2^0 THE PHYSICAL GEOGRAPHY OF THE SEA. 



are also different from those of the high seas. " The water which 

 evaporates from the sea is nearly pure, containing but very minute 

 traces of salts. Falling as rain upon the land, it washes the soil, 

 percolates through the rocky layers, and becomes charged with 

 saline substances, which are borne seaward by the returning cur- 

 rents. The ocean, therefore, is the great depository of every thing 

 that water can dissolve and carry down from the surface of the 

 continents ; and, as there is no channel for their escape, they of 

 course consequently accumulate."* They would constantly ac» 

 cumulate, as this very shrewd author remarks, were it not for the 

 shells and insects of the sea and other agents mentioned. 



467. How, therefore, shall we account for this sameness of com= 

 A general system of pouud, this structurc of coral (§ 465), this stability 



circulation required ^ • ^ t n ' i i ^ • 



for the ocean. as to animal life m the sea, but upon the supposi- 



tion of a general system of circulation in the ocean, by which, in 

 process of time, water from one part is conveyed to another part 

 the most remote, and by which a general interchange and com- 

 mingling of the waters take place ? In like manner, the constitu- 

 ents of the atmosphere, whether it be analyzed at the equator or 

 the poles, are the same. By cutting off and shutting up from the 

 general channels of circulation any portion of sea water, as in the 

 Dead Sea, or of atmospheric air, as in mines or wells, we can easi- 

 ly fill either with gases or other matter that shall- very much af- 

 fect its character, or alter the proportion of its ingredients, and 

 affect the health of its inhabitants ; but in the open sea or open 

 air we can do no such thing. 



468. The principal agents that are supposed to be concerned in 

 Dynamical agents, giviug circulatiou to the atmosphcrc, and in preserv- 

 ing the ratio among its components, are light, heat, electricity, and 

 perhaps magnetism. But with regard to the sea, it is not known 

 what ofB.ce, if any, is performed by electricity, in giving dynami- 

 cal force to its system of circulation. The chief motive power 

 from which marine currents derive their velocity has been ascribed 

 to heat ; but a close study of the agents concerned has suggested 

 that an important — nay, a powerful and active agency in the sys- 

 tem of oceanic circulation is derived from the salts of the sea wa- 

 ter, through the instrumentality of the winds, of marine plants, 

 and animals. These give the ocean great dynamical force. Let 



* Youman's Chemistry. 



