EARTH, TUE, the globe on which we live, being the third planet in order form the sun, and the larwest within the belt of the planetoids. We proceed to consider briefly the points of chief interest connected with it, and which do not fall to be treated under separate heads, viz., 1. Its form and magnitude; 2. Its mass and density; 3. Its motions; 4. Its temperature.
1. The Form and Magnitude of the Earth.—Tat, spectator so placed as to have an unob structed view all round, it appears a circular plain, on whose circumference the vault of heaven seems to rest. Accordingly, in ancient times, even philosophers looked long upon the earth as a flat disk swimming upon the water. But many appearances were soon observed to be at variance with this idea, and even in antiquity, the spherical form of the E. began to be suspected by individuals. It is only by assuming the E. to be spherical, that we can explain how our circle of vision becomes wider as our position is more elevated; and how the tops of towers, mountains, masts of ships, and the like, come first into view as we approach them. There are many other proofs that the E. is a globe. Thus, as we advance from the poles towards the equator, new stars, formerly invisible, come gradually into view; the shadow of the E. upon the moon during an eclipse is always round; the same momentary appearance in the heavens is seen at different hours of the day in different places on the E. 's surface; and lastly, the E., since 1519, has been circumnavigated innumerable tunes. The objection to this view that readily arises from our unthinking impressions of up and down, which immediately suggest the pic ture of the inhabitants of the opposite side of the E,—our antipodes—with their heads downwards, easily got over by considering that on all parts of the earth's surface down is towards the E.'s center.
It is not, however, strictly true that the E. is a sphere; it is slightly flattened or compressed at two opposite points—the poles—as has been proved by actual measure ment of degrees of latitude, and by observations of the pendulum. It is found that a
degree of a meridian is not everywhere of the same length (see DEGREE OF LATITUDE), as it would be if the E. were a perfect sphere, but increases from the equator to the poles; from which it is rightly inferred that the E. is flattened there. A pendulum, again, of a given length is found to move faster when carried towards the poles, and slower when carried towards the equator, which shows that the force of gravity is less at the equator than at the poles, or, in other words, that the center, the seat of gravity, is more distant at the former than at the latter. The diminished force of gravity at the equator has, it is true, another cause, namely, the centrifugal force arising from the rotation of the E., which acts counter to gravitation, and is necessarily greatest at the equator, and gradually lessens as we move northwards or southwards, till at the poles it is nothing. But the diminution of the force of gravity at the equator arising from the centrifugal force amounts to only of the whole force; while the diminution indicated by the pendulum is 3 The difference, or A, nearly, remains assignable to the greater distance of the surface from the center at the equator than at the poles. From the most accurate measurements of degrees that have been made, the flattening or ellipticity of the E. has been determined by Bessel at -g-6. Try, or nearly; or, the equatorial radius is to the polar as 300 to 299. These measurements of degrees determine not only the shape but the size of the earth. Bessel's calculations give a geographical mile, or the 60th part of a mean degree of the meridian, at 951.807 toises (2,029 yards, thus making the whole circumference 43,526,400 yards), and the equatorial and polar diameters at 6,875.6 and 6,8552.4 geographical in. (7,925.6 and 7,899.14 English imperial miles). The surface of the E. contains nearly 150 millions of square geographical miles.