SOUND. 187 



able suggestion for the discrimination of diseases of the chest, and of 

 various healthy and morbid conditions. By putting the ear to the 

 chest we can hear the rush of air along the bronchial tubes, the pulsa- 

 tions of the heart, &c., and can discover any aberration in the execu- 

 tion of their functions. This is what was called by the late distin- 

 guished Laennec, of Paris the proposer of the method immediate 

 auscultation. The direct application of the ear to the chest is, how- 

 ever, frequently inadmissible. In these cases he used a hollow cylinder 

 called a stethoscope, one end of which he applied to the chest the 

 other to the ear. This plan he termed mediate auscultation. The 

 suggestion has led to valuable improvements in diagnosis. 



MM. Hassenfratz and Biot have made some accurate experiments on 

 the comparative rapidity of the progress of sound through air and solid 

 bodies. The latter found, in the aqueducts of Paris, that a blow, 

 struck upon a pipe nine hundred and fifty-one metres, or about ten 

 hundred and forty yards, in length, was heard two seconds and a half 

 sooner through the sides of the pipe than through the air within; but 

 the sound did not extend so far. Ice conveys sound even better than 

 water; for if a cannon be fired from a distant post a frozen river 

 intervening each flash is followed by two distinct reports, the first 

 conveyed by the ice, the second by the air. 



It has been already stated, that the vibrations of air, caused by a 

 sonorous body, are capable of exciting corresponding or sympathetic 

 vibrations in solid bodies within their sphere of action. It was an old 

 observation, that such vibrations are excited only in bodies that are in 

 unison with the sonorous body; in other words, in those that are capa- 

 ble of producing the same tone. Unison, however, is not necessary. 

 When a sound is produced in air, every body receives a vibration, which 

 is a repetition of the one that occasioned the sound. This M. Savart 

 proved by using small membranes on which he placed fine sand. They 

 were agitated ; and the sand assumed various regular arrangements, 

 whenever a sound was produced in their vicinity. In other words, the 

 membrane was thrown into vibration, not as a whole, unless its funda- 

 mental note was in unison with the one sounded; but in distinct seg- 

 ments, every one of which reciprocated the sound. This law of physics 

 is important in its physiological relations. The apparatus of audition 

 consists of several membranous structures, which are thrown into oscilla- 

 tion, whenever the ear receives the impressions of sound. 



The vibrations, which produce sound, differ much as regards their 

 extent and rapidity ; and on these differences two of the qualities of 

 sound strength and tone are dependent. Strength or intensity de- 

 pends on the extent of the vibrations of a sonorous body. This is seen 

 in a musical string, the sound of which becomes weaker as the extent 

 of the oscillations diminishes. The tone, on the other hand, is depend- 

 ent on the rapidity of the oscillations ; on their number in a given 

 time. The tone, produced by .a string or other sonorous body that 

 vibrates quickly, is termed acute or sharp, when compared with that of 

 one which vibrates more slowly. The latter is called grave, when com- 

 pared with the former. The gravest sound that the ear can appreciate 

 is considered to result from thirty-two vibrations per second; the most 



