THE CLIMATE OF THE LAKE REGION. 213 



we know to be imprisoned within the earth. When the 

 temperature of the land sinks to 20 or 30 below zero, 

 that of Lake Michigan is 60 or 70 higher ; and the 

 vapor which ascends from its surface is the literal si- 

 militude of the steam rising from a kettle heated over 

 a domestic fire. 



Two local factors enter into the rational explanation 

 of the peculiarities of the climate of the lake region. 

 One of these is the equable temperature of great bodies 

 of water, the other is the prevailing direction of the wind. 

 To illusti-ate the latter more precisely than has been done, 

 let us consider the peninsula of Michigan. Were the at- 

 mosphere perpetually calm, the contiguous land and super- 

 incumbent atmosphere would only be very feebly warmed 

 during winter by direct radiation from the lake; and this 

 effect would be more than counterbalanced by a perpetual 

 land breeze as long as the lake should remain warmer 

 than the land. But the general atmosphere is always in 

 motion. Warmed in winter, while passing over the sur- 

 face of the lake, it conveys some part of the lake- warmth 

 to the land, and the rigor of the cold becomes amelio- 

 rated, on the principle of a hot-air furnace. As the wind 

 by turns moves from all directions, the lake exerts some 

 warming influence on all the surrounding land. This is 

 illustrated by the isothermal lines for the cold months, 

 which are bent northward on approaching the lake from 

 either side. Evidently that side of the lake which re- 

 reives most wind from the lake-surface will be most im- 

 pressed by the lake-influence. Now it happens that the 

 Michigan side of Lake Michigan receives most lake winds 

 during the cold season, because, as is well known, the 

 cold winds of the region approach from a westerly direc- 



