74 WRITINGS OF JOSEPH HENRY. [1855- 



temperature of a place is nearly equal to 70° instead of 60°, 

 since the curve of 70° reaches almost as far north. The 

 curve of the mean temperature of 60°, as has been stated, 

 terminates on the shores of the Pacific, at about latitude 34° ; 

 whereas, on the Atlantic, it commences at about 37°, indi- 

 cating a lower temperature along the 35th parallel of lati- 

 tude on the Pacific than on the Atlantic shore. The next is 

 the curve of 70°. This commences in about latitude 28° on 

 the coast of Florida, passes through New Orleans, and thence 

 to a point on the Pacific in the latitude of 30°. It presents 

 an upward curvature in that portion which passes through 

 the Gulf, indicating that New Orleans is warmer than a cor- 

 responding place on the Atlantic, or on the shores of Texas. 

 It thence curves rapidly to the north, though indicating the 

 greatest temperature near the eastern edge of the mountain 

 system. It terminates on the Pacific at a point at least two 

 degrees higher than its point of commencement on the At- 

 lantic, thereby indicating that along the 30th parallel the 

 mean temperature is a little greater on the east than on the 

 west side of the continent. It should be constantly borne in 

 mind, that the temperatures in these descriptions are those 

 which would be exhibited were the mountain system of the 

 country removed and the whole reduced to the level of the 

 ocean. This system of lines therefore exhibits the extraor- 

 dinary fact that eliminating the effect due to elevation, there 

 remains a cause of a remarkable degree of abnormal heating 

 beyond that due merely to the latitude of the place. In 

 other words, that at every point within the mountain system, 

 whatever may be its elevation, the temperature is far above 

 that of the same elevation of a point in free space having the 

 same latitude, when compared with the eastern and western 

 coast. 



The broken lines indicate the temperatures of summer. 

 The first of these given on the map is that of 70° and com- 

 mences near Long Island, ascends rapidly towards the north, 

 and then descends towards the large lakes, passing through 

 Lake Erie; it reaches its greatest northern declination at 

 about the 110th meridian, and thence turning nearly paral- 



