EARTH. 588 EARTH. which shows that the force of gravity is less at the equator than at the poles, or, in other worils, that the centre is more dislaiil at the former than at the latter. The diminished force of gravity at the equator has, it is true, an- other cause — namely, the centrifugal force aris- ing from the rotation of the earth, which acts counter to gravitation, and is necessarily great- est at the ei|Uator, where the linear vch)city of rotation is greatest, and gradually lessens as we move nortliward or southward, till at the poles it is nothing. But the diminution of the force of gravity at the c(]uator, arising from the centrifugal force, amounts to only ^J^, of the whole force; while the diminution indicated by the pendulum is jjj. The difference, or -^j nearly, remains assignable to the greater distance of the surface from the centre at the equator than at the poles. Krom the most accurate measure- ments of degrees that have been made, the flat- tening or eliiplicity of the earth lias hccii deter- mined at jio nearly; or the equatorial radius is to the polar as 300 to 299. These measurements of degrees determine not only the shape, but also the size of the earth. It is thus found that the equatorial semi-diameter amounts to 3903 miles, while the polar senii-diainetcr is 3950 miles. The Mass ..nd DEN.sixr of the Earth. There are several methods of weighing ^1"- earth: (1) The first method is by observing how much the attraction of a mountain deflects a |)lumiiiet from a vertical line. This licing observed, if we can ascertain the actual weight of the mountain, we can calculate that of the earth. In this way Maskel.yne, in the years 1774-70, by experi- ments at Sehehallien, in Scotland, a large moun- tain mass lying east and west, and steep on both sides, calculated the earth's mean density to be five times greater than that of water. The ob- served deflection of the iiluiiimet in these experi- ments was about si.x minutes. In the iiielbod just described there must always be uncertainty. however accurate the observations, in regard to the mass or weight of the mountain. (2) The method known as Cavendish's is much freer from liability to error. This method was first emiiloyed by Henry Cavendish on the suggestion of Michel, and has since lieen repeated by Reich, of Freiburg. ;iml by Baily. In the apparatus used by 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 tele- scope. Large balls of lead placed on a turning frame, the centre of which is in the prolonga- tion 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 toward them tlirough a small s])ace, which is carefully nieasiired. 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 the position of the small balls is again ob.served. Many ob- servations are made, till (be exact amount of the deviation of the small balls is ascertained beyond doubt. Then by calculation the amount of at- traction of the large balls to produce this devia- tion is easily obtained. Having reached this, the next question is. What would their attraction be if they were as large as the earth? This is easily answered : and hence, as we know the at- tr.ictivc force of the earth, we can at once com- jiare its mean density with that of lead. Baily'a ex|>eriments lead to the result that the earth's mean density is 5.07 times that of water. (3) - third mode, tried by .iry, consisted in ob- serving two invariable pendulums, one at the earth's surface, the other at the bottom of a pit at llarton Colliery, near Newcastle, 1200 feet below the surface. The density of the earth, as ascertained from this experiment, is six to seven 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 .iry himself was dissatisfied with it, and meant to repeat the experiment with new i)recautions. The most recent determination of the earth's density was made by Wilsing. at Potsdam, using a method not dissimilar from Cavendish's. Wil- sing found 5.59 as his final result. The density of the earth being thus known, its mass or weight is easily calculated and made a unit for measuring that of the other bodies in the solar system. The JloTioxs of the Earth. The earth, as a member of the solar system, moves along with the other planets round the sun from west to east. This journey round the sun is |)erformed in about 30514 da,vs, which we call a year (solar year). The earth's path or orbit is not a circle, but an ellipse of small eccentricity, in one of whose foci is the sun. It follows that the earth 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 Xortli- ern Hemisphere has winter; and at its greatest distance, or aphelion, about the middle of the year, or iluring the summer of the Northern Hemisphere, The <Iifference of distance, how- ever, is too snuill to exercise anv perceptible in- fluence on the heat derived from the sun, and the variation of the seasons has a quite difl'erent cause. The least distance of the sun from the earth is over 91.000.000 miles, and the greatest over 94,000.000 : the mean distance is commonly stated at 92.900.000 miles. (See Parallax, Solar.) If the mean distance be taken as unity, then the greatest and least are respectively rep- resented by 1.01677 and 0.98323. It follows thac (he earth ye:irly describes a path of upward of 500.000.000 miles, so that its velocity in its oroit is about 19 miles a second. Besides its annual motion round the sun the earth has a daily motion or rotation on its axis, which is i)erfonncd from west to east and occu- pies exactly 23 hours, 50 minutes, 4.090 seconds of ordinary mean solar time. On (his motion depend the rising and setting of the sun. or the changes of day and night. The relative lengths of day and night depend upon the angle formed by the earth's axis with the ])lane of its orbit. If the axis were perpendicular to (he plane of (he orbit, day and niirht would be equai during the whole year over all the earth, and there would be no change of seasons; but the axis makes with the orbit an angle of 23.5°. and the eonseqience of this is all that variety of sen- sons and of climates that we find on the earth's surface, for it is only for a small strip {{heoretic-
- illy for a mere line) lying under (he equa(or 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 (he celestial cipuitor — i.e. abiuit March 21st and ."September 23d. From March 21st the
