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24 establishment of the Law of Gravity had to wait all that time in consequence, just as I have been keeping you waiting to see the end of the experiment. It was only after twenty years' thinking, off and on, that Newton discovered that we must measure all our distances from the centre of the Sun, where the screw is. And now, measuring in this way, we will go again twice as near, and the pull is getting very strong (Fig. 3): twice as near again and it is almost too much for me to hold: nearer still and it is too much—the weight wins the battle, as you see.

Perhaps you will let me tell you the history of Newton and the Law of Gravity. We shall see from it how much human nature enters into such discoveries at times; and it will have the further advantage of introducing us to another experiment which I want to show you, proving the rotation of the Earth. Newton was born in the year 1642, the same in which Galileo died. (Perhaps we may like to remember that Galileo was born in Shakespeare's birth-year and died in Newton's.) He began thinking about Gravity when the apple fell in the autumn of 1665, when only twenty-three years old; but having got so far as the idea of a pull which altered with the distance, as our apparatus illustrated, Newton was beset by the difficulty we have noticed. It is all very well, he reflected, to talk of the pull altering with the distance, but how are we to measure distance from a large body like the Earth or the Sun? Are we to measure it from the nearest point of the Earth, or from its centre, or from some point in between? When the whole Earth is far away, it does not much matter: but when we are dropping