Page:Popular Astronomy - Airy - 1881.djvu/288

274 "inequalities of long period," but there are others sufficiently sensible, which are treated in just the same way. In this manner the masses of these distant little bodies are ascertained with reasonable accuracy. The largest of them (the third satellite) is about as large as our moon.

I have thus redeemed my pledge of explaining how the weights of the principal bodies of the Solar System are estimated by means of a pound avoirdupois. And I will here briefly recapitulate the principal steps.

First of all, I remarked that this estimation rests absolutely upon the truth of the Theory of Universal Gravitation, and I therefore pointed out the principal evidences of that theory. As these different evidences are nearly independent of each other, I shall not repeat them, but refer you back to what was said upon each of them.

Then the reference of weights to avoirdupois pounds begins with the weighing of the rocks of Schehallien for the Schehallien experiment, and the weighing of the large leaden balls for the Cavendish experiment; or, if you please, by weighing water, because the weights both of rocks and of lead are conveniently expressed by expressing the proportion which their weights bear to the weight of water.

The next step was this: by means of the Schehallien experiment and the Cavendish experiment, as well as by inferences from the ellipticity of the earth, we found that the mean density of the earth is between five and six times the density of water, and from that we were able to compute the weight of the earth.

The next step was this: having the dimensions of the moon's orbit round the earth, we could find