Curiosities of Sound. 43 



sensation of calmness and repose, is not as soundless as we 

 imagine; but even that would be oppressive if endured for 

 long ; and could we visit a planet without an atmosphere such 

 as our moon is supposed to be, how appalling would be the 

 dreariness of its great mountain shadows, throwing their huge 

 black pall over the scene, as the sun deserted vast regions of 

 crags and plains, in which not the faintest whisper of any 

 voice was heard. 



We have in several previous papers explained the nature of 

 ivaves, and their propagation. Sound is the result of vibrations , 

 or wave-movements, transmitted by the air to the delicate 

 apparatus of our ears, and then reaching our brains, where they 

 become transformed into sensations, of which the mind takes 

 note. In wave-motion the particles of matter first affected 

 vibrate or oscillate through small spaces, but they communicate 

 their own motion to other particles ; and so the wave-form 

 spreads and spreads, until it becomes too feeble to be dis- 

 cerned. A stone thrown in a pond illustrates these actions. 

 Circle after circle of ripples are formed, wider and wider, but 

 shallower and shallower, until they are stopped by the banks ; 

 or, if the pond be big enough, until, in acquiring great width, 

 they have lost so much depth, that they can no longer be seen. 

 Further illustration of wave-propagation, as a series of spherical 

 shells, will be found in the paper referred to ; and though the 

 subject may appear a little difficult at first sight, it will prove 

 very simple when approached step by step.* 



If we have an instrument capable of communicating strong" 

 vibrations to the air, such as a bell, and place it under the 

 receiver of an air-pump, and strike it while in that position^ 

 we shall have a full sound while the receiver contains its 

 ordinary quantity of air ; but keep the bell ringing, and at 

 the same time pump the air out, the bell sounds will grow 

 weaker and weaker, until at last, if we make the vacuum 

 sufficiently complete, they will no longer be heard at all. The 

 intensity of a sound in a given medium depends on the force with 

 which its particles are moved, or on the velocity of their motion. 

 ' e Fix your attention," says Professor Tyndall, cc upon a particle 

 of air as a sound-wavepasses over it ; it is urged from its position 

 of rest towards a neighbouring particle, first with an accele- 

 rated motion, and then with a retarded one. The force which 

 first urges it is opposed by the elastic force of the air, which 

 finally stops the particle, and causes it to recoil. At a certain 

 point of its excursion, the velocity of the particle is at its 

 maximum. The intensity of the sound is proportioned to the 

 square of this maximum velocity." 



The intensity of a sound is, as we have seen from the air- 

 * See " Kadiant Forces," Intellectual Obseevee, March, 1867. 



