DEPTH OP THE OCEAN DEEP SEA SOUNDINGS DEPTH OF THE OCEAN AND FORM OF SEA BOTTOM Before giving the main results of the more recent re searches as to the depth of the ocean, it may perhaps be advisable to present the student with a few particulars relative to the art of "deep-sea sounding," which has only recently been so perfected as to furnish us with accurate and reliable results.
At first sight, " sounding " seems to be one of the sim plest operations possible, for what could be more easy than to ascertain the depth of the water at any locality, by simply letting down a line with a weight at one end, and then noting the actual length of line run out, which would, of course, be the actual depth at that point. In still, shallow water, the depth may be pretty accurately measured in this way, but when tried in water of considerable depth, the plan will be found to fail, as it gives greatly exaggerated results. In the first place, in " sounding " by line and weight, it was taken for granted that when the weight reached the bottom, the line would cease to run out ; or, at any rate, that the observer would be able to tell, by a so-called "shock," when the weight touched the bottom. But repeated experiments proved the fallacy of both these suppositions. It was found impos sible to tell exactly when the weight touched the bottom, for in many cases the line would never cease to run out, and the " shock " was purely hypothetical—it being, indeed, a physical impossibility for a shock, caused by the weight falling on the bottom, to be communicated through thousands of fathoms of line to the surface. Reports of enormous depths in the open ocean were made by the early explorers ; lengths of 40,000 and 50,000 feet were ran out without reaching the "bottom." The subject now began to excite greater interest, and a deter mination to solve the matter satisfactorily,—the pioneers of deep sea-sounding being the officers of the United States Navy, acting under the direction of Captain Maury.
The immense length of line reported to be run out was often in itself a ground for suspicion, but the frequent affix of "no bottom" deepened the mistrust of results thus obtained. The attention of scientists was therefore directed to the root of the matter—it being evident that the errors were due to the method of sounding. Various projects were put forth to
obviate the known obstacles which had, as yet, rendered accurate and reliable "sounding" in the open ocean impossible. The uncertain elements in "sounding," as then practised, were three-fold:—(1.) Unless in water of no considerable depth, it was doubtful whether the weight had touched the bottom or not,—the line in many cases never ceasing to run out. (2.) Supposing the weight had reached the bottom, and was sta tionary, it was questionable whether the line had not been deflected from its vertical position by under-currents, and if so, what was the amount of such deflection, or when did the line begin to deflect 4 (3.) Taking it for granted that the weight had touched the bottom, and that the line was tense throughout, it was uncertain whether the vessel from which the "soundings" were taken had not shifted its position during the operation.
In the first place, the uncertainty as to whether the weight had touched the bottom or not was soon solved. When deep-sea soundings first attracted general attention, some asserted that the weight could never reach the bottom, at least in the deeper parts of the ocean, for it would be buoyed up, as it were, by the dense water at great depths. The absurdity of this opinion is evident, if we reflect that, water being all but incompressible, it would require a depth of not less than 93 miles to double its density at the surface. But the specific gravity of lead is about eleven times greater than that of water. Taking it for granted, then, that a depth of 93 miles were attained, the lead would still be five times as heavy as the water at the bottom, and, consequently, could not possibly be buoyed up even at that depth. But there are no such depths in the ocean, the deepest part probably not exceeding six miles: so that in most cases where "no bottom" was reported, the weight certainly had reached and rested on the bottom—the continued running out of the line being due to its deflection by under-currents, and not to its indefinite dragging down by the weight.