Page:Popular Astronomy - Airy - 1881.djvu/124

110 produce two forces acting in two different directions; yet we have plenty of familiar examples of the same thing. For instance, in driving a wedge by means of one force, we produce two force's in different directions. I mention that, as a case which must be notorious to everybody, and one which may well furnish food for thinking. By pushing at the back of the wedge with a small force you do exert two great forces at the sides of the wedge; and in like manner, by pulling at E (Figure 36) in a downward direction, you may exert a force even greater than your downward pull at the two inclined directions; and both these are accurate instances of what is called the resolution of forces. It must be understood, therefore, that having got a force in any one direction, we may say that instead of one force we have two forces acting in any two directions suited to the nature of the case, whose magnitudes are determined by certain laws depending upon the angles of inclination; and we may use those two forces instead of the one force which we had originally.

From this consideration, in combination with the considerations which I stated, relative to the dependence of curvature of path upon velocity and deflecting force, I shall endeavour to give you a little idea of the motion of a planet in its orbit. The thing that I wish specially to explain to you is, how it happens that when a planet has once begun to approach to the sun, it does not go quite to the sun, but after a time recedes again from it. If you understand this, you will understand the rest. I will suppose, if you please, that in Figure 30, a planet is moving from l, through M, towards L. The attraction of the sun pulls it in the direction of the line MS. Upon the principle of the resolution of forces, of which I have