-1859] WRITINGS OF JOSEPH HENRY, 73 



eating a great increase of temperature in this region beyond 

 that of the eastern and middle portion. Let us first con- 

 sider the dark Hues representing the mean temperature of 

 the year. These, and indeed all the lines, are given for 

 each ten degrees of Fahrenheit. Too much complication 

 would be introduced were lines drawn for intermediate de- 

 grees on so small a map, though such lines have been pro- 

 jected on a larger one from which this has been reduced. 



The first dark line, beginning at the top of the map, is 

 that of the mean temperature of 40°. It commences near the 

 northern part of Nova Scotia, passes through Canada and 

 the middle of Lake Superior, slightly diverging from paral- 

 lelism with the line of 45° of latitude until about the 95th 

 meridian, when it more rapidly curves northward and leaves 

 the United States for the British Possessions at about the 

 103d meridian, passing out at the top of the map at the 110th. 

 The next line of mean temperature is that of 50°. It com- 

 mences a little south of Nantucket, passes almost directly 

 west, nearly parallel to the line of the 40th degree of north 

 latitude, to about the 95th meridian of west longitude, 

 whence it curves more rapidly to the north, meeting the 

 coast of the Pacific in about the 48th degree of north latitude, 

 near Puget's Sound. It thus exhibits the fact that the mean 

 temperature of a point near Rhode Island is the same as that 

 of a point on the Pacific, at least six degrees of latitude 

 further north. The next line of mean temperature for the 

 year, given on the map, is that of 60° ; commencing near the 

 mouth of Chesapeake Bay it inclines a little downward 

 toward the 35th parallel of latitude until the meridian of 

 about 98°, whence it rapidly ascends to the north, gains its 

 greatest altitude at the 115th meridian, thence gradually de- 

 clines southward to about the 125th, and thence, with a re- 

 markably short bend, it passes parallel to the coast to about 

 the latitude of 34°. By comparing the course of this line 

 with that of the 35th parallel, it will be seen that the mean 

 temperature is a little less near the Mississippi river than it 

 is on the seaboard ; but that in the great mountain system, 

 in the same latitude as the mouth of the Chesapeake, the 



