PRESSURE AND WEIGHT OP THE OCEAN Another point closely connected with the depth of the sea is that of its pressure, or, in other words, the actual weight of the mass of water at various depths from the surface to the bottom. Knowing from common observation the weight of even small masses of water, one would naturally look upon the weight of the vast mass of the ocean as incalculable. But scientific researches and discoveries enable us in the present day to solve problems far more difficult and incredible than the determination of the pressure or weight of the mass of the sea. Of course, in calculating the mass or weight of all the waters of the ocean, we are obliged to work on more or less general and indefinite data For instance, it is virtually impossible to determine the exact mean depth of the whole ocean. The irregularities of the bottom are such as to preclude the thought of this being done, and even the superficial extent of the ocean can only be roughly estimated ; so that in all these and similar calculations a rather wide margin must be left for probable errors. Still the general estimates thus obtained are practically all that are required. The weight or mass of the ocean, in round numbers, may be easily de termined. Taking the entire area of the sea at 45,500,000 square miles, and the average depth at 2,000 fathoms, or 2f miles, the mass of the ocean will be 102,375,000 cubic miles. Now, if we take 65 lbs. as the weight of a cubic foot of sea water, the weight of a cubic mile multiplied by 102,375,000 will be the total weight of the ocean.
Again, the weight of a column of water of a given depth can be measured in the same way. For if 65 lbs. be the weight of a cubic foot of sea-water, the weight of 1,000 cubic feet will be 65,000 lbs. In other words, the pressure on a surface one foot square, at a depth of 1,000 feet, will be 65,000 lbs., or about 450 lbs. on the square inch. At a depth of 10,000 feet, the pressure will be 650,000 lbs. per square foot, or about 4,500 lbs. on the square inch ; while at 20,000 feet, the weight of the superincumbent mass of water will be equal to a pressure of 9,000 lbs. on the square inch, or a little
more than 4 tons ; or, as Sir Wyville Thomson puts it— at a depth of 2,000 fathoms, or 12,000 feet, a man would bear a weight equal to twenty locomotive engines, each with a long train loaded with pig-iron.
It is usual, also, to indicate the pressure of the water at various depths as so many atmospheres, that is, so many times the weight of the air at sea-level. Thus the pressure at a depth of one mile is equivalent to 160 atmospheres, and at 4,000 fathoms, or 4.5 miles, it amounts to 750 atmo spheres. The pressure thus stated can be readily converted into concrete quantities by multiplying the number of atmo spheres by the actual atmospheric pressure at sea-level, viz., 14i lbs. per square inch.
At first sight, the enormous pressure of water at great depths seems almost to justify the old notion that the lower strata of water in the open ocean are so dense, in consequence of the great pressure, that the heaviest substances would never fall to the bottom, but would be buoyed up by the heavier and denser bottom-water. We have already (Art. 71) shown the absurdity of this notion, founded on the supposition that the density or specific gravity of the water increases propor tionately to the pressure. The density certainly does increase, but in a fractional degree only. Thus, at a depth of one mile, under a pressure of 2,320 lbs. on the square inch, the water is compressed only of its bulk, and had there been a depth of twenty miles, the compression would then amount to only +. We thus find that, in spite of an enormous pressure or "dead weight" at various depths, the all but incompressibility of the water prevents any considerable increase in the density or specific gravity ; and that this increase in density, consequent on increased pressure, is so slight as scarcely to affect any calculations of the weight of the water, even at the greatest depths. This distinction is a most important one, being, as it were, the key to the otherwise insolvable problem of the con tinued existence of delicate organisms under the enormous pressure at the depths of the sea.