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Rh a connected story as they have done. What I shall endeavour to do is to bring before you some of the difficulties of making such voyages, so that you may understand as clearly as possible how they arise: which of them we may hope to get rid of in time, and which seem beyond our powers. And we shall begin to-day with the first difficulty of getting away from this Earth at all.

Why is it so difficult to get away? If we jump up we come down again. We call the reason "gravity," and sometimes we are tempted to let the matter rest when we have given a name to the difficulty. But I want us to think a little more about it than this: to think about what the nature of it is, and also about the great men who found out for us what gravity is. Because it took a great deal of finding. About 2000 years ago, in Greek times, they had some curious ideas about it. The great Greek philosopher Aristotle said that if you took a light thing and a heavy thing and dropped them together, the heavy thing fell much faster. Here are his actual words; some of you (very few, I hope) may not be able to read Greek, and so we will translate them; but it is better to show you the actual words that Aristotle used, because what he said seems so strange to us now.

"We see that bodies which have a greater tendency of heaviness or lightness, if they are otherwise