SOUND 311 



the resistance of the air and friction bring it gradually 

 to rest. 



A large vibration of any one string occupies very nearly 

 the same time as a smaller, because the more the 

 string is bent, the more forcibly it is pulled back again 

 by its elasticity ; hence the uniformity of a musical tone. 



To the perfection of a tone, it is of no consequence in 

 what way the pulses are produced, provided they follow 

 with sufficient regularity ; witness the pure sound pro- 

 duced by the motion of a fly's wing^- supposed by many 

 to be the voice of the insect. ThWclacking of a corn- 

 mill, and the noise of a stick, pulled along a grating, are 

 not musical only because the pulses follow too slowly. 



Where a continued sound is produced by impulses 

 which do not, like those of an elastic body, follow in reg- 

 ular succession, the effect ceases to be a clear, uniform 

 sound or tone, and is called a noite. Such is the sound 

 of a saw or grindstone ; the roar of waves, breaking on 

 a rocky shore, or of a violent wind in a forest ; the roar 

 and crackling of houses, or of a wood in flames ; the mixed 

 voices of a talking multitude; the diversified sounds of a 

 great city, including the rattling of wheels, the clanking 

 of hammers, the voice of street criers, the noises of man- 

 ufactories, &c ; which rough elements, however, at last 

 mingle with such uniformity, that the combined result is 

 often called the hum of men, from analogy to the smooth 

 mingling miniature sounds which constitute the hum of 

 a bee-hive. 



II. VIBRATIONS IN THE AIR. 



The vibrations of sounding bodies above described are 

 communicated to the air around them, and thence to the 

 ear. Without the air, therefore, although the bell would vi- 

 brate if struck, the vibrations would not be communicated 

 to us, and we should hear no sound. That these vibrations 

 are thus conveyed through the air is evident from a vari- 

 ety of facts and experiments. The following is, however, 

 a sufficient illustration. 



If two bass-viols, tuned so that corresponding strings 

 in each, will give the same sound, be placed in two rooms, 



