TILE AlASS AND DENSITY OF THE EARTH. There are several methods of weighing the earth: (1) The tirst method is by observing how much the attraction of a mountain dethoets a plummet from a if we can ascertain the actual weight of the mountain. we can calculate that of the earth. in this way Alaskelyne, in the years 1774-76, by experi ments at SehehaBien, in Seotlaml, a large moun tain mass lying east and west, and steep on both --ides, calculated the earth's mean density to be five times greater than that of water. The ob served deflection of the plummet in these experi ments was about six minutes. In the method just described there must always he 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 employed by Henry CavendiA on the suggestion of Michel, and has since been repeated by Reich, of Freiburg. and by Bail•. In the apparatus used by Bail•, two small balls at the extremities of a line rod are suspended by a wire, and their position carefully observed by the aid of a tele scope. Large halls 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 n 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 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 the position of the small balls is again observed. Many ob ser•ations are made, till the exact amount of the deviation of the small balls is ascertained beyond doubt. Then by enlenlation the amount of at traction of the large halls to produce this devia tion is easily obtained. !laving reached this, the next question is. What would their attraction be if they wore as large as the earth? This is (asily answered; and hence- as we know the at tra•tive force of the earth. we can at m•e colon bare its mean density with that of lead. Bally's experiments lead to the result that the earth's mean density is 5.67 times that of water. (3)
A third mode, tried by Airy, consisted in ob serving two invariable pendulums. one at the cart l's surface, the other at the bottomu of a pit at Harton near Newcastle. 1260 feel 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 1iry himself was dissatisfied with it, and meant to repeat the experiment with new precautions. The most recent determination of the earth's density was made by Wilsing, at. l'otsdam. using a Method not dissimilar front Wil sing found 5.59 as loie final result. The density of the earth being thus known. its mass or weight is easily ealeidated and made a unit for measuring that of the ether bodies in the solar system.
Mornees OF TIIE EARTH. Tht• earth, as a member of the solar system, moves along with the other planets round the sun from west to ens!. This journey round the sun is performed in about days, which we calla vear (solar year). The earth's path or orbit is not a circle, but ono ellipse of small eccentricity, in one of whose food is the sun. It ((dhows that the earth is not equally distant from the sun at all tunes of the year; it is nearest, or in perihelion, at the beginning of the year. or when the North ern Hemisphere has whiter; and at its greatest distance, or aphelion, about the middle of the year, or during the summer of the Northern demisphere. The difference of distance, how ever, is too small to exercise any perceptible in fluence on the beat derived from the sun, and the variation of the seasons has a quite different 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 mike. (See SoLide.) 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 that the earth yearly describes a path of upward of 560.000,000 miles, so that its in its orbit is about 19 miles a second.