CLOUDS AND RIVERS, ICE AND GLACIERS. 19 



the frost, and melts it. Hence the wisdom of darkening, 

 instead of the flask itself, the mixture within the flask. 



55. This experiment proves to demonstration the 

 statement in paragraph (36) : that it is the dark waves 

 of the sun that melt the mountain snow and ice, and 

 originate all the rivers derived from glaciers. 



There are writers who seem to regard science as an 

 aggregate of facts, and hence doubt its efficacy as an 

 exercise of the reasoning powers. But all that 1 have 

 here taught you is the result of reason, taking its stand, 

 however, upon the sure basis of observation and experi- 

 ment. And this is the spirit in which our further 

 studies are to be pursued. 



§ 6. Oceanic Distillation. 



56. The sun, you know, is never exactly overhead in 

 England. But at the equator, and within certain li- 

 mits north and south of it, the sun at certain periods of 

 the year is directly overhead at noon. These limits are 

 called the Tropics of Cancer and of Capricorn. Upon 

 the belt comprised between these two circles the sun's 

 rays fall with their mightiest power; for here they 

 shoot directly downwards, and heat both earth and sea 

 more than when they strike slantingly. 



57. When the vertical sunbeams strike the land they 

 beat it, and the air in contact with the hot soil becomes 

 heated in turn. But when heated the air expands, and 

 when it expands it becomes lighter. This lighter air 



