Page:Popular Astronomy - Airy - 1881.djvu/122

108 behind a cannon when it is fired, as you can then see the ball from the time that it leaves the cannon's mouth to a distance of a half a mile or more, you will perceive that its path is curved, but very much less curved than the path of a stone ; in fact it is nearly straight, but still not quite straight. The ball dropped downwards through the same space as the stone in one second of time ; but the ball travelled further in the horizontal direction than the stone in a second of time. Now, from this consideration we shall be able to explain something of the most puzzling matters in the motion of planets.

First, however, we must proceed to another consideration, which is called the resolution of forces. This may be illustrated by a model, represented in Figure 36. Suppose A and B to be two pullies fixed



upon an upright frame, and suppose two cords to pass over them, carrying the two weights C and D at their ends ; and where they meet at E, let a third cord be attached, carrying the weight F ; then you will see