Part 30
EARTH, the planet which we inhabit, a nearly spherical body which every twenty-four hours rotates from west to east round an imaginary line called its axis--this axis having as its extremities the north and south poles--while in the course of a year it completes a revolution round the sun. To an observer whose view is not obstructed, the visible part of the earth appears as a circular and horizontal expanse, on the circumference of which the heavens appear to rest. Accordingly, in remote antiquity, the earth was regarded as a flat, circular body, floating on the waters. But even in antiquity the spherical form of the earth began to be suspected. It is only on this supposition that we can explain how the horizon of vision grows wider and wider the higher the position we choose, how the tops of towers and mountains at a distance become visible before the bases, how the hull of a ship first disappears as she sails away, and how, as we go from the poles towards the equator, new stars become visible. Besides these proofs there are many others, such as the circular contour of the earth's shadow seen on the moon during an eclipse. The mere fact that the earth can be circumnavigated does not, as is sometimes assumed, prove it to be globular. But its surface, land and ocean, has been almost all explored and accurately mapped, and the relative distances and directions found to obtain between the places on its surface are consistent only with its possessing such a shape.
The earth is not, however, an exact sphere, but is very slightly flattened at the poles, so as to have the form known as an _oblate spheroid_. In this way the _polar diameter_, or diameter from pole to pole, is shorter than the diameter at right angles to this--the _equatorial diameter_. The most accurate measurements make the polar diameter almost 27 miles less than the equatorial, the equatorial diameter being found to be 7926.7 miles, and the polar 7900 miles. The earth is regarded as divided into two halves--the northern and the southern hemisphere--by the _equator_, an imaginary line going right round it midway between the poles. In order to indicate with precision the position of places on the earth additional circles are imagined to be traced upon the surface in such a manner that those of the one set all pass through both poles, while those of the other are drawn parallel to the equator. The former are called _meridians_, the latter _parallels of latitude_, and by reference to them we can state the latitude and longitude, and thus the exact position, of any place.
Many experiments by various methods have been made in order to determine the average density of the earth, and the total quantity of matter it contains. Amongst these methods may be mentioned: (1) that of measuring the deflection of a plumb-line due to a mountain's attraction, and thereby comparing the mass of the earth with that of the mountain; (2) that founded on the difference of oscillation period of a pendulum when placed at the summit of a mountain and when at the sea-level; (3) by the determination of the difference of gravity at the top and the bottom of a deep mine, by pendulum experiments; (4) Cavendish's experiment with the torsion balance, which attempts to compare the attractive force of two large lead balls upon two small lead balls with that exercised by the earth. From these and other experiments it has been calculated that the mean density of the earth is to that of water as about 5-1/2 to 1.
The earth, in common with the other planets, moves round the sun, completing its revolution in about 365 days and 6 hours. The orbit of the earth is an ellipse, with the sun in one of its foci. Hence the earth is not equally distant from the sun throughout the year; it is over 3,000,000 miles nearer at one time than another, its least distance (_perihelion_ distance), according to recent calculations, being about 91,340,000 miles; its greatest (_aphelion_ distance), 94,450,000; and the mean distance, 92,897,000 miles. From this it may be calculated that the velocity of the earth in its orbit is about 18-1/2 miles a second. About 3rd Jan. the earth is nearest the sun, and about 4th July farthest from it. This position of matters, which is subject to slow alteration in the course of ages, at present tends to moderate the seasonal variations in the northern hemisphere, and to intensify them in the southern. The passage of the earth round its orbit causes the sun to appear as if it described an annual circuit of the heavens; and hence it is that at one time of the year one group of stars is seen in the neighbourhood of the sun near sunrise or sunset, and at another time another group. This apparent path of the sun is the _ecliptic_, and corresponds with what would be the path of the earth as seen from the sun; and the groups of stars through which the sun successively passes form the _zodiac_.
[Illustration: The Hemispheres, showing the Greatest Masses of Land and Water]
The earth's daily motion about its own axis takes place in twenty-three hours, fifty-six minutes, and four seconds of mean time. This diurnal revolution is the occasion of the alternation of day and night. As the axis on which the earth performs its diurnal rotation is inclined towards the plane of its path about the sun at an angle of 66-1/2deg, and the angle between the plane of the ecliptic and the plane of the earth's equator is therefore 23-1/2deg, the sun ascends in the heavens, as seen from our northern latitudes, from 21st March to 21st June (the summer _solstice_), to about 23-1/2deg above the celestial equator, and descends again towards the equator from 21st June to 23rd Sept.; it then sinks till 22nd Dec. (the winter _solstice_), when it is about 23-1/2deg below the equator, and returns again to the equator by 21st March. This arrangement is the cause of the seasons, and the inequality of day and night attending them. For all places removed from the equator, day and night are equal only twice in the year (at the _equinoxes_). At the summer solstice in the northern hemisphere the north pole of the earth is turned towards the sun, and the south pole away from it, and for places within 23-1/2deg of the former there is a period of longer or shorter duration during which the sun is continually above the horizon throughout the twenty-four hours of each day, while round the latter there is an equal extent of surface within which the sun for similar periods is below the horizon. (See _Day_.) The reverse state of matters occurs at the winter solstice. The circles bounding these regions are called respectively the _arctic_ and the _antarctic_ circle, and the regions themselves the _polar_ or _frigid zones_. Throughout a region extending to 23-1/2deg on each side of the equator the sun is directly overhead at any place twice in the year. The circles which bound this region are called the _tropics_, that in the northern hemisphere being the tropic of _Cancer_, that in the southern the tropic of _Capricorn_, while the region between is the _torrid zone_. The regions between the tropics and the polar circles are respectively the _north_ and _south temperate zones_.
From the evidence furnished by volcanoes, hot springs, sinking of mines, &c., it is known that the earth has a high internal temperature. Taking the average of the various observed rates of increase this temperature seems to increase 1deg F. for every 60 feet of descent. Assuming this to continue, the rocks at a depth of 2 miles would be as hot as boiling water, and at a depth of 50 miles the heat would be such as at the surface would melt every known solid. This being so, various theories as to the internal condition of the earth have been proposed: (1) that a thin envelope or crust surrounds a molten interior. It can be shown, however, that as tides must be produced in such a molten mass the cool outer crust would be unable to withstand the enormous force of these unless it were about 2000 miles thick. (2) That the interior is solid, with spaces here and there filled with liquid or gaseous material. This theory assumes that there are within the earth enormous cavities filled with molten rock, which escapes, when local pressure is removed, in the form of volcanic outbursts. (3) That the earth consists of a thin crust, a large solid nucleus, and a liquid film between the nucleus and the crust. (4) That the earth is solid to the centre, but any part may become liquid if local pressure is removed. On this theory it is supposed that if water should percolate to liquefied rocks, it would be converted into steam, and produce the various volcanic phenomena.
The question of the constitution of the earth's interior has in recent years been much investigated by means of seismographic records. These appear to indicate that there are three distinct divisions. The outer crust has a thickness of from 20 to 40 miles. It possesses a high power of resistance to all kinds of stress. Beneath it is a large shell possessing a density and elasticity resembling fine steel. This shell has a high rigidity against changing forces of shorter duration, like tidal action, but in its outer parts at least yields in time to unvarying long-continued stress. The third or innermost division of the earth is probably molten, as it can transmit compressional waves, but yielding immediately to distortional or twisting forces, is unable to transmit distortional waves. This innermost portion appears to be a sphere of radius approximately one-half that of the earth as a whole. The transition between the crust and intermediate shell is abrupt, but that between the latter and the central portion is more gradual.
The earth (like the other planets) is believed to have condensed and solidified from a gaseous or nebular condition, and to have once had a far higher temperature than now. If such were the case, the outer surface, losing heat by radiation, would be the first part to cool quickly; while the interior, losing its heat by conduction, would not cool so rapidly, and, therefore, would naturally have a higher temperature than the portion at the surface. This is what all observations indicate the condition of the earth to be, and the shape of the earth also indicates that it must once have been in a fluid state. Calculations have been made of the time which has elapsed since solidification commenced, the estimates being in general of the order of hundreds of millions of years. See _Nebular Hypothesis_.
Another feature that the earth as a whole presents is its magnetism. When a magnetic needle is balanced on a point, it remains at rest in one position only, pointing then nearly due north and south. This can be explained only on the supposition that the earth acts as a great magnet. It has, in fact, two poles--a north and a south magnetic pole--which are not very far from the geographical poles. The magnetic equator, where the vertical force is zero and the dipping needle takes a horizontal position, does not diverge greatly from the geographical equator. The earth acts upon all magnets as they act upon each other, and it is for this reason that they point north and south.
The surface of the earth contains over 196,000,000 sq. miles, of which about two-sevenths is dry land, the remaining five-sevenths being water. The land is arranged into masses of irregular shape and size, the greatest connected mass being in the eastern hemisphere. The chief masses receive the name of continents, detached masses of smaller size being islands. The surface of the land is variously diversified, exhibiting mountains, valleys, plains, plateaus, deserts, &c. The water area of the earth is divided into oceans, seas, bays, gulfs, &c., while rivers and lakes may be regarded as features of the land surface. The great phenomena of the oceans are currents and tides. The population of the whole earth is estimated at from 1600 to 1700 millions. The earth is attended by the moon as a subordinate or secondary planet. See also such articles as _Climate_, _Currents_, _Ocean_, _Earthquake_, and _Seasons_.--BIBLIOGRAPHY: A. von Humboldt, _Cosmos_; E. Reclus, _The Earth and its Inhabitants_; T. G. Bonney, _The Story of our Planet_; T. M. Reade, _The Evolution of Earth Structure_; _Theory of Geomorphic Changes_; A. T. Swaine, _The Earth: its Genesis and Evolution, considered in the Light of the most Recent Scientific Research_.
EARTHENWARE, a name applied to the commoner sorts of pottery-ware. The older kinds of earthenware, such as Majolica, Delft-ware, Faience, and Palissy-ware, are not only glazed, but are besides elaborately coloured and enamelled and ornamented with raised figures of various kinds. See _Pottery_.
EARTH-HOUSES, a name generally given throughout Scotland to underground buildings, also known as 'Picts' houses' or 'Picts' dwellings'. The earth-house in its simplest form consists of a single irregular-shaped chamber, formed of unhewn stones, the side walls gradually converging towards the top until they can be roofed by stones of 4 or 5 feet in width, all covered in by a mound of earth rising slightly above the level of the adjacent ground. In the more advanced form of these structures two or three chambers are found. Earth-houses are frequent in the north-east of Scotland, occasionally thirty or forty being found in the same locality. Querns, bones, deers' horns, earthen vessels, cups and implements of bone, stone celts, bronze swords, and the like, are occasionally found in connection with them. Very similar structures, known as beehive-houses, occur also in Ireland and Cornwall.
EARTH-NUT, the _Conopodium denudatum_, an umbelliferous plant common in woods and fields in Britain. The leaves are ternately divided, and the small white flowers are in terminal umbels. The tuber or nut is about 4 or 6 inches below the surface, at the termination of a long slender root. It is brown, the size of a chestnut, of a sweetish farinaceous nature, resembling in taste the common chestnut. Swine are very fond of the nuts, and fatten rapidly where they are abundant. The name is frequently applied to _Carum Bulbocast[)a]num_, which has a similar tuber. See _Ground-nut_.
EARTHQUAKE, a shaking of the earth's surface, propagated from place to place by a wave motion. It may vary in intensity from the slightest perceptible tremor to a violent shock which bursts open chasms and changes the appearance of the ground. Earthquakes originate in the crust of the earth, generally at only a very few miles depth, and probably never lower than about 30 miles. The point of origin is called the centre or seismic focus, and the place on the surface vertically over it the epicentre. It is rather difficult to tell the depth of the focus. Mallet estimated this by projecting backward the direction of travel of the wave at different points, as judged from the inclinations of the rents in buildings, &c., assumed to be at right angles to the line of propagation. The accuracy of this method has been improved by substituting evidence of direction as given by seismographs. The focus of an earthquake is often submarine, and subsequent to the shock transmitted through the solid earth a great sea-wave may invade the land and produce far more disastrous effects.
In some cases an earthquake may be caused by a fall of rock in some subterranean cavity. This gives only a minor and local shock. The vast majority of earthquakes are certainly _tectonic_, originating from the snapping of strata under great strain, or the further slipping of portions of the earth's crust along previously existing fault planes. Such dislocations probably arise sometimes from the variations of weight supported by the earth's crust in neighbouring regions, due to the transport by rivers of material, which they erode at one place and deposit at another. A further cause is the contraction undergone by the earth in its secular cooling. There are also earthquakes of _volcanic_ origin, accompanying eruptions, but these are not usually of any great violence, nor do they involve any large area. The coasts of the Pacific Ocean--American, Asiatic, and East Indian--are much visited by earthquakes, in especial the Japanese Islands. The other band of greatest frequency has a direction outlined by the Azores, Alps, Mediterranean, and the Caucasus and Himalaya Mountains. It may be noted that all the regions specially affected are distinguished by steep gradients of the earth's surface.
[Illustration: Diagram of Earthquake
AB, Surface of earth. F, Focus. E, Epicentre. 1-4, Successive positions of earthquake wave. S, S, Cracked walls; the cracks being at right angles to CF and DF give some indication of the depth of focus.]
In recent years much information has been obtained by the investigation of earthquakes by various kinds of seismograph. One single instrument at a
## particular station, e.g. a Milne seismograph, will enable the distance of
the epicentre to be calculated. From the results of three stations, the precise locality can practically always be told. With additional or
## particular forms of instruments, this may even be possible by means of the
records at one station. Earthquake waves are found to consist of distinctly defined types. The first to arrive are the preliminary tremors or first-phase waves, then the second-phase waves, next the third-phase or large waves, and lastly the concluding waves, consisting largely of 'echoes' or reflected vibrations. The speed of the preliminary tremors is found to be only about 2 miles per second for very short distances, but for a quadrant of the earth's surface they travel at an average of about 7 miles per second, a speed which is only slightly exceeded for still greater distances. The second-phase waves travel with a little under two-thirds of these velocities. These two classes of wave have been proved to travel through the earth, approximately along chords, but with the path slightly bent, convex towards the earth's centre. The first-phase waves are longitudinal, or waves of compression; the second-phase are transverse, or waves of distortion. The greater speed for greater distances is due to the track being more through the earth's interior and less through its outer portions, as the interior transmits wave-motion much more rapidly than the crust. The rigidity at some depth from the surface has been shown to be of the same order of magnitude as the rigidity of steel. The third-phase waves are of much longer vibration period and wide amplitude, and have been compared to a groundswell on the sea. Their time of passage from the epicentre to any place is proportional to the distance measured round the earth's surface, and it is clear that they travel on the surface, and not through the interior. Their speed is nearly 2 miles per second. The difference in time between the arrival of the preliminary tremors at any station and the arrival of the second-phase waves, or between the second-phase and third-phase waves, enables the distance of the epicentre to be easily found, as these differences, of course, become greater with increasing distance.
The number of earthquakes has been found to be enormously greater than was at one time supposed; in fact, small tremors are occurring daily in one part or another of the earth. Great and destructive shocks are generally preceded by minor shocks in the same district, and they are always followed for months afterwards by a series of gradually lessening after-shocks. Among the most remarkable earthquakes of modern times were those which destroyed Lima in 1746, and Lisbon in 1755; more recently destructive earthquakes visited Calabria in 1857, Peru and Ecuador in 1868, the Island of Ischia in 1884, Japan in 1896, North India and Calabria in 1905, San Francisco in 1906, Messina and Reggio in 1908. One of the greatest earthquakes of recent times was that which visited the provinces of Kansu and Shensi in North-West China, on 16th Dec., 1920. (See _Seismograph_.)--BIBLIOGRAPHY: J. Milne, _Earthquakes and other Earth-Movements_; C. Davison, _A Study of Recent Earthquakes_; C. G. Knott, _The Physics of Earthquake Phenomena_.
EARTHS, a term applied in geology to certain loosely aggregated siliceous and aluminous materials, the detritus of pre-existing rocks. In chemistry the term earth is given to certain metallic oxides, such as the 'alkaline earths' lime, baryta, and strontia; also to alumina and a series known as the 'rare earths'. The earths were regarded as simple bodies until Sir H. Davy proved them to be compounds of oxygen with metals.
EARTH-SHINE, in astronomy, a name given to the faint light visible on the part of the moon not directly illuminated by the sun, due to the illumination of that portion by the sunlight which the earth reflects on her. It is most conspicuous when the illuminated part of the disc is small, as soon after new moon. This phenomenon is popularly described as 'the old moon in the new moon's arms'.
EARTH-TONGUE. See _Geoglossum_.
EARTHWORKS (in fortification) are military works formed chiefly of earth and designed either as permanent or temporary defences. They are cheaper, more easily repaired, and expose their defenders to less risk from broken stone than stone-works. See _Entrenchments_.
EARTHWORM, the name applied to segmented worms (Annelids) that burrow in the soil, and belong to the ord. Oligochaeta, a subdivision of the bristle-worms (Chaetopoda). They have a long, cylindrical body, divided by transverse furrows into numerous rings. The mouth is destitute of jaws, and they have no eyes, tentacles, or other head appendages. They are hermaphrodite. The commonest British forms are chiefly species of Lumbricus and Allolobophora. They feed on earth and various kinds of animal and vegetable matter, and move by the contractions of successive parts of the body aided by a double row of bristles. They are of great service to the agriculturist by loosening the soil and increasing its depth. This is chiefly the result of their mode of nourishment, since they deposit the soil they have swallowed, after digestion, in heaps called _worm castings_ which bring up rich fine soil to the surface, gradually covering the upper layer sometimes to the extent of several inches.
EAR-TRUMPET, an artificial instrument for aiding the collection of the vibrations or waves of sound, and carrying them in an intensified form to the internal parts of the ear. They are generally made of tin, vulcanite, or gutta-percha, and are of various forms. A small kind known as ear-cornets or acoustic auricles, attached to the ear by a spring, is sometimes used in slight cases of deafness.
[Illustration: Earwig (_Forfic[)u]la auricularia_). Male and female]
EARWIG (_Forfic[)u]la_), a common orthopterous insect whose name is derived from its supposed habit of insinuating itself into the ears of persons. This is practically impossible, yet the notion is widely spread, as appears from the names given to the earwig in different languages, as in Fr. _perceoreille_ (pierce-ear), in Ger. _ohrenh[:o]hler_ (ear-borer). Much damage is sustained by gardeners from the depredations of these insects among fruit and tender vegetables, which constitute their proper food. The earwig is about three-quarters of an inch in length, having the wings folded under very short and truncate elytra or wing-cases, and the extremity of the abdomen armed with a horny forceps.
EASEMENT, in law, a right or privilege which one proprietor may have to use the land of another in connection with the needs of his own land, as the use of a way, a water-course, &c. The right to an easement may be acquired either by grant or by uninterrupted enjoyment for a period of years.