THE ATMOSPHERE. 101 



thither by the winds, resides the chief source of the dynamical 

 power which gives them motion. In some aspects vapour is to 

 the winds what fuel is to the steam-engine : they carry it to the 

 equatorial calm belt; there it rises, entangling the air, and 

 carrying it up along with it as it goes. As it ascends it expands, 

 as it expands it grows cool ; and as it does this its vapour is 

 condensed, the latent heat of which is thus liberated ; this raises 

 the temperature of the upper air, causing it to be rarefied and to 

 ascend still higher. This increased rarefaction calls for in- 

 creased velocity on the part of the inpouring trade-winds below. 



262. The effect of the deserts upon the trade-winds. — Thus the 

 vapours uniting with the direct solar ray would, were there no 

 counteracting influences, cause the north-east and south-east 

 trade-winds to rush in with equal force. But there is on the 

 polar side of the north-east trade-winds an immense area of arid 

 plains for the heat of the solar ray to beat down upon, also an 

 area of immense precipitation. These two sources of heat hold 

 back the north-east trade-winds, as it were, and, when the two 

 are united, as they are in India, they are sufficient not only to 

 hold back the north-east trade-wind, but to reverse it, causing 

 the south-west monsoon to blow for half the year instead of the 

 north-east trade. 



2G3. Indications of a crossing at tJie calm belts. — We have, in this 

 difference as to strength and stability (§ 254) between the north- 

 east and south-east trade-winds, another link in the chain of 

 facts tending to show that there is a crossing of the winds at the 

 calm belts. The greatest amount of evaporation takes place in 

 the southern hemisphere, which is known by the simple circum- 

 stance that there is so much more sea-surface there. The 

 greatest quantity of rain falls in the northern hemisphere, as 

 both the rain-gauge and the rivers show. So likewise does the 

 thermometer ; for the vapour which affords this excess of pre- 

 cipitation brings the heat — the dynamical power — from the 

 southern hemisphere; this vapour transports the heat in the 

 upper regions from the equatorial cloud-ring to the calms of 

 Cancer, on the polar side of which it is liberated as the vapour 

 is precipitated, thus assisting to make the northern warmer than 

 the southern hemisphere. In those northern latitudes where the 

 precipitation of vapour* and liberation of heat take place, aerial 

 rarefaction is produced, and the air in the calm belt of Cancer, 

 Avhich is about to blow north-east trade, is turned back and 



