chap, xxxr.i SOUXD. 419 



the other, the maximum declining to a minimum, till 

 it finally disappears when the particle comes to rest. 

 The whole history, then, of a particle sharing in the 

 propagation of a wave of sound is, that under the 

 influence of a shock it moves forward from its 

 position of rest for a certain distance, then moves 

 backward past its position of rest, but in the same 

 line, and finally returns to its middle position. Each 

 particle thus oscillates to and fro on each side of its 

 middle position. While this is the history for each 

 particle, the combined movements of the series of 

 particles produce an alternate condensation and rare- 

 faction, the two forming a sound wave. 



The movement of a particle from its position of 

 rest to one extreme, then to the opposite extreme, 

 and finally back to its original position, is one com- 

 plete oscillation or vibration, the time occupied in 

 completing the movement being the period of oscilla- 

 tion, while the distance between the period of rest 

 and either of the extremes gives the amplitude of the 

 vibration. Two particles which are moving in the 

 same direction and are at the moment occupying the 

 same position, are said to be in the same phase ; and 

 the distance between one particle and the succeeding 

 particle in the same phase is the wave length, the 

 distance between one condensation and the succeeding 



o 



one, or between one rarefaction and its successor. 



There is another difference between the propaga- 

 tion of sound waves and light waves. For the propa- 

 gation of light it was found necessary to assume the 

 existence of a subtle ether pervading all space and all 

 bodies, an ether which was present even where no 

 atmosphere existed. A simple experiment shows 

 how different is the case with sound. Let an alarum 

 bell be placed within the jar of an air pump. It 

 must be suspended in the jar by threads, and not 

 simply placed in the jar of the air pump. Suppose 



