THE WORLD'S WONDERS. 27 



from the ocean, in the form of vapor. At least three-fourths of 

 this immense volume is raised within the tropics, and a great 

 part falls beyond them. If the extent of the tropical ocean 

 were diminished by half, there is no part, perhaps, of the tem- 

 perate zones which would not be parched by excessive drought, 

 and hardly a river but whose bed would be a dry ravine. 



The water which fills the great lakes of North America and, 

 thundering down the cataract of Niagara, finds its way through 

 the St. Lawrence River into the ocean almost on the verge of the 

 Polar World, only a few weeks before, perhaps, laved the coral 

 reefs of the tropical seas. 



If any considerable part of the tropical ocean were converted 

 into land, the heat of the Torrid Zone would become so greatly 

 increased that no animal life, such as now exists, could endure 

 it ; and, as the vegetation of a climate is adapted to the prevail- 

 ing temperature, the trees and plants which now flourish would 

 become extinct. Water, in being converted into a gaseous form 

 by the process of evaporation, absorbs heat from surrounding 

 objects, or, as we may say, produces cold. Thus the burning 

 rays of a vertical sun, pouring down upon the ocean, in a measure 

 quenc/i themselves. The same rays, which, falling upon the 

 ocean, never raise the water beyond a grateful temperature, falling 

 upon the land produce an intolerable heat. 



The great extent of the tropical seas is the cause of those 

 mighty ocean currents which sweep from the equatorial to the 

 polar regions. Cool as the waters of the tropics are, they are 

 warm when compared with the other parts of the ocean. The 

 water thus heated becomes specifically lighter than that of colder 

 regions, is lifted up, and, in obedience to the laws of gravitation, 

 *'uns off in both directions towards the poles. There, having 

 become cooled, the salt waters are heavier than the comparatively 

 fresh ones of the polai regions, and sinking beneath them, return 

 in an undercurrent to their starting-place. 



This great equatorial current, or rather series of currents, is 

 the marvel of physical geography. Let us follow that of the 

 Atlantic in its long career. Starting on the line of the equator, 



