LAWS OF HEAT. WATER. 81 



mixture, reduce tjie whole to a temperature at 

 least as low as that of the surface. And this 

 equalization of temperature by means of such cur- 

 rents, is an operation of a much more rapid nature 

 than the slow motion of conduction by which 

 heat creeps through a solid body. Hence, alter- 

 nations of heat and cold, as day and night, sum- 

 mer and winter, produce in water inequalities of 

 temperature much smaller than those which occur 

 in a solid body. The heat communicated is less, 

 for transparent fluids imbibe heat very slowly ; 

 and the cold impressed on the surface is soon dif- 

 fused, through the mass by internal circulation. 



Hence it follows that the ocean, which covers 

 so large a portion of the earth, and affects the 

 temperature of the whole surface by its influence, 

 produces the effect of making the alternations of 

 heat and cold much less violent than they would 

 be if this covering were removed. The different 

 temperatures of its upper and lower parts produce 

 a current which draws the sea, and by means 

 of the sea, the air, towards the mean tempera- 

 ture. And this kind of circulation is produced, 

 not only between the upper and lower parts, but 

 also between distant tracts of the ocean. The 

 great Gulf Stream which rushes out of the gulf 

 of Mexico, and runs across the Atlantic to the 

 western shores of Europe, carries with it a portion 

 of the tropical heat into the northern regions : and 

 the returning current which descends along the 



w. 5 G 



