768 



HISTORY OF THE SEA. 



of its waters, are very important in the study of the influence 

 of the sea upon the land. By these the circulation of the 

 waters of the globe is carried on. The warm water of the 

 equatorial regions seeking the poles, and a counter movement 

 from the poles to the equator, is established. By their mean* 

 a constant mingling of the waters on the face of the whole eartfc 

 is maintained, and the wonderful similarity of its different por- 

 tions, in their composition, appearance, and the substances 

 held in solution, is produced. The chief causes of this grand 

 circulation are found in the heat of the sun and in the rotation 

 of the earth upon its axis. By the evaporation of the waters 

 in the tropics the surface of that portion of the ocean is esti- 

 mated to be lowered more thau fourteen feet yearly. By this 

 means not only is the atmosphere provided with its store oi 

 vapor, to be dispensed in rain upon the land, and thus returned 

 again to the sea, but this lowering of the surface of the ocean, 

 bi one part, leads to the curreuts flowing from the others to 

 restore the equilibrium. The same cause leading also to the 

 o irculation of the atmosphere, produces the trade winds, which 

 ad in producing the currents in the ocean. 



Now that by study and observation mankind have arrived 

 a' the conception of the form of the earth, at its general fea- 

 1 1 ires, and can, in idea, grasp it as a whole, the opportunity is 

 .vepared for the methodical study of its parts, and their rela- 

 L/on to each other; and this is the subject which for the first 

 time in the history of mankind is offered to the physical geog- 

 rapher, w ith the certainty that none of his observations can be 

 lost, but that they all are important, and can each be referred 

 to its proper place. Another movement of the ocean is the 

 tides. To the ancients, unacquainted with the form of the 

 earth, its position in space, or its relations with the other bodies 

 of the solar system, the tides were naturally inexplicable. It 

 has been possible only in modern times to attempt their expla- 

 nation. Kepler first indicated the course to be followed; and 



