CLOUDS AND RIVERS, ICE AND GLACIERS. 25 



now fix your eye upon those mixed currents of air and 

 aqueous vapour which rise from the warm tropical ocean. 

 They start with plenty of heat to preserve the vapour 

 as vapour; but as they rise they come into regions 

 already chilled, and they are still further chilled by 

 their own expansion. The consequence might be fore- 

 seen. The load of vapour is in great part precipitated, 

 dense clouds are formed, their particles coalesce to 

 rain-drops, which descend daily in gushes so profuse 

 that the word 'torrential 3 is used to express the 

 copiousness of the rain-fall. I could show you this 

 chilling by expansion, and also the consequent precipi- 

 tation of clouds. 



75. Thus long before the air from the equator reaches 

 the poles its vapour is in great part removed from it, 

 having redescended to the earth as rain. Still a good 

 quantity of the vapour is carried forward, which yields 

 hail, rain, and snow in northern and southern lands. 



Illustrative Experiments. 



76. I have said that the air is chilled during its ex- 

 pansK in. Prove this, if you like, thus. With a con- 

 densng syringe, you can force air into an iron box 

 furnished with a stopcock, to which the syringe is 

 screwed. Do so till the density of the air within the 

 box is doubled or trebled. Immediately after this con- 

 densation, both the box and the air within it are warm, 

 and can be proved to be so by a proper thermometer. 



