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now it seems wonderful that it should have been overlooked so long. That is often the case in science: once the way is shown, it is easy enough to follow it; but it may require a genius to show the way in the first instance. Have you ever hunted for seabirds' eggs on the shore? They are so nearly the colour of the stones that it takes a very sharp eye to find them, though when once they are pointed out they are easily seen.

Another thing is that the arrows of the star movements do not converge accurately to a point: indeed many of them are wildly wrong. That is what made the great discovery so difficult. It is only by taking careful averages of hundreds of stars that the general tendencies were discovered. This means that, although there is on the whole a tendency to the centre of our cluster, many of the stars have had knocks which carry them to one side or the other, just as our billiard balls had after the first few swings. There is no difficulty in this, however; indeed, it removes a difficulty that might have otherwise existed, for it shows how too-frequent collisions are avoided by the stars passing to one side or other of the centre. Some of them, indeed, may never go near the centre at all; they may whirl round and round it just as I can make this pendulum-bob whirl round and round instead of oscillating to and fro. To make it describe a circle I need only give it enough sideways motion, and now you see it never goes near the centre. But if all the stars moved like this, there would be none near the centre, except some almost stationary, as the pendulum is now. There