II. TO HIGHGATE. 33 



- 



ther when the vacuum is complete; but 

 on the re-admission of fresh air, they again 

 become audibJe. 



Thus the atmosphere, which surrounds 

 us on all sides, and which, to a superficial 

 observer, would appear to present an ob- 

 stacle to the propagation of sound, is ab- 

 solutely necessary to its transmission. But 

 how is that transmission produced ? It is 

 generally admitted to be effected by undu- 

 lations : but you must not suppose, my 

 children, that it is the gross air, which we 

 term wind, that experiences those tremors 

 and undulations, but the more pure and sub- 

 tile portion of the atmosphere. This fact 

 may easily be ascertained by placing our 

 ear to the side of a large bell when it is 

 struck ; in which position, we experience a 



violent commotion in the surrounding air ; 



t? * 



and yet the flame of a taper, placed in the 

 same situation, is not at all agitated, though 

 the slightest breath of wind is sufficient to 



I 



render it unsteady. 



But 



