OF NATURAL HISTORY. 169 



To enable us to underftand the manner in which founds are pro- 

 pagated through the air, philofophers have had recourfe to the un- 

 dulations produced by a ftone thrown into a pond of ftagnating wa- 

 ter. Thefe undulations affume the form of circular waves, which 

 fucceffively proceed from the place where the ftone ftruck the water, 

 as from a center, and continually dilate, and become greater and 

 greater as they recede from that center, till they reach the banks of 

 the water, where they either vanifli or are refleded. Now, as air 

 is likewife a fluid, fimilar undulations, though to us invifible, are 

 produced in it by the vibrations of fonorous bodies, and are alfo 

 propagated to great diftances in fucceffive waves or rings. Thefe 

 undulations of the air, when they come into contadt with our or- 

 gans of hearing, make fuch a tremulous impreffion upon them as 

 excites in our minds the fenfation of found. This analogy, though 

 not altogether perfed, is fufficient to illuftrate thofe invifible mo- 

 tions of the air by which founds are conveyed from one place to 

 another, and to give an idea of echoes, or refleded undulations of 

 that fluid. 



The celerity with which founds, or undulations of air, move, has 

 been exa£tly computed. All founds, whether acute or grave, ftrong 

 or weak, move at the rate of 1 142 feet in a fecond of time. Hence, 

 whenever the lightning of thunder, or the fire of artillery, are feen, 

 their adual diftances from the obferver may be eafily afcertained by 

 the vibrations of a pendulum. This velocity, it is true, may be a 

 little augmented or diminifhed by favourable or by contrary winds, 

 and by heat or cold. But the difference, even in high winds, is fo 

 trifling, that, for any ufeful purpofe, it fcarcely merits attention. 



Infants hear bluntly, becaufe the bones of their ears are foft and 



cartilaginous ; and, of courfe, the tremulations excited in them by 



the motions of '•the air are comparatively weak. Young children, 



I t Y accordingly, 



