162 THE PHYSICAL GEOGRAPHY OF THE SEA. 



ries of meteorological observations has been carried on at the mil- 

 itary posts of the United States since 1819. Rain 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. Cooledge, 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 Val- 

 ley. The winds coming from the southwest, and striking upon 

 the coasts of California and Oregon in winter, precipitate there 

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

 their moisture. Of course, after watering the Pacific shores, they 

 have not as much vapor to make rains of, especially for the upper 

 Mississippi Yalley, 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 Mis- 

 sissippi Valley, and vice versa. Blodget's maps show that such is 

 actually the case. Meteorological observations in the " Red River 

 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 in- 

 clude between them all places that have a mean annual tempera- 

 ture between 45° and 65°. I have drawn, for the sake of com- 

 parison, 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 in- 

 tertropical from the extra-tropical plants and productions. I have 

 drawn these two lines across America in order to give a practical 

 exemplification of the nature of the advantages which the indus- 

 trial pursuits and the political economy of the country would de- 

 rive by the systematic extension of our meteorological observa- 

 tions from the sea to the land. These lines show how much we 

 err when we reckon climates according to parallels of latitude. 

 The space that these two isotherms of 45° and 65° comprehend 

 * See Army Meteorological Observations, published 1855. 



