442 Cattfe of acute and grave Sbwids. [Book V. 



tides be in fome meafure removed from the point of 

 contact. 



There are then two vibrations which take place in 

 fbnorous bodies j the general vibration, which changes 

 the form of the body, and the particular vibration, 

 -which affects the- minute particles, in confequence of 

 the former. M. de la Hire has proved *, that the 

 found is not owing to the general vibration, but rather 

 to the vibration of the particles \ for whenever the two 

 vibrations can be feparated, it is found that the for- 

 mer produces no found; but when the general vibra- 

 tion is accompanied with a, vibration of the particles, 

 the latter it is that regulates the duration, the force, 

 and th modulation of the found: if, on the contrary, 

 thefc vibrations are flopped or interrupted by touch- 

 ing the fonorous body, the found immediately ceafes. 

 On this account clock-makers attach to the hammer* 

 which (rrikes the bell of the clock, a (hull fpring* 

 which elevates it again the moment it has ft ruck, ard 

 prevents it from remaining upon the bell, which would 

 conficlerably deaden or deilroy the found. 



Acute founds are produced, when the vibrations of 

 the founding body are more frequent; grave or deep 

 founds, when they are lefs fo: no medium between 

 acqte and grave founds can be found. Sonorous bo- 

 dies arc faid to be in unifon when they vibrate with 

 the fame frequency; when one vibrates twice as fait 

 as the other, they differ by an octave; and other ratios* 

 with refpect to the quicknefs of vibration, are diflin- 

 guilhed by other names. Cords, which are fhort and 

 tightly ftr etched, produce acute founds; thofe which 

 are long and lax, grave founds. 



The motion or vibration of bodies at a diftance 

 from us would not affect our fenfe of hearing without 



Mem. de 1'Acad. 1716, p. 264. 



the 



