THE NON-LIVING WORLD 149 



sion and greater density of its lower strata, owing to the 

 pressure on them of the air above. The atmosphere is 

 supposed to form a layer over the earth's surface of 

 probably between forty and fifty miles in thickness. At 

 the sea level, 100 cubic inches of air at 60 F. weigh 

 about 30 grains, but at an elevation of 20,000 feet the 

 pressure on them would be diminished one-half; for there 

 is as much air in the lower 3 \ miles of the atmosphere as 

 in all the superior portion. The weight of the atmosphere 

 at any spot is tested by the barometer.* 



As has been pointed out,f the sun's rays hardly at 

 all raise the temperature of the atmosphere ; it is 

 warmed by the earth's surface, which is itself heated 

 by the rays passing to it through the air. Then the 

 superincumbent mass of air, becomes gradually warmed 

 by convection,! and so currents upwards and down- 

 wards are produced. Now the earth's surface is very 

 unequally heated, that of tropical lands being vastly 

 hotter than at the arctic regions, and the air being 

 most expanded by the warmer surface will rise to a 

 greater elevation above it, than elsewhere. 



But no fluid, either liquid or aeriform, can heap itself 

 up. It must overflow, and then it will immediately 

 be pressed upon by a rush of colder and therefore 

 heavier air. Thus it is winds are produced, which 

 are of the greatest utility in lessening extremes of tem- 

 perature. There is a constant rise of warm air from the 

 hottest regions of the earth, which then flows north- 

 wards and southwards at a high altitude towards the 

 poles, while lower currents of cold air rush simul- 

 taneously towards the equator. This is, on a gigantic 



* See ante, p. 82. t See ante, p. 98. 



J See ante, p. 95. 



