Page:Collier's New Encyclopedia v. 03.djvu/515

EARTH 468 EARTHQUAKE the eye it appears as if the earth was in the center of the universe, the sun and the stars revolving round it. The phe- nomena are much better accounted for by supposing the apparent revolution of the celestial vault to be produced by an actual rotation of the earth on its axis in about 24 hours, producing day and night. In possessing a satellite (the moon) the earth resembles various other plan- ets, except that they have more attendant bodies than one. In fact, the earth is a planet, and, like other planets, its figure is not far from spherical, as is proved by its having been sailed round. Magellan led the way, having circumnavigated a great part of the globe between 1519 and 1521, being killed in the Philippine Is- lands in the last-named year. Sebastian del Cano, one of his officers, completed the enterprise. Sir Francis Drake re- turned alive from a similar enterprise successfully carried out between 1577 and 1579 or 1580. The sight of the masts of a vessel ap- pearing before the hull comes in sight is a proof that at least that portion of the world visible to us is a curve. Moreover, in an eclipse of the moon the shadow of the earth obscuring the face of the lu- minary is found to be circular, and there are other arguments in the same direc- tion. Only in a broad sense can the earth be described as spherical; it is really an oblate spheroid — i. e., the dis- tance between the two poles is less than that between two extremities of a diam- eter drawn through the equator. This form may have been produced by the ro- tation of a partially fluid sphere. Ac- cording to Bessel, the greater or equa- torial diameter is 7,925.604 miles, the lesser or polar one 7,899.114 miles; the diflference of diameter, or polar com- pression, is 26.471 miles, and the pro- portion of the equatorial to the polar diameter as 299.15 to 298.15. The di- mensions given by Sir R. Airy slightly differ from these. The force of gravity at the poles is to that at the equator very nearly as 180 to 179. It is not of uniform density, the French mathematician, Clairvault, as- suming it to consist of ellipsoidal strata increasing in density as they approached the earth's center, and, taking it for granted also that the attractive force might be calculated on the law of liquids, proved that the amount of gravity at the poles to that at the equator is as 180 to 179, and that the earth's polar axis was to its equatorial one as 299 to 300, which almost exactly agi'ees with the result of observation. Clairvault believed the mean density of the earth, taken as a whole, to be about twice that of the parts near the surface. Henry Cavendish, Dr. Reich, and Francis Baily considered the density of the earth to be 5.67, and Sir R. Airy believed it 6.665, that of water being 1. The number of cubic miles in the earth is about 259,800,000,000, each cubic mile containing 147,200,000,000 cu- bic feet. The surface of the land is to that of the water on the earth in the proportion of one to three. The land is unequally distributed, most of it being in the N. hemisphere. The universal opinion of geologists is that the earth is of immeasurable an- tiquity. The old view that our planet is but a few thousand years old now exists only among the uninformed. It is not yet proved that astronomical changes have ever taken place since the first establish- ment of the solar system seriously to modify the state of things existing on the earth. The action of the earth on magnetic substances is like that of a magnet, and it has two poles different from the ordi- nary poles. EARTH, originally, in the opinion of the ancient chemists, or alchemists, one of the four elements, of which all ma- terial things in the world were held to be composed, the others being fire, air, and water. Not even one of the four is really a simple substance. Later, a name given to various sub- stances, opaque, insipid to the taste, in- combustible, and, when dry, friable, i, e., easily separated into particles. Five divisions of them were recognized: (a) Boles, (b) Clays, (c) Marls, (d) Ochers, and (e) Tripolis. Under these cate- gories were ranked the oxides of the metals, cerium, aluminium, beryllium, zirconium, yttrium, erbium, thorium, etc. These oxides are insoluble in water, and are all very rare except aluminum. They are diflScult to separate from each other, occurring together in rare minerals, and hence the number of metals belonging to this class is not known. EARTHQUAKE, the term applied to any tremor or vibration of the ground produced by subterranean cau.ses. Many earthquakes are so gentle as to pass al- most unrecognized, others excite general alarm without causing damage, while some spread destruction over wide areas. Probably no part of the earth's surface is wholly free from vibration, but de- structive earthquakes are confined to comparatively limited regions. The al- most universal succession of phenomena recorded in the case of notable earth- quakes is first a trembling or vibration, next a severe shock, or several in quick succession and then a vibration which gradually but rapidly becomes insensib. .