160 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOPvOLOGY. 



calms, should not, after ascending, rather retimi to the south than 

 go on to the north ? Where and what is the agency by which its 

 course is decided ? Here I found circumstances which again in- 

 duced me to suppose it probable that it neither tm-ned back to the 

 south nor mingled with the air which came fi'om the regions of 

 the north-east trades, ascended, and then flowed indiscriminately 

 to the north or the south. But I saw reasons for supposing that 

 what came to the equatorial calms as the south-east trade-winds 

 continued to the north as an upper current, and that what had 

 come to the same zone as north-east trade-winds ascended and con- 

 tinued over into the southern hemisphere as an upper ciu-rent, 

 bomid for the calm zone of Capricorn. And these are the prin- 

 cipal reasons and conjectures upon which these suppositions were 

 based : At the seasons of the year when the area covered by the 

 south-east trade-winds is large, and when they are evaporating most 

 rapidly in the southern hemisphere, even up to the equator, the 

 most rain is falling in the northern. Therefore it is fair to sup- 

 pose that much of the vapom- which is taken up on that side of the 

 equator is precipitated on this. The evaporating smface in the 

 southern hemisphere is greater, much greater, than it is in the 

 northern ; still, all the gi^eat rivers are in the northern hemisphere, 

 the Amazon being regarded as common to both ; and this fact, as 

 far as it goes, tends to corroborate the suggestion as to the cross- 

 ing of the trade-winds at the equatorial calms. Taking the laws; 

 and rates of evaporation into consideration, I could find (Chapter 

 V.) no part of the ocean of the northern hemisphere fi^om which 

 the sources of the Mississippi, the St. Lawrence, and the other 

 great rivers of om^ hemisphere could be supplied. A regular 

 series of meteorological observations has been carried on at the mili- 

 taiy posts of the United States since 1819. Kain maps of the 

 whole country* have been prepared from these observations by 

 Mr. Lorin Blodget at the sm-geon general's office, and under the 

 direction of Dr. Cooledge, IT. S. A. These maps, as far as they 

 go, sustain these views in a remarkable manner, for they bring- 

 out facts in a most striking way to show that the diy season in 

 California and Oregon is the wet season in the J\rississippi Val- 

 ley. The winds coming from the south-west, and striking upon 

 the coast of California and Oregon in Vv^nter, precipitate there 

 copiously. They then pass over the momitains robbed in part of 

 their moistm^e. Of com^se, after watering the Pacific shores, they 

 * See Army Meteorological Observations, publislied 1835. 



