76 THE CREATION OF MATTER 



Production of sound. Any body made to vibrate is 

 capable of producing the sensation of sound. Any kind 

 of blow, as a hammer striking a nail, the explosion of a 

 rifle, a flash of lightning, a bow drawn across a string, a 

 reed agitated, any force acting so as to produce vibrations 

 in sufficient numbers creates sound. 



A medium of conveyance is necessary. Where there is 

 nothing to strike or be struck, there can be no emission 

 of sound. Where there is nothing to receive or convey 

 the motions produced by a stroke, they cannot be con- 

 veyed. A glass jar is made to rest on the plate of an air- 

 pump. Within it is a bell associated with clockwork. 

 The jar is emptied of air by exhaustion. By means of 

 a rod descending into the glass, the detent holding the 

 hammer is loosed, the hammer strikes the bell, arid 

 sound motions are created in it. But as there is no air 

 to receive and forward them, they are not forwarded. 

 They cannot therefore be heard. Putting the ear to the 

 jar, a dull thud strikes it. The clock is hung by strings, 

 and that noise passes through them to the outside. Were 

 the air admitted, the bell would ring clearly. 



Sound is carried by wave motions. Their characteristics 

 have already been described in treating of light, and have 

 been illustrated by those of water. In the waves of the 

 sea, as we saw, the water does not advance, but the rising 

 and falling. If a flexible string, fastened at one end, be 

 held in the hand, and be drawn quickly to one side and back 

 again, a wave will run along it. In this case it is very 

 easy to see that the particles of the string do not run 

 along, but the motions only. And so sound motions are 

 carried, not by the first particles of air struck being borne 

 forward to the ear, but by their striking the next, and 

 they the next, and so onward. The first, moving forward 

 a little, stop ; they then return, and so oscillate to and 



