50 PHYSIOGRAPHIC GEOLOGY. 



An increase of density from an addition of carbonic acid would increase 

 proportionally the amount of heat absorbed, the absorptive power of this 

 gas being 90 times that of the atmosphere. The presence of aqueous vapor 

 also increases the absorptive power. 



The winds are a means of distributing heat and moisture, and thus tend 

 to equalize the temperature of the globe. But, at the same time, they make 

 local areas of extreme heat and cold, of extreme precipitation and dryness. 



(2) Surface movements of the winds. — For theoretical views and details 

 on this subject, reference should be made to meteorological treatises, the 

 remarks here being confined to a few general facts. They illustrate well 

 the dependence of effects on the east-west characteristics of the earth derived 

 from its rotation. 



In general, the courses of the winds are nearly coincident with those of 

 the great oceanic currents, and it is held by many that the winds are the 

 motive power of the currents. In the tropics, the prevailing course of the 

 winds is from the eastward (these winds being called the trades), and in 

 the higher temperate latitudes from the westward. There is a tendency to 

 calms (1) along the equator; (2) in mid ocean between the parallels of 25° 

 and 35° ; and (3) about the poles ; but the equatorial area of calms is some- 

 times in part a region of a counter-current. 



The trades strike the east side of the continent, and then, bending away 

 from the equator, curve around to become the westerly winds. And the 

 reverse is true for the westerly winds ; but where they strike the west side 

 of a continent, only part of the wind may be deflected toward the equator 

 and the rest curve around poleward ; and when so, the former gradually 

 warms up, since it goes toward warmer regions, and the latter loses heat 

 because going into higher latitudes. These two parts vary in their rela- 

 tive amounts or force according to the trends of the coast-lines or form of 

 the land. 



The Indian Ocean makes an exception under the system, because the 

 region there existing to the north of the equator is occupied by a continental 

 mass, Asia, which pushes the circuit to the south, the winds that blow there 

 from the eastward corresponding to the trades of the other oceans. 



(3) Distribution of moisture. — The capacity of air for moisture — that is, 

 its power of taking up moisture without a loss of transparency — varies with 

 the temperature. When saturated, a loss of heat causes condensation, and 

 thence, mist, clouds, rain. On the contrary, an increase of heat increases 

 capacity for moisture, and the wind, instead of dropping moisture, gathers 

 moisture from the surface it passes over. 



In the south Pacific the wind from the west is a cold wind, charged with 



•moisture derived from the ocean; as it divides on striking South America it 



becomes in its northern branch desert-making, in its southern, rain-giving. 



The branch going north passes into regions of increasing warmth, and the 



vhi 1 gathers up the moisture beneath and makes the desert region of 



