Page:Popular Astronomy - Airy - 1881.djvu/279

Rh same proportion; and thus the mean density of the earth is found to be 5·67 times the density of water.

The near agreement of this result with that found from the Schehallien experiment, and that found from the theory of the figure of the earth, (taking the observed ellipticity of the earth in combination with such a law of density as would produce that ellipticity,) shows, beyond doubt, that the same law of gravitation which regulates the attraction of the sun upon the planets, and the attraction of the earth upon the moon, does also apply to the attraction of a leaden ball upon another ball within a foot of it. In regard to the slight difference of results, it is probable that the result of the Cavendish experiment is the more accurate of the two, simply because there is always some uncertainty upon the constitution of the rocks and mineral veins forming the interior of such a mountain as Schehallien.

I have gone into this subject, "the evidence for the theory of gravitation," at some length. First, because the law of gravitation is the most extensive in its application of all known laws; for we are certain that it applies to every body, and to every portion of a body, in the Solar System, and we have strong reason to believe that it applies to the mutual action of those stars which are observed to revolve in binary systems, the two stars revolving each round the other. Secondly, because the explanation of the methods of calculating its effects, in several instances of varied character, leads us to the consideration of several very interesting principles and applications of them. And thirdly, because the assumption of this