THE LENGTH OF OUR VOYAGE
hydrogen in the uppermost layers. Unfortunately these gases often escape before the chemists can get at them; they burst through the outer covering. In some of Mr. Gregory's specimens you see the holes through which the gases have burst out before we could examine them.
In one way and another, therefore, we have learnt a good deal about the upper air lately, so that we know what we should have to pass through in leaving port when we start on our voyage. The air will get thinner and colder and change ultimately into hydrogen, and then—what after that? We shall get out into the cold and silence of space—the great Silence. Perhaps it had not occurred to you that we should lose all sensation of sound? Our ancestors thought that the heavenly bodies were making beautiful music—the "music of the spheres"; but there is nothing to carry music in space. Most sounds we hear are carried by air; though they can be carried by other things as well. Here is a wooden rod which goes down through the floor into the basement where there is a musical-box. The sound of the music is coming up this rod into this room, though only those close to the rod can hear it, even faintly. But if two of my audience will kindly lift this wooden tray on to the top of the rod, the tray will act as a resonator and the whole room will hear the musical-box through the instrumentality of the rod. (Experiment as indicated.) So that materials other than air can carry sound. But in outer space there is no material at all, not even air; so that no sounds can travel. We can realize this by another experiment with this bell,