WRITINGS OP JOSEPH HENRY. 



[1855- 



along the northwest coast of the North American continent, 

 to receive a new accession of heat and be again conveyed to 

 the north. The total result of this circulation together with 

 those of lesser influence in the northern hemisphere, is 

 shown in the annexed polar projection, in which the series of 

 irregular lines, marked 50°, 32°, 16°, and 0°, indicate the 

 mean annual temperature of the points through which they 

 pass, and are called the yearly isothermal lines, or lines of 

 equal heat. 



The darker line, marked 32°, indicates the boundary of 

 the region within which the average temperature is below 

 the freezing point. It will be seen at a glance that, instead 

 of being circular in its outline, it has the form of an irreg- 

 ular elongated ellipse, the greater diameter of which is across 

 the pole, from the southern extremity of Hudson's Bay to 



