Physical Geography Of The Sea 1855/11
CHAPTER XI. — THE DEPTHS OF THE OCEAN.
The Depth of blue Water unknown, § 421. — Results of former Methods of Deep-sea Soundings not entitled to Confidence, 422. — Attempts by Sound and Pressure, 423. — The Myths of the Sea, 424. — Common Opinion as to its Depths, 425. — Interesting Subject, 427. — The deepest Soundings reported, 428. — Plan adopted in the American Navy, 429. — Soundings to be made from a Boat, 431. — Why the Sounding twine will not stop running out when the Plummet reaches Bottom, 432. — Indications of under Currents, 433. — Rate of Descent, 434. — Lieutenant John Mercer Brooke’s Deep-sea Sounding Apparatus, 437. — The greatest Depths at which Bottom has been found, 438.
421. Until the commencement of the plan of deep-sea soundings, as now conducted in the American Navy, the bottom of what the sailors call “blue water” was as unknown to us as is the interior of any of the planets of our system. Ross and Dupetit Thouars, with other officers of the English, French, and Dutch navies, had attempted to fathom the deep sea, some with silk threads, some with spun-yarn (coarse hemp threads twisted together), and some with the common lead and line of navigation. All of these attempts were made upon the supposition that when the lead reached the bottom, either a shock would be felt, or the line, becoming slack, would cease to run out.
422. The series of systematic experiments recently made upon this subject shows that there is no reliance to be placed on such a supposition, for the shock caused by striking bottom can not be communicated through very great depths, and therefore it does not follow that the line will become slack and cease to run out when the plummet reaches the bottom. Furthermore, the lights of experience show that, as a general rule, the under currents of the deep sea have force enough to take the line out long after the plummet has ceased to do so. Consequently, there is but little reliance to be placed upon deep-sea soundings of former methods, when the depths reported exceeded eight or ten thousand feet.
423. Attempts to fathom the ocean, both by sound and pressure, had been made, but in “blue water” every trial was only a failure repeated. The most ingenious and beautiful contrivances for deep-sea soundings were resorted to. By exploding heavy charges of powder in the deep sea, when the winds were hushed and all was still, the echo or reverberation from the bottom might, it was held, be heard, and the depth determined from the rate at which sound travels through water. But, though the explosion took place many feet below the surface, echo was silent, and no answer was received from the bottom.
Ericsson and others constructed deepsea leads having a column of air in them, which, by compression, would show the aqueous pressure to which they might be subjected. This was found to answer well for ordinary purposes, but in the depths of “blue water,” where the pressure would be equal to several hundred atmospheres, the trial was more than this instrument could stand. Mr. Baur, an ingenious mechanician of New York, constructed, according to a plan which I furnished him, a deep-sea sounding apparatus.
To the lead was attached, upon the principle of the screw propeller, a small piece of clock-work for registering the number of revolutions made by the little screw during the descent; and, it having been ascertained by experiment in shoal water that the apparatus, in descending, would cause the propeller to make one revolution for every fathom of perpendicular descent, hands provided with the power of self-registration were attached to a dial, and the instrument was complete. It worked beautifully in moderate depths, but failed in blue water, from the difficulty of hauling it up if the line used were small, and from the difficulty of getting it down if the line used were large enough to give the requisite strength for hauling up.
424. But, notwithstanding these failures, there was encouragement, for greater difficulties had been overcome in other departments of physical research. Astronomers had measured the volumes and weighed the masses of the most distant planets, and increased thereby the stock of human knowledge. Was it creditable to the age that the depths of the sea should remain in the category of an unsolved problem? It was a sealed volume, abounding in knowledge and instruction that might be both useful and profitable to man. The seal which covered it was of rolling waves many thousand feet in thickness. Could it not be broken? Curiosity had always been great, yet neither the enterprise nor the ingenuity of man had as yet proved itself equal to the task. No one had succeeded in penetrating, and bringing up from beyond the depth of two or three hundred fathoms below the aqueous cover ing of the earth, any specimens of solid matter for the study of philosophers. The sea, with its myths, has suggested attractive themes to all people in all ages. Like the heavens, it affords an almost endless variety of subjects for pleasing and profitable contemplation, and there has remained in the human mind a longing to learn more of its wonders and to understand its mysteries. The Bible often alludes to them. Are they past finding out? How deep is it? and what is at the bottom of it? Could not the ingenuity and appliances of the age throw some light upon these questions? The government was liberal and enlightened; times seemed propitious; but when or how to begin, after all these failures, with this interesting problem, was one of the difficulties first to be overcome.
425. It was a common opinion, derived chiefly from a supposed physical relation, that the depths of the sea are about equal to the heights of the mountains. But this conjecture was, at best, only a speculation. Though plausible, it did not satisfy. There were, in the depths of the sea, untold wonders and inexplicable mysteries. Therefore the contemplative mariner, as in mid ocean he looked down upon the gentle bosom of the sea, continued to experience sentiments akin to those which fill the mind of the devout astronomer when, in the stillness of the night, he looks out upon the stars, and wonders.
426. Nevertheless, the depths of the sea still remained as fathomless and as mysterious as the firmament above. Indeed, telescopes of huge proportions and of vast space-penetrating powers had been erected here and there by the munificence of individuals, and attempts made with them to gauge the heavens and sound out the regions of space. Could it be more difficult to sound out the sea than to gauge the blue ether and fathom the vaults of the sky? The result of the astronomical undertakings[426] lies in the discovery that what, through other instruments of. less power, appeared as clusters of stars, were, by these of larger powers, separated into groups, and what had been reported as nebulae could now be resolved into clusters; that, in certain directions, the abyss beyond these faint objects is decked with other nebulae, which these great instruments may bring to light, but can not resolve; and that there are still regions and realms beyond, which the rays of the brightest sun in the sky have neither the intensity nor the force to reach, much less to penetrate.
[426] See the works of Herschel and Ross, and their telescopes.
427. So, too, with the bottom of the sea, and the knowledge seeking mariner. Though nothing thence had been brought to light, exploration had invested the subject with additional interest, and increased the desire to know more. In this state of the case, the idea of a common twine thread for a sounding-line, and a cannon ball for a sinker, was suggested. It was a beautiful conception; for, besides its simplicity, it had in its favor the greatest of recommendations — it could be readily put into practice. Well directed attempts to fathom the ocean began now to be made, and the public mind was astonished at the vast depths that were at first reported.
428. Lieutenant Walsh, of the United States schooner “Taney,” reported a cast with the deep-sea lead at thirty-four thousand feet without bottom. His sounding-line was an iron wire more than eleven miles in length. Lieutenant Berryman, of the United States brig “Dolphin,” reported another unsuccessful attempt to fathom mid ocean with a line thirty-nine thousand feet in length. Captain Denham, of her Britannic majesty’s ship “ Herald,” reported bottom in the South Atlantic at the depth of forty-six thousand feet; and Lieutenant J. P. Parker, of the United States frigate “Congress,” afterward, in attempting to sound near the same region, let go his plummet, and saw a line fifty thousand feet long run out after it as though the bottom had not been reached. The three last-named attempts were made with the sounding twine of the American Navy, which has been introduced in conformity with a very simple plan for sounding out the depths of the ocean. It involved for each cast only the expenditure of a cannon ball, and twine enough to reach the bottom. This plan was introduced as a part of the researches conducted at the National Observatory, and which have proved so fruitful and beneficial, concerning the winds and currents, and other phenomena of the ocean. These researches had already received the approbation of the Congress of the United States; for that body, in a spirit worthy of the representatives of a free and enlightened people, had authorized the Secretary of the Navy to employ three public vessels to assist in perfecting the discoveries, and in conducting the investigations connected therewith.
429. The plan of deep-sea soundings finally adopted, and now in practice, is this: Every vessel of the Navy that will, when she puts to sea, is, if she desires it, furnished with a sufficient quantity of sounding-twine, carefully marked at every length of one hundred fathoms — six hundred feet — and wound on reels of ten thousand fathoms each. It is made the duty of the commander to avail himself of every favorable opportunity to try the depth of the ocean, whenever he may find himself out upon “blue water.” For this purpose he is to use a cannon ball of thirty-two pounds as a plummet.
Having one end of the twine attached to it, the cannon ball is to be thrown overboard from a boat, and suffered to take the twine from the reel as fast as it will. The reel is made to turn easily. A silk thread, or the common wrapping-twine of the shops would, it was thought, be strong enough for this purpose; for it was supposed there would be no strain upon the line, except the very slight one required to drag it down, and the twine having nearly the specific gravity of sea water, this strain would, it was imagined, be very slight. Moreover, when the shot reached the bottom, the line, it was thought (§ 421), would cease to run out; then breaking it off, and seeing how much remained upon the reel, the depth of the sea could be ascertained at any place and time, simply at the expense of one cannon ball and a few pounds of common twine.
430. But practical difficulties that were not suspected at all were lurking in the way, and afterward showed themselves at every attempt to sound; and it was before these practical difficulties had been fairly overcome that the great soundings (§ 428) were reported. In the first place, it was discovered that the line, once started and dragged down into the depths of the ocean, never would cease to run out (§ 422), and, consequently, that there was no means of knowing when, if ever, the shot had reached the bottom. And, in the next place, it was ascertained that the ordinary twine (§ 427) would not do; that the sounding-line, in going down, was really subjected to quite a heavy strain, and that, consequently, the twine to be used must be strong; it must be subjected to a test which required it to bear a weight of at least sixty pounds freely suspended in the air. So we had to go to work anew, and make several hundred thousand fathoms of sounding-twine especially for the purpose. It was small, and stood the test required, a pound of it measuring about six hundred feet in length.
431. The officers intrusted with the duty soon found that the soundings could not be made from the vessel with any certainty as to the depth. It was necessary that a boat should be lowered, and the trial be made from it; the men with their oars keeping the boat from drifting, and maintaining it in such a position that the line should be “up and down” the while.
432. That the line would continue to run out after the cannon ball had reached bottom, was explained by the conjecture that there is in the ocean, as in the air, a system of currents and counter currents one above the other, and that it was one or more of these submarine currents, operating upon the bight of the line, which caused it to continue to run out after the shot had reached the bottom. In corroboration of this conjecture, it was urged, with a truth-like force of argument, that it was these under currents, operating with a swigging force upon the bights of the line — for there might be several currents running in different directions, and operating upon it at the same time — which caused it to part whenever the reel was stopped and the line held fast in the boat.
433. A powerful train of circumstantial evidence was this (and it was derived from a source wholly unexpected), going to prove the existence of that system of oceanic circulation which the climates, and the offices, and the adaptations of the sea require, and which its inhabitants (§ 293) in their mute way tell us of. This system of circulation commenced on the third day of creation, with the “gathering together of the waters,” which were “called seas,” and doubtless will continue as long as sea water shall possess the properties of saltiness and fluidity.
434. In making these deep-sea soundings, the practice is to time the hundred fathom marks as they successively go out; and by always using a line of the same size and “make,” and a sinker of the same shape and weight, we at last established the law of descent. Thus the mean of our experiments gave us, for the sinker and twine used,
2 m. 21 s. as the average time of descent from 400 to 500 fathoms.
3 m. 26 s. as the average time of descent from 1000 to 1100 fathoms.
4 m. 29 s. as the average time of descent from 1800 to 1900 fathoms.
435. Now, by aid of the law here indicated, we could tell very nearly when the ball ceased to carry the line out, and when, of course, it began to go out in obedience to the current and drift alone; for currents would sweep the line out at a uniform rate, while the cannon ball would drag it out at a decreasing rate.
436. The development of this law certainly was an achievement, for it enabled us to show that the depth of the sea at the places named (§ 428) was not as great as reports made it. These researches were interesting; the problem in hand was important, and it deserved every effort that ingenuity could suggest for reducing it to a satisfactory solution.
437. As yet, no specimens of the bottom had been brought up. The line was too small, the shot was too heavy, and it could not be weighed. In this state of the case, Passed Midshipman John Mercer Brooke, United States Navy, who, at the time, was associated with me on duty at the Observatory, proposed a contrivance by which the shot, on striking the bottom, would detach itself from the line, and send up a specimen of the bottom.
BROOKE’S DEEP—SEA SOUNDING AND CORE SAMPLING APPARATUS.
This beautiful contrivance, called Brooke’s Deep-sea Sounding Apparatus, is represented in Plates II. And III. opposite. A is a cannon ball, having a hole through it for the rod B. Plate II. represents the rod, B; the slings, D D, with the shot slung, and in the act of being lowered down. Plate III. represents the apparatus in the act of striking the bottom, and shows how the shot is detached, and how specimens of the bottom are brought up, by adhering to a little soap or tallow, [437] called “arming,” in the cup, C, at the lower end of the rod, B. With this contrivance specimens of the bottom have been brought up from the depth of more than two miles.
438. The greatest depths at which the bottom of the sea has been reached with the plummet are in the North Atlantic Ocean, and the places where it has been fathomed do not show it to be deeper than twenty-five thousand feet. The deepest place in this ocean (Plate XI.) is probably between the parallels of 35° and 40° north latitude, and immediately to the
PAGE 207 THE DEPTHS OF THE OCEAN.
southward of the Grand Banks of Newfoundland. No satisfactory deep-sea soundings, either in the Pacific or Indian Oceans, have as yet been made by those who are cooperating in this admirable plan of research.[438] A few have been made in the South Atlantic, but not enough to justify deduction as to its depths or the shape of its floor.
- [438] Since the above was written, I have received a letter from Captain Ringgold, commanding the Surveying Expedition in the Pacific, informing me that, on his way out, he had obtained, in the southern hemisphere, a deep-sea sounding, with bottom at the depth of eight thousand fathoms. The notes and details of this cast have not yet been received.