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which is ringing plainly enough even when I cover it with a glass jar. But now, if we exhaust the air from the jar the sound will die away until you cannot hear it at all, though you can see that the hammer is still striking the bell as vigorously as ever. (Experiment.) And if we gradually let the air in again the sound comes back. All the apparatus for making the sound was the same throughout; only the air which carries it was removed, and the other materials round (such as the table) did not carry it sufficiently for you to hear.

Hence we must be prepared for a great silence on our voyage. When Jules Verne's travellers were shot out of the enormous cannon to the Moon, they were puzzled because they did not hear even the sound of the explosion which started them; and they reasoned it out that they travelled more quickly than sound, so that the sound was unable to catch them up. Of course, they talked to one another inside the projectile because they took air with them; but they were cut off from all outside sounds. Other projectiles might have blown foghorns louder than any steamer that ever passed us on the sea, but the warning would not be of the slightest use because it could not be heard.

It seems almost better for us not to venture into this great silence, doesn't it? Especially if I am to continue these lectures. You will not be able to hear me if we have no air, for several reasons. Consequently I shall next time propose a method of making our voyage which will avoid this difficulty, and allow us to stay comfortably in this room all the time.