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a weight as Galileo did, from the top of a tower, as at T in Fig. 4, then part of the Earth near F, the foot of the tower, is pretty close to us, and we have just seen how enormous the pull is when we come close to the attracting body. The other side of the Earth at K is 8000 miles away, and the pull will be much less. Suppose, for instance, the tower were a mile high, which is far higher than any tower we

Voyage in Space page025.png

Fig. 4.

are likely to ascend. Even then, the point K is 8000 times as far away as F, and its pull would be 8000 × 8000, or 64 million times weaker. So that when he wanted to add up the pulls from all parts of the Earth together, Newton naturally thought that the far side of the Earth near K would count for very little, and the part near F would be all-important. Even Newton, therefore, did not at first suspect the truth which he found out twenty years later, and which is, that we must measure from the centre C, which is just as near to K as to F. He thought that the correct starting-point would be at some point like R, nearer to F than to K; and he could not make any such point fit in with what was known about gravity at the time when he saw the apple fall, which was in the autumn of 1665. Hence, after puzzling over it for some time, he put the matter aside. Fourteen years later, Mr. Hooke, Secre-