Dr. Forry on the Climate of the United States, i^'c. 45 



As the mean annual temperature of Fort Vancouver is 51° -75, 

 and that of the assumed point between Fort Gibson and Jeffer- 

 son Barracks is about 61°, it follows that the hnes of equal win- 

 ter cut isothermal lines which differ more than 9° of Fahrenheit. 

 (See Plate II.) 



In Europe a greater deviation from the terrestrial parallels is 

 caused by the inflections of the isocheimal than by the isothermal 

 lines ; for while two points having the same winter temperature 

 may differ as much as 11° in latitude, a difference of not more 

 than 5° is found between any two places having an equal annual 

 temperature — disparities which increase as the eastern coast of 

 Asia is approached. In the United States the same law obtains; 

 for between the isothermal line of Fort Vancouver and the same 

 in the Atlantic region, the difference is only 4° of latitude. (See 

 Plate II.) 



The isotheral curves or lines of equal summer follow a di- 

 rection opposite to that of the isocheimal lines. The region 

 about Moscow and that about the mouth of the Loire, in France, 

 notwithstanding differing 11° in latitude, present the same sum- 

 mer temperature. Although this result, as regards difference of 

 latitude, is not discovered in the United States, yet the most ex- 

 traordinary results in this respect have been demonstrated on the 

 same parallel running from the Atlantic through the great lakes. 

 In the United States, the heats of summer are every where in- 

 tense. At Fort Snelling, notwithstanding the isocheimal line is 

 54° lower than at Key West, the isotheral is only 8° lower. 

 (See Plate II.) At Fort Vancouver, the mean summer temper- 

 ature is 2° or 3° higher than on the same parallel in the region 

 of the Atlantic and the great lakes, and about 7° lower than in 

 the excessive climates of the same region. In tracing an isother- 

 mal line around the globe, we find that the same causes which, 

 on the Atlantic coast of North America and in the north of China, 

 depress the curves of equal annual heat, tend to elevate the iso- 

 theral curves or lines of equal summer. Thus, in following the 

 isothermal line of 51° around the globe, and adding the indica- 

 tions of the mean temperature of summer and winter at its sum- 

 mits and depressions, we find it marked in England, gg^rgo' ^^ 

 Hungary, ^^J ; in China, ?|^ ; in western America, at Fort 



