THE ATMOSPHERE. 



83 



except that which is precipitated in the region of equatorial 

 calms. 



The South Seas, then, according to § 119, should supply mainly 

 the water for this engine, while the northern hemisphere condenses 

 it ; we should, therefore, have more rain in the northern hemi- 

 sphere. The rivers tell us that we have — at least on the land : 

 for the great water-courses of the globe, and half the fresh water 

 in the world, are found on our side of the equator. This fact 

 alone is strongly corroborative of this hypothesis. 



The rain gauge tells us also the same story. The yearly aver- 

 age of rain in the north temperate zone is, according to Johnston, 

 thirty-seven inches. He gives but twenty-six in the south tem- 

 perate. 



125. Moisture is never extracted from the air by subjecting it 

 from a low to a higher temperature, but the reverse. Thus, all the 

 air which comes loaded with moisture from the other hemisphere, 

 and is borne into this with the southeast trade-winds, travels in 

 the upper regions of the atmosphere (§ 100) until it reaches the 

 calms of Cancer : here it becomes the surface wind that prevails 

 from the southward and westward. As it goes north it grows 

 cooler, and the process of condensation commences. 



We may now liken it to the wet sponge, and the decrease of 

 temperature to the hand that squeezes that sponge. Finally reach- 

 ing the cold latitudes, all the moisture that a dew-point of zero, 

 and even far below, can extract, is wrung from it ; and this air 

 then commences " to return according to his circuits" as dry at- • 

 mosphere. And here we can quote Scripture again : " The north 

 wind driveth away rain." This is a meteorological fact of high 

 authority and great importance in the study of the circulation of 

 the atmosphere. 



126. By reasoning in this manner, we are led to the conclusion 

 that our rivers are supplied with their waters principally from the 

 trade-wind regions — the extra-tropical northern rivers from the 

 southern trades, and the extra-tropioal southern rivers from the 

 northern trade-winds, for the trade-winds are the evaporating 

 winds. 



Taking for our guide such faint glimmerings of light as we can 

 catch from these facts, and supposing these views to be correct, 

 then the saltest portion of the sea should be in the trade-wind re- 



