2. The Mass and Density of the Earth.-.-We have now seen that the E. is a sphere slightly flattened at its poles—what is called by geometers an elliptical spheroid—of a mean radius of somewhat less than 4,000 miles. We have next to consider its mass and density. Nothing astonishes the young student more than the idea of weighing the E.; but there are several ways of doing it; and unless we could do it, we never could know its density. (1.) The first method is by observing how much the attraction of a mountain deflects a plummet from the vertical line. This being observed, if we can ascertain the actual weight of the mountain, we can calculate that of the earth. In this way, Dr. Maskelyne, in the years 1774-76, by experiments at Schihallion, in Perthshire, a large mountain mass lying e. and w., and steep on both sides—calculated the E.'s mean density to be five times greater than that of water. The observed deflection of the plummet in these experiments was between 4" and 5". (2.) In the method just described, there must always be uncertainty, however accurate the observations, in regard to the mass or weight of the mountain. The method known as Cavendish's experiment is much freer from liability to error. This experiment was first made by Henry Cavendish on the suggestion of Michel, and has since been repeated by Reich of Freyberg, and Mr. Francis Bally. In the apparatus used by Mr. Baily two small balls at the extremities of a fine rod are suspended by a wire, and their position carefully observed by the aid of a telescope. Large balls of lead placed on a turning-frame, the center of which is in the prolongation of the suspending wire, are then brought near them in such a way that they can affect them only by the force of their attraction. On the large balls being so placed, the small ones move towards them through a small space, which is carefully measured. The position of the large balls is then reversed i.e., they are placed at the same angular distance on the other side of the small balls— and the change of position of the small balls is again observed. Many observations are made, till the exact amount of the deviation of the small balls is ascertained beyond doubt. Then by calculation the amount of attraction of the large balls to produce this deviation is easily obtained. Having reached this, the next question is, what would their attraetion be if they were as large as the earth? This is easily answered, and hence, as we know the attractive force of the E., we can at once compare its mean density with that of lead. Mr. Baily's experiments lead to the result that the E.'s mean density is 5.67 times that of water. (3.) A third mode has lately been adopted by the astronomer-royal, by comparison of two invariable pendulums, one at the E.'s surface, the other at the bottom of a pit at Harton colliery, near Newcastle, 1260 ft. below the surface. The density of the E., as ascertained from this experiment, is 6 and 7 times that of water; but for, various reasons this result is not to be accepted as against that of the Cavendish experiment, and it is said that the astronomer-royal was himself dissatified with it, and meant to repeat the experiment with new precautions. The density of the E. being known, its mass is easily calculated, and made a unit of mass for measuring that of the other bodies in the system. It is found that the mass of the E. compared with that of the sun is .0000028173.
3. the Motions of the Earth.—The E., as a member of the solar system, moves along with the other planets round the sun from w. to east. This is contrary to our sensible impressions, according to which the sun seems to move round the E.; it was not till a few centuries ago that men were able to get over this illusion. See COPERNICAN SYSTEM. This journey round the sun is performed in about 365f days, which we call a year (solar year). The E.'s path or orbit is not strictly a circle, but an ellipse of small eccen tricity, in one of the foci of which is the sun. It follows that the E. is not equally distant from the sun at all times of the year; it is nearest, or in perihelion, at the beginning of the year, or when the northern hemisphere has winter; and at its greatest distane, or aphelion, about the middle of the year, or during the summer of the north ern hemisphere. The difference of distance, however, is comparatively ,too small to exercise any perceptible influence on the heat derived from the sun, and the variation of the seasons has a quite different cause. The least distance of the sun from the E. is over 94 millions of miles, and the greatest over 96 millions; the mean distance is com monly stated at 95 millions of miles. If the mean distance be taken as unity, then the greatest and least are respectively represented by 1.01679, and 0.98321. It follows that the E. yearly describes a path of upwards of 596 millions of miles, so that its velocity in its orbit is al4cnit ft., or 19 nt. in a second.
Besides its annual motion round the sun, the E. has a daily motion or rotation on its axis, or shorter diameter, which is performed from w. to e., and occl:pies exactly 23
hours, 56 minutes, 4 seconds of mean time. On this motion depend the rising and set ting of the sun, or the vicissitudes of day and night. The relative lengths of day and night depend upon the angle formed by the E.'s axis with the plane of its orbit. If the axe., were perpendicular to the plane of the orbit, day and night would be equal during the whole year over all the E., and there would be no change of seasons; but the axis makes with the orbit an angle of and the consequence of this is all that variety of seasons and of climates that we find on the E.'s surface; for it is only for a small strip (theoretically, for a mere line) lying under the equator that the days and nights are equal all the year; at all other places, this equality only occurs on the two days in each year when the sun seems to pass through the celestial equator, i.e., about the 21st of Mar., and the 23d of September. From Mar. 21, the sun departs from the equator towards the n., till, about June 21, he has reached a n. declination of 23°, when he again approaches the equator, which he reaches about Sept. 23. He then advances southward, and about Dec. 21, has reached a s. declination of 23e, when he turns once more towards the equator, at which he arrives, Mar. 21. The 21st of June is the longest day in the northern hemisphere, and the shortest in the southern; with the 21st of Dec., it is the reverse.
The velocity of the E.'s rotation on its axis evidently increases gradually from the poles to the equator, where it is about equal to that of a musket-hall, being at the rate of 24,840 m. a day, or about 1440 ft. in a second.
A direct proof of the rotation of the E. is furnished by its compression at the poles. There are indubitable indications that the E. was originally fluid, or at least soft; and in that condition it must have assumed the spherical shape. The only cause, then, that can be assigned for the fact that it has not done so, is its rotation on its axis. Calculation also shows that the amount of compression which the E. actually has, corresponds exactly to what its known velocity and mass must have produced. Experi ments with the pendulum, too, show a decrease of the force of gravity from the poles towards the equator; and though a part of this decrease is owing to the want of perfect sphericity, the areatest part arises from the centrifugal force caused by the motion of rotation. Another direct proof of the same hypothesis may be drawn from the observation that bodies dropped from a considerable height deviate towards the e. from the vertical line. This fact has been established by the experiments of Benzenberg and others. In former times, it was believed that if the E. actually revolved in the direction of e., a stone dropped from the top of a tower would fall, not exactly at the foot of the tower,but to the w. of it. Now, as experience, it was argued, shows that this is not the case—that the stone, in fact, does fall at the bottom—we hate here a proof that the pretended rotation of the E. does not take place. Even Tycho Brahe and Riccioli held this objection to the doctrine to be unanswerable. But the facts of the case were just the-reverse. Newton, with his wonted clearness of vision, saw that, in consequence of the E.'s motion from w. to e., bodies descending from a height must decline from the perpendicular, not westward, but eastward; since, by their greater distance from the B.'s center, they acquire at the top a greater eastward velocity than the surface of the E. has at the bottom, and retain that velocity during their descent. He therefore proposed that more exact observations should be made to ascertain the fact; but it was not till more than a century afterwards that experiments of sufficient delicacy were made to bring out the expected result satisfactorily. It is difficult to find an elevation sufficiently great for the purpose, as several hundred feet give merely a slight deviation, which it requires great accuracy to observe. If a height of 10,000 ft. could be made available, the devia tion would be not less than 7+ feet. The analogy of our E. to the other planets may also be adduced, the rotation of which, with the exception of the smallest and the most distant, is distinctly discernible. Finally, an additional proof of the E.'s rotation was lately given by Leon Foucault's striking experiment with the pendulum. The principle of the experiment is this: that a pendulum once set in motion, and swinging freely, continues to swing in the same plane, while at any place at a distance from the equator the plane of the meridian continues to change its position relative to this fixed plane.—The objection taken to the doctrine of rotation from the fact that we are uncon scions of any motion, has little weight. The movement of a vessel in smooth water is not felt, though far less uniform than that of the E.; and as the atmosphere acconi. panics the E. in its motion, there is no feeling of cutting through it to break the illusion of rest.