Page:Popular Astronomy - Airy - 1881.djvu/280

266 law is necessary for the estimation of the weight of the bodies of the Solar System, the part of my subject to which I now proceed.

I must first allude to the weight of the earth, because the weights of all other bodies of the Solar System are necessarily referred to it as a standard. Taking the dimensions of the earth as I have stated them before, the number of cubic miles in the earth is about 259,800,000.000; each cubic mile contains 147,200,000,000 cubic feet; and each cubic foot, upon the average, weighs 5·67 times as much as a cubic foot of water, or 354 lbs. 6 oz. avoirdupois. I will leave the combination of these numbers to you, and will only remark at present, that I have shown you how the first step is made in referring the weights of the bodies of the Solar System, to the pound weight avoirdupois.

Next, I shall proceeed with the estimation of the weight of the sun as compared with the weight of the earth. And this I shall do by comparing the attraction produced by the sun with the attraction produced by the earth at the same distance. And here is involved an important principle, namely that the weight of a body is proportional to the attraction which it exerts. In order to explain this, it is necessary to remark, that every calculation of perturbation in the Solar System requires us to suppose, that the attraction of one body A upon another body B is not a mysterious influence by which the presence of A causes a movement in B, without any reciprocal influence upon A, but is a real mechanical action which exerts equal strains upon both, just as if they were connected by a contracting spring. Thus, every strain which a large body A produces upon a small body B; is accompanied by an equal strain, produced