THE ATMOSPHERE. 91 



of square miles, thus leaving an evaporating surface of atout twen- 

 t j-five millions of square miles in the northern against about sev- 

 enty-five millions in the southern hemisphere. 



183. According to the hypothesis, illustrated by Plate I., p. 75, 

 as to the circulation of the atmosphere, it is these northeast trade- 

 winds that take up and carry over, after they rise up in the belt 

 of equatorial calms, the vapors which make the rains that feed the 

 rivers in the extra-tropical regions of the southern hemisphere. 



184. Upon this supposition, then, two thirds only of the north- 

 east trade-winds are fully charged with moisture, and only two 

 thirds of the amount of rain that falls in the northern hemisphere 

 should fall in the southern, and this is just about the proportion 

 (§ 173) that observation gives. 



185. In like manner, the southeast trade-winds take up the va- 

 pors which make our rivers, and as they prevail to a much greater 

 extent at sea, and have exposed to their action about three times 

 as much ocean as the northeast trade-winds have, we might ex- 

 pect, according to this hypothesis, more rains in the northern — 

 and, consequently, more and larger rivers — than in the southern 

 hemisphere. A glance at Plate VIII. will show how very much 

 larger that part of the ocean over which the southeast trades pre- 

 vail is than that where the northeast trade-winds blow. 



186. This estimate as to the quantity of rain in the two hem- 

 ispheres is one which is not capable of verification by any more 

 than the rudest approximations ; for the greater extent of south- 

 east trades on one side, and of high mountains on the other, must 

 each of necessity, and independent of other agents, have their ef- 

 fects. Nevertheless, this estimate gives as close an approxima- 

 tion as we can make out from our data. 



187. The rainy seasons, how caused. — The calm and trade- 

 wind regions or belts move up and down the earth, annually, in 

 latitude nearly a thousand miles. In July and August the zone 

 of equatorial calms is found between 7° north and 12° north; 

 sometimes higher ; in March and April, between latitude 5° south 

 and 2° north. 



188. With this fact and these points of view before us, it is 

 easy to perceive why it is that we have a rainy season in Oregon, 



