Page:Physical Geography of the Sea and its Meteorology.djvu/193

Rh 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 great rivers of our hemisphere could be supplied. A regular series of meteorological observations has been carried on at the military 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 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 watering 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 versá. 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 include between them all places that have a mean annual temperature between 45° and 65°. 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-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 industrial pursuits and the political