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Rh coin rests stolidly at the bottom. Here, again, is a thing worth noting that is not immediately before us—this rapid inrush of the air from outside into the exhausted space. If the jar had been full of air and the outside empty the rush would of course have been the other way and would soon have left very little in the jar. Now in Jules Verne's book, of which I spoke early in the lecture, the brave men who went in the projectile took with them a dog, and the poor dog died. They thereupon opened a trap-door and put it outside; but I fancy this would have been much more difficult and dangerous than Jules Verne thought, because though the travellers had taken the precaution to carry plenty of air with them inside their projectile, there would be very little outside it when once they had got some distance from the Earth. We shall see in the next lecture how very shallow our atmosphere—is the projectile would pass right through it in a few seconds. Hence, when the trap-door was opened to put out the body of the poor dog, I fear all the air would have rushed out almost at once, and the men would have died too, and so would the story.

If we want the story to go on, we must let Jules Verne tell it in his own way: but there is no harm in making mental reservations, and we will take the opportunity to make one more, because it brings to our notice another fact about Gravity, in which we are specially interested to-day.

We have already said that when we jump up we come down again because of Gravity—because the Earth pulls us to itself. Now when Jules Verne's travellers opened the trap-door and put the poor dog