170 SPECIAL SENSES. 



strata, after a single oscillation, return to their original quies- 

 cent condition. The first stratum receives the shock, and the 

 last communicates the shock to the ear. The oscillations of 

 sound, produced in this way, are to and fro in the direction 

 of the line of conduction, and are said to be longitudinal. 

 In the undulatory theory of light, the vibrations are supposed 

 to be at right angles to the line of propagation, or transversal. 

 A complete oscillation to and fro is called a sound-wave. 



It is evident that vibrating bodies may be made to per- 

 form and impart to the atmosphere oscillations of greater or 

 less amplitude. The intensity of the sound is in proportion 

 to the amplitude of the vibrations. If we cause a tuning- 

 fork to vibrate, the sound is at first loud, or intense ; but the 

 amplitude gradually diminishes, and the sound dies away until 

 it is lost. In a vibrating body capable of producing a definite 

 number of waves of sound in a second, it is evident that the 

 greater the amplitude of the wave, the greater is the velocity 

 of the particles thrown into vibration. It has been ascer- 

 tained by experiment, that there is an invariable mathemati- 

 cal relation between the intensity of sound, the velocity of 

 the conducting particles, and the amplitude of the waves ; and 

 this is expressed by the formula, that the intensity is propor- 

 tional to the square of the amplitude. It is evident, also, 

 that the intensity of sound is diminished by distance, as the 

 amplitude of the waves and the velocity of the vibrating par- 

 ticles become weaker, the farther we are removed from the 

 sonorous body. The sound, as the waves recede from the 

 sonorous body, becomes distributed over an increased area. 

 The propagation of sound has been reduced also to the for- 

 mula, that the intensity diminishes in proportion to the square 

 of the distance; 



Sonorous vibrations are subject to many of the laws of 

 reflection which we have studied in connection with light. 

 Sound may be absorbed by soft and non-vibrating surfaces, as 

 certain surfaces absorb the rays of light. It is in this way 

 that we explain the deadening of sound in apartments fur- 



