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pass through it, and swing out to the other side. Then it would come back again as the pendulum does; and that is what I think the stars are doing in our great cluster, some of them moving alone, some in pairs (the double stars) and others in groups; but all swinging to and fro through the centre of the whole bunch.

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Fig. 95. The experiment with the billiard balls, idealized by a member of the audience.

But now how is it that they keep clear of one another? If I take, instead of a single pendulum as before, a whole lot of pendulums all swinging through the same centre, they will be liable to strike one another. Let us try the experiment. Here we have several billiard balls hung by long strings to the theatre roof, and we will see how