PNEUMATICS. 281 



BUTTON'S Mathematical Dictionary, article Gunnery, 

 p. 567. Both these propositions are empirical, or 

 deduced solely from experiment. 



Besides the works already quoted, see the article Re- 

 Distance, by Professor ROB i SON, in the Encyclopce- 

 dia Britannica. 



SECT. II. 



AIR AS THE VEHICLE OF SOUND. 



Vibration of Sonorous Bodies. 



377. -I HE bodies which we consider as sonor- 

 ous or as causes of the sensation of sound, at the 

 time when they produce that sensation, are observ- 

 ed to be in a state of tremor or vibration. 



This is exemplified in a bell, and in a drinking glass, 

 when sound is produced by rubbing its edge with 

 a wet finger, in a musical string, &c. The latter 

 affords the simplest case of this fact, and that in 

 which the laws of the vibration are most easily inves- 

 tigated. 



378. If a musical string stretched between two 

 fixed points, be struck any where between those 



points, 



