SOUND. 



313 



Let A B C D, represent 

 the strings of two bass-viols, 



5 laced fifteen or twenty feet 

 istant ; o x y z, particles of 

 air through which the sound 

 made by drawing a bow over 

 the string A B, of the first 

 viol, is communicated to the 

 string C D, of the second 

 viol. When the string A B, 

 is put in motion it comes in 

 contact with the particle of 

 air o, or sufficiently near to 

 give it an impulse. This 

 particle receives an impulse 

 sufficient to carry it forward 

 in the direction of the dotted 

 line e, from thence it returns 

 an equal distance on the other side of the point from whence it 

 started. 



The second particle of air, x is impelled forward in a direction 

 towards y, by the first particle o, and the same with all the 

 others of' the series. The sound made by drawing a bow over 

 the string A B, of the first viol, is thus transmitted through 

 the successive particles of air, and at length reaches the string C D 

 of the second viol, and causes it to sound. Here it will be eviden 

 that there is no progressive motion of the particles. Each one moves 

 slightly and then returns to its place. It probably goes back farther 

 than to its first place, and after various oscillations comes to a stale of 

 rest in its original position. If, however, the whole mass of air 

 moved onwards, it would be wind not sound. 



That sound is thus conveyed by vibrations of the air, is 

 evident from many other facts. The concussion thus 

 produced is sometimes very great. A discharge of artil- 

 lery will sometimes break windows. The thunder has 

 often the same effect. An explosion of a powder mill 

 produces a shock in the air for many miles, though un- 

 doubtedly in this case the concussion is not altogether 

 produced by the sound. 



It follows from what has been said above, that without 

 air there would be no sound. This fact has been proved 

 by many experiments. If a bell be placed under a glass 

 vessel from which the air has been exhausted, any efforts 

 to produce a sound by striking the bell, will be in vain. 

 A faint sound may sometimes be heard ; but this would 

 not be the case if the air could be completely exhausted 

 from the glass vessel, and the connexion with surround- 

 VOL. i. NO. xin. 28* 



