Chap. 10.] How Air transmits Sound. 443 



the medium of fome other body, which receives an 

 impulfe from this motion, and communicates the vi- 

 brations to our organs. Thus a hard blow upon an 

 anvil or upon a bell could not be heard by us, even at 

 a very fmall diftance, if there was not a medium be- 

 tween thofe objects and us capable of tranfmitting the 

 vibrations to our auditory nerves. Elaftic fluids are the 

 moft effective mediums for this purpofe, and confe- 

 quently the air is the moft common vehicle of found, 

 which is very eafily proved by ringing a bell under the 

 receiver of an air-pump, the found it affords being 

 found gradually to diminim as the air becomes ex- 

 haufted, till at length it ceafes to be heard at all. 

 That the air is capable of being agitated with great 

 force appears from the violent concufiions produced by 

 explofions of gunpowder, as well as from the power, 

 which fome perfons are known to poffefs, of breaking 

 drinking glaffes, by means of their voice, when found- 

 ed in unifon with the note which the glafs would have 

 produced when ftruck. The tremulous motion ex- 

 cited in the air by founding bodies has been fuppofed 

 analogous to the fucceffive rings which are produced 

 by disturbing the furface of water. This hypothe- 

 fis, however, was difproved by the obfervation that 

 founds, whether weak or loud, always travel with the 

 fame velocity, which does not hold true with refpedt 

 to the rings on the furface of water, fince thefe move 

 fafter or flower according to the force of the caufe 

 which excited them. 



Every found is rendered ftronger or weaker, and may 

 be heard at a greater or lefs diftance, according to the 

 denfity* or rarity of that elaftic fluid, by which it is 



* That fome degree of denfity is necefiary in a fluid, to enable, 

 it to convey founds, is evident from this faft, that light, which is 

 a fluid extremely rare, is totally d?fli:ute of this power. Tralte 

 $km. de Pkjfiiue, torn. ii. p. rfo. 



propagated. 



