greater—four times as great, as Newton tells us. It is not very strong even yet, but it is beginning to be felt. Here is a spring-balance with which we can measure the pull at C, and you will see that the spring is scarcely moved. If again I let C (the comet) come twice as near to the Sun the pull is four times as great again, or sixteen times as great as at
Fig. 3.
first. But now that we are getting nearer to the Sun, which we represent by a cardboard circle, one very important matter forces itself on our attention. Where are we measuring our distances from? Is it from the top of the card, which is the nearest point of the Sun? or is it to be from the centre where the screw is? In a sense we are twice as near the card when we are twice as near the nearest point of it; and there is a good deal to be said for measuring in this way, since the very point we are illustrating is that gravity is so immensely greater when we are close to the attracting body than when we are far away; and therefore we might expect that the part of the Sun nearest would be the important part. At any rate this seems to have been what first occurred to Newton, and he could not get the idea out of his head for twenty years: and the complete
