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tary of the Royal Society, wrote to Newton asking him whether he had any interesting suggestion for one of the meetings. Newton replied in a famous letter, which became even more famous because it disappeared, and was only quoted from memory. Fortunately it was recovered by Trinity College (Newton's College at Cambridge) about thirty years ago, and is now in their library. They have kindly allowed me to show you a picture of part of it, in which Newton makes the suggestion Hooke asked for. He suggested a method of proving that the Earth is turning round on its axis. Nowadays we are all so well convinced of this fact that it seems to require no proof, but the case was very different in Galileo's time, when to state the fact got him into serious trouble. Newton came after Galileo, as we have noticed, and in his day the truth was better known, but even then it was not so firmly established but that he thought it worth proving. And so he suggested that a weight should be dropped from a tower T, much as Galileo had dropped weights from the leaning tower of Pisa; but with a different object in view. This time it was to be carefully noted, not how quickly the weight fell, but exactly where it fell. If the Earth were not turning round then it would fall at F, the foot of the tower: but if the Earth is turning round, then the weight would be flicked forward a little to G, just as a splash of mud is flicked forward by a rotating wheel (Fig. 4).

The rotation of the Earth can indeed be shown in this way, as Newton thought and as we will presently mention. But unfortunately Hooke misunderstood the point. His mind was full of a different problem,