248 



time, produces no sound, unless in its course, it meets with a body which 

 vibrates from the percussion which it experiences. 



The force of sound depends, entirely on the extent of the vibrations ex- 

 perienced by the molecules of the sonorous body. In a large bell struck 

 violently, the agitation of the molecules is such, that they are transmit- 

 ted to considerable distances, and that the form of the body is evidently 

 changed by it. Acute or grave sounds are produced by the greater or small- 

 er number of vibrations, in a given time, and the vibrations will be more 

 numerous, the smaller the length and diameter of the body. Two catgut 

 strings, of the same length and thickness, and with an equaldegree of ten- 

 sion, will vibrate an equal number of times, in a given time, and produce 

 the same sound. This, in music, is called unison. If one of the strings is 

 shortened by one half, it vibrates as often again as the other, and gives out 

 a sound more acute, or higher by one octave. The same result may be ob- 

 tained by reducing the string one half of its original thickness, without 

 taking from its length. The vibrations will, in the same manner, be 

 accelerated, by giving a greater degree of tension to the sonorous 

 cord. The difference of the sounds produced by a bass, a harp, or any 

 other stringed instrument, depends on the unequal tension, length, and 

 size of the strings. 



This division of the elementary sound is an act of the understanding, 

 which distinguishes, in a noise apparently monotonous, innumerable va- 

 rieties, and shades expressed by signs of convention. But in the same 

 manner as light, refracted by a prism, presents innumerable intermediate 

 shades, between the seven primitive colours, and as the transition is gra- 

 dual, from the one to the other of these colours : so the division of the 

 primitive sound into seven tones expressed by notes, is not absolute, and 

 there are a number of intermediate sounds which augment or diminish 

 their value, Sec. 



Sound has, therefore, been analysed as well as light ; the use of the ear, 

 with regard to sound, corresponds to that of the prism with regard to 

 light, and the modifications of which sound is capable, are as numerous 

 and as various, as the shades between the primitive colours. 



Sound is propagated with less velocity than light. The report of a can- 

 non fired at a distance, is heard only a moment after the eye has per- 

 ceived the flash of the explosion. Its rays diverge and are reflected, like 

 those of light, when they meet with an obstacle at an angle equal'to that 

 of incidence. The force of sound, like that of light, may be increased 

 by collecting and concentrating its rays. The sonorous rays which 

 strike a hard and elastic body, when reflected by it, impart to it a vibra- 

 tory motion, giving rise to a secondary sound, which increases the force 

 of the primitive sound. 



When these secondary sounds, produced by the percussion of a body 

 at a certain distance, reach the ear, they give rise to what is called an 

 echo. Who is unacquainted with the ingenious allegory, by which its 

 nature is expressed in ancient mythology, in which echo was called, 

 daughter of the air and of the earth ? 



CXXII. Of the Organ and Mechanism of Hearing. The organ of hear- 

 ing in man, consists of three very distinct parts ; the one placed exter- 

 nally, is intonded to collect and to transmit the sonorous rays which are 

 modified in passing along an intermediate cavity, between the external 

 and internal ear. It is within the^cavities of this third part of the organ, 

 excavated in the substance of the petrous portion of the bone, that the 





