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out—probably with a fairly good push, because they would be in a hurry not to let more air escape than they could help—when they pushed the dog away from the projectile, would it fall back again on to the projectile as we fall back on to the Earth when we push ourselves away by jumping? We know that the Earth pulls things to itself, but do other bodies also pull other things to themselves in the same way? We can scarcely believe that they do because we never feel the pull of anything but the Earth. If I stand near an object such as this desk in front of me I do not feel any pull towards it, and yet, as a matter of fact, it is pulling me and so is everything else in the room, though the pulls are too slight for me to feel. That is one of the laws of Gravity, that everything is pulling or attracting everything else; but the amount of pull or attraction depends firstly on the size of the body that pulls, and secondly on its distance away. The great Sir Isaac Newton first proved these laws some 250 years ago, and we will presently recall how he did it, just as we have recalled Galileo's experiment to memory. But for the moment I only want you to notice one point—that a large body like our Earth exerts a greater pull than a small one like this desk or like the projectile from which Jules Verne's travellers pushed the dog. Undoubtedly the projectile would try to pull the dog backwards, but owing to its small size it might not succeed, and then the dog would go farther and farther away. We cannot tell what would happen without knowing how hard the dog was pushed out. If a thing is pushed hard enough it can get away even from the Earth—indeed, that is exactly the point of