SOUND. 125 



than thirty in a second, they are perceived as 

 separate throbs, and not as a continued sound ; 

 and there is a certain limit of rapidity, beyond 

 which the vibrations become inaudible. This 

 limit is different to different ears, and we are 

 thus assured by one person's ear that there are 

 vibrations, though to that of another they do not 

 produce sound. How was the human ear adapted 

 so that its perception of vibrations as sounds 

 should fall within these limits? the very limits 

 within which the vibrations fall, which it most 

 concerns us to perceive : those of the human 

 voice for instance? How nicely are the organs 

 adjusted with regard to the most minute mecha- 

 nical motions of the elements ! 



CHAPTER XV. 

 The Atmosphere. 



WE have considered in succession a number of 

 the properties and operations of the atmosphere, 

 and have found them separately very curious. 

 But an additional interest belongs to the subject 

 when we consider them as combined. The 

 atmosphere under this point of view must appear 

 a contrivance of the most extraordinary kind. 

 To answer any of its purposes, to carry on any 

 of its processes, separately, requires peculiar 



