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22. Take two equal wooden bars SA and SB (Fig. 2) and put a screw through them at S, into the side of a ladder. The holes in the bars should be larger than the screw shaft so that they will turn freely. At A and B tie the ends of two exactly equal cords ACB and AKB. To the middle (K) of one of them hang the weight W, and hold up the other yourself at its middle point C. You may put something at C to represent a comet—say a bundle of horsehair; and something at S to represent the Sun—say a card circle. For the Sun at S will attract the comet with a force like that of gravity—"the inverse square of the distance" SC. [For those who like a mathematical proof, I put one at the end of the chapter, but the great thing is to try the experiment for yourselves.]

Now when you go up the ladder and hold C high up, as in Fig. 2, you will find that it is quite easy to support the weight W perhaps with one finger. You may have to ask some one to lift up the weight for you in the first instance, but once lifted, it is very easy to hold it up. If a pair of reins were made on this principle, a child could hold back a runaway horse.

But now drop C down to halfway so that the bars open out. You will find that the pull is much