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Show I f ' ~ . 116 GRAVITATION OF DISTANCE. We must not, however, misunderstand what i~ m~ant by distance from a body in respect of gravitatwn. It is not from the surfaces of the bodies that the distance is estimated, it is from their cen~res of gravity, or of weight. Those centres, in bodies, are the points into which the whole would be sgueezed, if the different parts of the body itself gravitated so strongly as to be able to reduce the whole to one point; and they may or may not be the measured centres of bulk in the bodies. In a perfect globe. or round ball of uniform matter, the centre of gravity is the same as the centre of the ball ; but if half the ball were lead, and the other half cork, the centre of gravity would be so far into the leaden half that it would scarcely be possible to roll the ball, and it would always lie on the centre of the leaden surface. Very amusing toys for children are made upon that principle, by carving little figures in the pith of elder, or any other very light substance, and gluing a half leaden bullet on the bottom. The figures are so much lighter than the lead that they get up again when they are upset. Toys for children of an elder growth, such as bowls for playing on the green, are loaded in the same manner, which gives them a bias, or makes them run crooked; so that an expert player can bring his bowl in at a side, and take the mark · away from a bowl that touches it. On the same principle, ships' boats are ballasted, to prevent them from upsetting by the action of the wind on their sails; and coaches that have a box below, for heavy luggage, are much safer than those that carry a much smaller weight of luggage on the top. The gravitation of distance, or of the position which one body has with regard to another, depends, like specific gravity, on two elements,-the absolute weight of the body, and the distance; and it varies with every change in either of these. It is inversely as the square of the distance, and directly as the absolute gravity or quantitv of mattflr. But the .. - ~ . USE OF GRAYITATION. 117 ~avitatio_n of distance is not affected by the specific gravity ; for if it were not that the air resists ~he one mo~e than it _resists the other, because there Is r_nore weight of air opposed to the same wei<Yht of It, a cobweb would fall as fast and as straight fro~ the top of St. Paul's, or any other heirrht as a mill-stone. 0 ' . But though the place of no substance can be even m ~he leas~ changed without a change in the gravitatiOn of distance, yet the alterations produced by small . changes at long distances are very small. !he distanc~ of the mean surface of the earth from /it.s centre IS about four thousand miles and the 1 h1ghest mountain kl}own is less than five miles more, so that the weight of an ordinary man ~ould not be ~alf a po~nd less if he were on the top of the mountam than If he w~r~ on the seashore. Substances that have elasticity or spring in them, so that they are a.ffected by pressure, show even a much_ less elevatiOn than that. The air is altogether a sprmg, pres~ed down br its own weight, and it ~h?1ws ~he changes of height verv nicely. water 01 s with less. heat too; and the human bod is ~ffe?ted!-the mr which is with the blood or ofher hqmds, m the very small vessels, under delicate skin shwells the vessels, and sometimes bursts them and t ey bleed. Gravitation is the gr~nd principle by which N a~~ re, on the .great scale, IS .he~d together; and, under e same circum~tanc~s, It IS uniform in its a a~~ nt effects. In Itself It is always the same, foifcfwlmg mat~er thr~ugh all its changes, dead or alive at rest or m. mot~on. The paper of this book has' in the same Identical particles which now form it b~on ~~fnge·1 ~rom one .visible substance to another, 'pro"b-y ml ho_ns of hmes since it was created. and it ha~ ;ery likely been scattered through miliions of ~~ i~ h~c~s at 1 the same. insta_nt ; but not one atom s een ost ; and In all Its changes the amount |