76 WRITINGS OF JOSEPH HENRY. [1855- 



of 50° is also similar to that of the last ; also the line of 

 60°, which indicates in the Gulf of Mexico a lower degree 

 of temperature in winter than exists on the Atlantic or 

 Pacific coasts. In examining these winter lines attentively, 

 it will be seen that the rise is not uniform from the 95th to 

 the 105th degree, but the bend is most sudden about the 

 103d ; which is probably caused by the occasional descent 

 along this region of the polar winds to the Gulf of Mexico. 



It has been stated that in reducing the lines to the level 

 of the sea, 333 feet of elevation have been taken for each 

 degree of Fahrenheit's scale. Therefore the actual tempera- 

 ture of any part of the United States may be readily deter- 

 mined, provided its elevation above the sea is known, by 

 subtracting from the temperature given on the chart as many 

 degrees as there are spaces of 333 feet in the elevation. Let 

 us take, for example, the junction of the Kansas with the 

 Missouri river, on the 95th meridian. This point, it will 

 be seen by inspecting the map, is midway between the mean 

 isothermal lines of 50° and 60°, and its temperature will 

 therefore be approximately 55°. It has an elevation of 

 about a thousand feet, which will give three degrees for the 

 reduction; and hence its temperature will be about 52°. 



On a little reflection it will be clear that it would have 

 been impossible to draw these lines on the uneven surface 

 of the earth. The variation of temperature due to height 

 would mask that due to latitude and other climatological 

 causes. For example, a greater elevation of mountain peaks 

 at the south would represent a colder local temperature than 

 regions further north, would entirely hide from view the re- 

 sults which are due exclusively to the peculiarities of con- 

 formation of the country, and would give no means of com- 

 parison. 



Winds of North America. — We have said that the whole 

 mountain system of the western portion of the United States 

 presents a remarkable abnormal elevation of temperature 

 above the eastern and middle portions of the continent, and 

 the question naturally presses itself upon us as to the cause 

 of this surprising difference. The simple stateraant that the 



