202 pnysicAL GEocRAriiY or the sea, and its meteorology. 



more tinder currents. But in what direction these currents were 

 running is not known. 



404. Tlie compressibility of ivater — effect of in the oceanic circula- 

 tion. — Vertical circulation is as important in the sea as it is in 

 the air (§ 231). In striving to understand the physical machinery 

 of our planet and to comprehend its workings, we must, if wo 

 would learn, proceed 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 pro- 

 perties so proportioned and so adjusted as exactly to answer its 

 purposes in the grand design. And, consequently, we are en- 

 titled 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 required of fluid matter, and which, in 

 the terrestrial economy, solid matter was not adapted to perform. 

 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 required to reject as insufficient, any 

 hypothesis touching the s^^stem of aqueous circulation on our 

 planet which ignores, even in the profoundest depths of the ocean, 

 an interchange of its particles between the bottom and the top. 

 Were such interchange not to take place — were the water in the 

 sea which once sinks below the level of its horizontal circulation 

 doomed to remain there for ever, 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 lighter, until the one became 

 saturated with salt, the other entirely fresh. To prevent this 

 state of things, we recognize the influences 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 gravity, 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 pre- 

 cisely 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 has 

 pointed out exactly such a power for the atmospheric ocean, — a 



