THE CLIMATE OF THE LAKE KEGIOX. 207 



being 627 feet higher than the Atlantic, must experience 

 a diminution of temperature of nearly two degrees. At 

 the level of Lake Michigan, whose altitude is 587 feet, 

 the temperature should be one and three-fourths degrees 

 less than at the sea-level. As the mean height of the 

 lower peninsula of Michigan is about 750 feet above the 

 sea-level, its mean temperature is diminished two and 

 one-fourth degrees. 



Of all local influences affecting climate none are more 

 efficient or more interesting to study than the relations 

 of a locality to extensive continental areas, to oceanic 

 currents, and to large bodies of water. The ocean is the 

 great equalizer of temperatures. By a providential ar- 

 rangement, watery surfaces absorb and radiate solar heat 

 less rapidly than land surfaces. Continental areas, con- 

 sequently, become more heated in summer and in trop- 

 ical latitudes, and more refrigerated in winter and in 

 arctic latitudes, than the oceanic areas, in the same sea- 

 sons and latitudes. These unequal temperatures affect 

 unequally the superincumbent masses of atmospheric air. 

 From this source arise movements of the air, which, com- 

 bined with the rotation of the earth on its axis, generate 

 trade- winds and the other prevailing winds of different 

 reofions. Prevailins^ winds movine^ over the surface of 

 the sea set its waters in motion. Thus ocean currents 

 are established which, reflected northward and southward 

 by continental shores, serve to transfer tropical warmth 

 to the polar regions and polar cold to the tropical re- 

 gions. From these causes it happens that in tropical 

 latitudes the open sea is cooler than the land, while in 

 polar latitudes it is warmer than the land. In the tem- 

 perate zones the temperature of the sea exceeds that of the 



