253. =Sound Transmitted through Various Substances.=--In ordinary hearing sound, as you have seen, is transmitted through various substances before the vibration arrives at the liquid in the halls of audience. But sound need not take this course in all cases to arrive at the nerve of hearing. If, for example, you place a watch between your teeth, the sound will go through the solid teeth and the bones of the jaw directly to the halls of audience by a short cut, instead of going round through the outer ear-passage to the drum, and so through the chain of bones. Fishes in hearing receive the vibration through water. If you place your ear at the end of a timber, while some one scratches with a pin at the other end, you hear the sound distinctly, for the vibration is transmitted through the timber; as in the case of the watch between the teeth, it goes through the solid bone. [Illustration: Fig. 185.] 254. =Sound not Transmitted through a Vacuum.=--As sound is a vibration of some substance it can not be transmitted through absolute space. This can be proved by an experiment with the air-pump, as represented in Fig. 185. The apparatus in the receiver is so arranged that the bell, _a_, can be struck by pressing down a sliding rod, _h_. If it be struck before the air is exhausted the sound is heard through the glass. But the more you exhaust the air the fainter will be the sound; and at length, if you keep on pumping, it can not be heard at all. The same experiment can be tried with a music-box. It is from the thinness of the air on high mountains, and at the great heights reached by balloons, that all sounds are so faint. The report of a pistol fired off on top of Mont Blanc is a mere crack compared with its report when fired in the valley below. 255. =Motion of the Heavenly Bodies without Noise.=--Sound is often heard at a very great distance on the earth. The sound of an eruption of a volcano has been heard in one case at the distance of 970 miles. But suppose that the same sound should occur at the same distance from the earth, that is, over 900 miles beyond the atmosphere that enrobes the earth, no inhabitant of our world could hear it, for the same reason that you do not hear the bell ringing in an exhausted receiver. If, therefore, any sound, however loud, should be given forth by any of the heavenly bodies we could not hear it. The course of these bodies in their orbits is noiseless, because they meet with no resistance from any substance. Bodies passing rapidly through our atmosphere cause sound, from the resistance which the air gives to their passage. The whizzing of a ball is an example of this. It is the passage of the electric fluid through the air which produces the thunder. But the heavenly bodies, having no such resistance, make no sound in their course, though their velocity be so immense. In the expressive language of the Bible, "their voice is not heard."