Page:Physical Geography of the Sea and its Meteorology.djvu/228

202 more under currents. But in what direction these currents were running is not known.

404. The compressibility of water—effect of in the oceanic circulation.'—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 we 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 properties so proportioned 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 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 system 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 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 has pointed out exactly such a power for the atmospheric ocean,—a