THE STARTING-POINT, OUR EARTH
Jules Verne's story, how a very hard push, from a huge cannon, sent the projectile right away from the Earth. The push had to be very hard indeed because the Earth is so big that it pulls very strongly: the pull of the little projectile would be very feeble indeed so that even a very slight push would send the dog right away, never to fall back. This, however, is not what happens in the story: the dog neither goes right away nor falls back, but goes round and round the projectile as a "satellite," that is, as our Moon goes round us: and here I am sorry to say Jules Verne makes another mistake, though it would take too long to explain why. Let us be satisfied for the moment to notice this great fact that a large body pulls harder than a small one. Perhaps you think that is not surprising, perhaps it seems natural to expect more from a large body. Let me remind you that it was just by following what seemed natural that Aristotle made his big mistake: it seemed to him natural to expect a large body to fall quicker than a small one, and he was wrong, as Galileo proved by trying it. If I had no better reason than that it seemed natural, I should not ask you to believe that a large body pulls harder than a small one; but there is a much better reason, namely that the experiment has been tried many times.
One famous experiment was made in 1798 by Henry Cavendish (of the same family as the Dukes of Devonshire), but it is by no means the only one: the experiment is really being made by astronomers every day, for if the fact were not true none of their calculations would come right as we know they do.