Page:Popular Astronomy - Airy - 1881.djvu/234

220 stands. When we speak of the evidence of these things, we cannot give the evidence simply as it applies to any one law, but we can give it in combination with other laws.

The second law of motion we have endeavoured to illustrate (page 104) by apparatus, which showed that when a ball, was allowed to fall freely it was carried to the ground in the same time as a ball projected horizontally. The third law of motion relates to the effect of pressure, with which I have no occasion however to trouble you at present.

Having said so much on these subjects, we now come to gravitation. It is necessary to make this mention of the laws of motion first, because the movements connected with gravitation are but an instance of the application of the laws of motion to the movements produced by a certain force. The planets and satellites are in motion, and, according to the first law of motion, they would move on in straight lines, if they were not bent-aside by some force. This force, according to the theory of gravitation, is the attraction of another body. Let us now examine whether such a force, following the law of decreasing as the square of the distance increases, will account for some of these motions; and we will begin, as Newton did, with the moon.

The moon's motion with respect to the earth is influenced (according to this theory) almost entirely by the attraction of the earth; because though the sun attracts both the earth and the moon, yet it attracts them nearly in the same degree, and therefore produces little disturbance in their relative motions. And though the moon attracts the earth, still the moon is so much smaller than the earth, that we may omit the consideration of that at present. We shall