358 HEARING. LECT. XIX. 



number of impulses in a given time which belong to the 

 movements of the sonorous body. The acuity or gravity 

 of a sound, is more or less due to the great rapidity with 

 which the vibrations succeed each other. The intensity 

 depends on the extent of excursions of the vibrating parts. 



Wollaston and Savart have endeavoured to determine 

 the limits within which sounds remain perceptible, or cease 

 to be so, to our ear, by acquiring too great an acuity or 

 gravity. Savart showed, that these limits were much more 

 considerable than had been previously supposed ; and that, 

 in order to perceive, either very acute or very grave sounds, 

 it is requisite merely to increase the intensity. By making 

 a long bar of iron pass, with a certain rapidity, through a 

 longitudinal chink, which it almost completely filled, we 

 obtain a very intense sound when this bar goes and comes 

 seven or eight times in a minute ; and, as at each passage 

 of the bar there is a compression of the air, to which a 

 rarefaction succeeds, the undulations which constitute the 

 sound are only to the extent of forty or fifty per second. 

 If, on the contrary, we employ a toothed wheel of very 

 large diameter, and hold an elastic plate in contact with 

 the teeth when the wheel is rotated, we perceive a very 

 acute sound when it has twenty-four thousand impulses per 

 second, in which case the sound is formed of forty-eight 

 thousand undulations. How complicated must be the or- 

 gan of hearing, when we reflect that its sensibility is pre- 

 served within limits so far apart, and that its principal parts 

 must vibrate in unison with sounds which vary from four- 

 teen to forty-eight thousand vibrations per second. 



According to the definition given of sound, we explain 

 without difficulty how it happens that, by the coexistence of 

 two sounds, whose vibrations stand to each other in a sim- 

 ple ratio, we hear a graver sound. When this takes place, 

 there are some moments in which the shakings, produced 



