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other; and if we try once or twice we can find a position when they are just balanced, so that neither pulls the other. It is quite a nice game to balance them. Now I think we have them. The little one is doing most of the revolving, but the big one is by no means at rest, you see. Fig. 50. So you must remember that our big Earth is itself waltzing a little with the Moon, though the Moon does most of the dancing.

Our next spinning experiment shall be a rough imitation of the detaching of a satellite. It requires some delicate adjustment, and you owe your thanks to Mr. Green for making us this mixture of alcohol and water, so carefully made that a little oil will