Page:Popular Astronomy - Airy - 1881.djvu/143

Rh any falling body is accelerated in its motion. This shows that the effect of gravitation is not to create a sudden velocity, but to add velocity to velocity, and continually to increase velocity. Now, that is a thing which you must consider in regard to the motion both of bodies falling in a straight line and of bodies projected and allowed to fall in a curve.

It will be remembered that I exhibited an experiment to this effect—that if any body were projected horizontally at the same time that another body was allowed to fall freely, they would both reach the ground at the same time. The apparatus, Figure 32, by which the experiment was made, is so constructed that, of necessity, when one body is projected forward the other is allowed to drop at the same time. Now. let us consider what sort of a curve the projected body would describe. Suppose a shot is projected from a cannon, as A in Figure 35; as I said before, if a ball would fall from the cannon's mouth to the point C in a second of time, then the shot which was fired out of the cannon would have dropped to D in one second of time. What sort of course would it have described? It would have fallen from its original direction in exactly the same proportion, so far as regards the divisions of the time, as the ball which dropped from the cannon's mouth. The ball dropping from the cannon's mouth does not acquire all its velocity downwards at once, but by degrees. In like manner, if this other ball is supposed to be thrown out horizontally towards B, it does not begin to drop suddenly, but drops more and more rapidly; it follows, therefore, that the path of the cannon-shot begins to turn more and more downwards, and assumes the form of the curve which is shown in the figure. You see that the form differs very little from