MAGNETISM AND CIRCULATION OF THE ATMOSPHERE. 133 



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. 



334. These army observations, as expressed in Blodget's maps, 

 reveal other interesting features, also, touching the physical geog- 

 raphy 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°. 



335. I have drawn similar lines on the authority of Dove and 

 Johnston (A. K., of Edinburgh), across Europe and Asia, for the 

 sake of comparison. The isotherm of 65° skirts the northern lim- 

 its of the sugar-cane, and separates the inter-tropical from the extra- 

 tropical plants and productions. I have drawn these tv/o lines 

 across America in order to give a practical exemplification of the 

 nature of the advantages which the industrial pursuits and the 

 political economy of the country would derive by the systematic 

 extension of our meteorological observations from the sea to the 

 land. These lines show how much we err when we reckon cli- 

 mates according to parallels of latitude. The space that these 

 two isotherms of 45° and 65° comprehend between the Missis- 

 sippi and the Rocky jMountains, owing to the singular effect of 

 those mountains upon tlie climate, is larger than the space they 

 comprehend between the Mississippi and the Atlantic. 



Hyetographically it is also different, being dryer, and possessing 

 a purer atmosphere. In this grand range of climate between the 

 meridians of 100° and 110° W., the amount of precipitation is 

 just about one half of w^hat it is between those two isotherms east 

 of the Mississippi. In this new country west of it, winter is the 

 dry, and spring the rainy season. It includes the climates of the 

 Caspian Sea, which Humboldt" regards as the most salubrious in 

 the world, and where he found the most delicious fruits that he 

 saw during his travels. Such was the purity of the air there, 

 that polished steel would not tarnish even by night exposure. 

 These two isotherms, with the remarkable loop which they make 

 to the northwest, beyond the Mississippi, embrace the most choice 

 climates for the olive, the vine, and the poppy ; for the melon, the 

 peach, and almond. The finest of wool may be grown there, and 

 the potato, with hemp, tobacco, maize, and all the cereals, may be 



