198 THE PHYSICAL GEOGKAPHY OF THE SEA. 



currents. But in what direction these currents were running is 

 not known. 



404. Vertical circulation is as important in the sea as it is in 

 The compressibility the air (§ 231). In striving to understand the 

 il tiTe^lce7ni^'ScS- phjsical machinery of out- planet and to compre- 

 ^''^'°°- hend its workings, we must, if we would learn, pro- 



ceed upon the principle (§ 351) that at creation the waters were 

 measured, the hills weighed, and the atmosphere meted out, and 

 that each was endowed with its peculiar properties so proportion- 

 ed and so adjusted as exactly to answer its purposes in the grand 

 design. And, consequently, we are entitled to infer that fluidity 

 instead of solidity was imparted to a certain quantity of matter 

 which we call water, to enable it to perform the offices to be re- 

 quired of fluid matter, and which solid matter was not adapted to 

 perform in the terrestrial economy. By this mode of reasoning 

 we are taught to regard the fluidity of all the water in the sea as 

 a physical necessity — and by this mode of reasoning we are re- 

 quired to reject as insufficient any hypothesis touching the sys- 

 tem of aqueous circulation on our planet which ignores, even in 

 the profoundest depths of the ocean, an interchange of its parti- 

 cles between the bottom and the top. Were such interchange 

 not to take place — were the water in the sea which once sinks be- 

 low the level of its horizontal circulation doomed to remain there 

 forever, it would not be difficult to show that the sea would lose 

 its balance and its counterpoises ; that, not being able to preserve 

 its status, the water at the bottom would have grown heavier and 

 heavier, while that at the top would have become lighter and 

 lisrhter, until the one became saturated with salt, the other entire- 

 ly fresh. To prevent this state of things, we recognize the in- 

 fluences of the winds and tides, as well as the necessity of vertical 

 movements in the sea. Whence, therefore, let us inquire, when 

 a given quantity of water once finds its way to the bottom of the 

 sea, whence — since it goes there by virtue of its own specific gravi- 

 ty — whence is power to be derived for bringing it up again? for 

 sooner or later, according to this view, up it must come. We thus 

 arrive precisely at one of those points (§ 287) at which hypothesis 

 becomes absolutely necessary if we would make further progress. 

 Here, therefore, let us pause to search among the physics of the 

 sea for such a power and the foundation for hypothesis. Leslie 



