EASTING OF THE TRADE-WINDS, ETC. 167 



borate the suggestion as to the crossing 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 from which the sources of the Mississippi, 

 the St Lawrence, and the other gi'eat rivers of our hemisphere 

 could be supplied. A regular series of meteorological obser- 

 vations has been carried on at the military posts of the United 

 States since 1819. Eain maps of the whole country* have been 

 prepared from these observations by Mr. Lorin Blodget at the 

 Surgeon-General's office, and under the direction of Dr. Cool- 

 edge, U.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 dry season in California and 

 Oregon is the wet season in the Mississippi Valley. The winds 

 coming from the south-west, and striking upon the coast of 

 California and Oregon in winter, precipitate there copiously. 

 They then pass over the mountains robbed in part of their 

 moisture. Of course, after w^atering the Pacific shores, they 

 have not as much vapour to make rains of, specially for the 

 upper Mississippi Valley, as they had in the summer-time, when 

 they dispensed their moisture, in the shape of rains, most 

 sparingly upon the Pacific coasts. According to these views, 

 the dry season on the Pacific slopes should be the "wet, especially 

 in the upper Mississippi Valley, and vice versa. Blodget's maps 

 show that such is actually the case. Meteorological observations 

 in the "Eed Eiver country" and other parts of British America 

 would throw farther light and give farther confirmation, I doubt 

 not, both to these views and to this interesting question. These 

 army observations, as expressed in Blodget's maps, reveal other 

 interesting features, also, touching the physical geography of 

 the country. I allude to the two isothermal lines 45° and 65° 

 (Plate VIII.), which include between them all places that have 

 a mean annual temperature between 45° and (jo^. I have drawn, 

 for the sake of comparison, similar lines on the authority of 

 Dove and Johnston (A. K., of Edinburgh), across Europe and 

 Asia. The isotherm of 65° skirts the northern limits of the 

 sugar-cane, and separates the intertropical from the extra-trojDical 

 plants and productions. I have drawn these two lines across 

 America in order to give a practical exemplification of the nature 

 of the advantages w^hich the industrial pursuits and the political 

 * See Amiy Meteorological Observations, published 1855. 



