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21 Gravity. You have heard the story of Newton and the apple, which must surely be true, because here is a bit of the actual apple tree from Newton's garden at home: it now belongs to the Royal Astronomical Society and they have kindly lent it to us for this occasion. In the entrance hall you will find a beautiful picture of Newton under the apple tree, thinking about Gravity. But though the story of the apple may be quite true, it may suggest to you what is not true, namely, that he thought out the law of Gravity very quickly after seeing the apple fall. This was not the case: it took him 20 years to work out the great law fully in all its bearings. When he saw the apple fall, he began to think; he wondered if the pull of the Earth which made the apple fall, and which brings us down again when we jump up, would still be the same if we could go farther away, or how it would alter; and his thoughts took him farther and farther away till he thought of the Moon, and whether it was feeling the pull of the Earth. And then he must have thought of the movement of the Moon and of other bodies in the heavens, and naturally he would think of Kepler's three laws: and then the third of them, which we have just been talking about, led him to see that if the movements of the planets were in any way controlled by a pull from the Sun, the pull must be weaker at a distance, and stronger close to the Sun: and he calculated how much stronger it would be. Instead of stating the law in the language he used, let us illustrate it with this piece of apparatus, because it is easier to understand a thing when we have something to look at. You can easily make the apparatus for