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we set machinery to work and buzzed the hoops round at a terrific pace we could make the portions ab break away and fly off. It is only a question of speed to break even the strongest flywheel.

Fig. 48.

Here is another experiment, which shows us something else. We generally say that the satellites go round the planets, as though the planets themselves did not move. But this is not strictly true. The Earth and Moon are like a pair of partners waltzing, one partner being very much heavier than the other. The little partner almost flies round the big one, but the big one has to move a little. Fig. 49. Here is a big ball A and a little one B tied together (Fig. 48), and both are sliding on the same wire. If we put them at equal distances from the centre C and then spin the apparatus, the big ball immediately flies to the end a, pulling the little one with it. But if we put A at the centre (Fig. 49) and spin again, the little one flies to its end b and pulls the big one. If we are to get a proper balance we must put the big one, not at the centre, but certainly nearer than the