gutenberg's The Eruption of Vesuvius in 1872, by Luigi Palmieri