24 THE FORMS OF WATEK IN 



tended; at the top of the mountain it is fully distended, 

 and has evidently to bear a pressure from within. 

 Returning to the sea level you find that the tightness 

 disappears, the bladder finally appearing as flaccid as 

 at first. 



72. The reason is plain. At the sea level the air 

 within the bladder has to bear the pressure of the 

 whole atmosphere, being thereby squeezed into a com- 

 paratively small volume. In ascending the mountain, 

 you leave more and more of the atmosphere behind ; 

 the pressure becomes less and less, and by its expansive 

 force the air within the bladder swells as the outside 

 pressure is diminished. At the top of the mountain 

 the expansion is quite sufficient to render the bladder 

 tight, the pressure within being then actually greater 

 than the pressure without. By means of an air-pump 

 we can show the expansion of a balloon partly filled 

 with air, when the external pressure has been in part 

 removed. 



73. But why do I dwell upon this ? Simply to make 

 plain to you that the unconfined air, heated at the earth's 

 surface, and ascending by its lightness, must expand 

 more and more the higher it rises in the atmosphere. 



74. And now I have to introduce to you a new fact, 

 towards the statement of which I have been working 

 for some time. It is this : — The ascending air is chilled 

 by its expansion. Indeed this chilling is one source of 

 the coldness of the higher atmospheric regions. And 



