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284 steadily moving in a particular direction—making a sort of migration. Whence did they come and whither are they going? Will they go on for ever or will they come back after a time as migrating birds do? These are questions which suggest themselves at once, though they are not so easily answered. I think, however, that an answer can be given: an answer which suggests itself if we go back to that notion of our belonging to a large cluster of stars to which we were led earlier in the lecture. You remember that our "audience" of stars, as we called it, thins off in the back rows, giving us the notion that we form part of a definite cluster such as we see in the picture (Fig. 85). Now let us think out the consequences of this arrangement, remembering that by the great Law of Gravitation each star is pulling every other star. We have already said that even with three or four stars it is so difficult to tell what will happen in detail that the best mathematicians have not yet been able to work it out: so you may think that with millions and millions of stars it would be quite hopeless. But curiously enough it makes the problem in some ways simpler to have a very large number, and I do not think we shall find it very hard to think out some of the consequences. For instance, it is easy to see that a star right in the middle of the bunch will be pulled in all directions at once—to right as well as to left, up as much—as down so that it will not know which way to move; and if it had already no movement it would not acquire any. But it would be very different for a star on the boundary: all the pulls would be