CLOUDS AND RIVERS, ICE AND GLACIERS. 23 



balance each, other. Then place for a moment the 

 flame of a spirit-lamp beneath the open base of one of 

 them ; the hot air ascends from the lamp and instantly 

 tosses upwards the cone above it. 



G8. Into an inverted glass shade introduce a little 

 smoke. Let the air come to rest, and then simply 

 place your hand at the open mouth of the shade. 

 Mimic hurricanes are produced by the air warmed by 

 the hand, which are strikingly visible when the smoke 

 is illuminated by a strong light. 



69. The heating of the tropical air by the sun is 

 indirect. The solar beams have scarcely any power to 

 heat the air through which they pass ; but they heat 

 the land and ocean, and these communicate their heat 

 to the air in contact with them. The air and vapour 

 start upwards charged with the heat thus communi- 

 cated. 



§ 7. Tropical Rains. 



70. But long before the air and vapour from the 

 equator reach the poles, precipitation occurs. Wherever 

 a humid warm wind mixes with a cold dry one, rain 

 falls. Indeed the heaviest rains occur at those places 

 where the sun is vertically overhead. We must enquire 

 a little more closely into their origin. 



71. Fill a bladder about two-thirds fall of air at the 

 sea level, and take it to the summit of Mont Blanc. As 

 you ascend, the bladder becomes more and more dis 



