Page:Popular Astronomy - Airy - 1881.djvu/117

Rh accounts for the curved form of their orbits, and for all their motions in those orbits. Now, in speaking of this, I must observe that there is a term frequently used by persons not acquainted with its real meaning; persons speak of "projectile force" as if such a thing were constantly in action. The planets are in motion; they have been put in motion somehow; but there is no force to maintain their motion afterwards, that we know of, and there is no necessity for us to suppose the existence of a force which keeps up that motion. But, having been once started with a certain velocity, it is necessary to suppose that there is a force constantly pulling them towards the sun. The planets will sometimes go away from the sun, the sun will pull them back, and afterwards they will go away again, and be again pulled back, and so on.

In order to explain this, I must proceed with a very rude experiment on what is called the second law of motion. The first law of motion is simply this: if a body be once set in motion, and if it have a certain velocity given to it, it will continue to move (if not acted on by any force) in a straight line with unabated velocity. We cannot make experiments proving this law in the simple form in which I have mentioned it, but we can make experiments on it in combination with other laws; and we are compelled to believe that the law is true, that if a body were started in motion, and if nothing were acting upon it, it would continue in the same motion. The second law of motion is that which may be illustrated by a very rough experiment. Suppose a body to be projected horizontally, like a cannon ball, or like a stone thrown horizontally, you will observe that it begins to curve in its path, under the attraction of gravity, and it falls to the ground. The