Popular Science Monthly/Volume 3/July 1873/How the Sea-Depths are Explored
|HOW THE SEA-DEPTHS ARE EXPLORED.|
ONE of the most recent and impressive examples of the interaction of science and art by which knowledge is extended, and man's control over Nature increased, is furnished by the late remarkable investigations into the depths and life of the sea. The taking of soundings is, of course, as old as navigation, and is an indispensable portion of the mariner's art. The record of these soundings was embodied in charts by which sailors were guided in unknown waters. As commerce extended, such observations became more full, and resulted in systematic coast-surveys in which the depth of water, currents, magnetic conditions, temperatures, tides, and winds, were taken into account, and the knowledge thus accumulated gave rise at length to a great science—the Physical Geography of the Sea. About twenty-five years ago a new step was taken toward the extension of our knowledge of sea-depths. Science had given to the world the electric telegraph, and commerce demanded that it should be laid across the ocean. For this purpose the bed of the North Atlantic required to be carefully examined and mapped, and the configuration of the sea-bottom and the nature of its material determined. This gave a new impulse to the art of sea-sounding. The transatlantic cable was laid, got broken, and the end of it was then fished up from a depth of nearly two miles. A great victory was thus gained; the bottom of the sea was no longer inaccessible, and the possibility of its scientific exploration became established. Hitherto, sea observations had main reference to the advantages of navigation and commerce; but, from this time forward, the idea was entertained of pursuing the investigation in the interest of science alone. At the instance of the Royal Society, the British Admiralty, in 1868, granted a small government vessel, the gunboat Lightning, to Dr. William B. Carpenter and Prof. Wyville Thomson, to be used for dredging the bottom of the sea, and investigating its animal life. So promising were the results of this experiment, that a second expedition was arranged in 1869, and the government surveying-vessel Porcupine was assigned to the naturalists to carry on the work. This expedition was also so highly successful, that the ship Challenger has now started out on a four years' voyage around the world to carry out a comprehensive plan of deep-sea observations. We noticed very briefly last month the admirable work of Prof. Wyville Thomson on "The Depths of the Sea," giving a history of what has been lately done in the investigation of the subject. We propose now to lay Prof. Thomson's work under contribution for the benefit of our readers, and especially to give some account of the instruments of ocean-research, and the way the exploration is conducted.
It may be remarked, in passing, that, when the dredging of the deep seas was found to be feasible, questions of large scientific interest and moment, which had been hitherto regarded as inaccessible, were suddenly brought within the range of practical solution. It was a popular opinion, shared also by men of science, that the bottom of the sea was a dark and desolate waste, subject to such tremendous pressure as to render all life impossible. Prof. Thomson observes: "The enormous pressure at these great depths seemed at first sight alone sufficient to put any idea of life out of the question. There was a curious popular notion, in which I well remember sharing when a boy, that, in going down, the sea-water became gradually under the pressure heavier and heavier, and that all the loose things in the sea floated at different levels, according to their specific weight: skeletons of men, anchors, and shot, and cannon, and, last of all, the broad gold-pieces lost in the wreck of many a galleon on the Spanish Main, the whole forming a kind of false bottom to the ocean, beneath which there lay all the depth of clear, still water, which was heavier than molten gold. The conditions of pressure are certainly very extraordinary. At 12,000 feet a man would bear upon his body a weight equal to 20 locomotive-engines, each with a long goods-train loaded with pig-iron. We are apt to forget, however, that water is almost imcompressible, and that, therefore, the density of sea-water at a depth of 12,000 feet is scarcely appreciably increased."
Contrary to all anticipation, it was found that highly-organized representatives of all the invertebrate classes do live under these conditions of enormous pressure. The bottom of the ocean is, therefore, to be regarded as habitable, and is proved to be actually inhabited by numberless forms of animal life. A new world was thus opened to the naturalist, which, although difficult of access, was yet accessible and must be investigated. The pioneers in the exploration of course encountered very formidable obstacles; but the field was too vast and the promise too rich to be neglected, and how it was regarded by the devotees of research may be gathered from the following words of Dr. Thomson:
"Still the thing is possible, and it must be done again and again, as the years pass on, by naturalists of all nations working with improved machinery and with ever-increased knowledge. For the bed of the deep sea, the 140,000,000
Fig. 1. Brooke's Deep-Sea Sounding-Apparatus square miles which we have now added to the legitimate field of natural-history research, is not a barren waste. It is inhabited by a fauna more rich and varied on account of the enormous extent of the area, and with organisms in many cases apparently even more elaborately and delicately formed, and more exquisitely beautiful in their soft shades of coloring and in the rainbow-tints of their wonderful phosphorescence, than the fauna of the well known belt of shallow water, teeming with innumerable invertebrate forms, which fringes the land. And the forms of these hitherto unknown living beings, and their mode of life and their relations to other organisms, whether living or extinct, and the phenomena and laws of their geographical distribution, must be worked out."
There are two principal operations in exploring the bottom of the ocean: first, sounding to ascertain depth; and, second, dredging to bring up materials. Although much ingenuity has been expended in devices to bring up samples of the sea-bottom by the sounding-apparatus, yet dredging contrivances are now mainly relied upon for that purpose. To determine the depth with a sounding-line, it is customary to graduate it by attaching slips of different-colored cloths or leather which mark it off into sections, and give the means of determining the distance to which the weight runs down. Another method of measuring the depth consisted in running down a weight attached to a line, which was cut at the surface as soon as the weight was supposed to have
Fig. 2. The Bull-dog Sounding-Machine. reached bottom, from a sudden change in the rate of running out, and the depth was then calculated by the length of cord left on the reel.
The ordinary system of sounding fails at great depths, and cannot be depended upon for more than 6,000 feet. The weight is not sufficient to carry the line rapidly and vertically to the bottom, and, if a heavier weight be used, the line is in danger of breaking. No impulse is felt when the lead strikes the bottom, and the line goes on running out, and, if stopped, is liable to break. Sometimes the line is carried along by submarine currents, forming loops or bights, and it often continues to run out and coil itself in a tangled mass directly over the lead. These sources of error vitiate very deep soundings, so that the reports that have been made of measurements in the Atlantic of 39,000, 46,000, and 50,000 feet, without reaching bottom, are now regarded as exaggerations. In the last charts of the North Atlantic, on the authority of Rear-Admiral Richards, no soundings are entered beyond 24,000 feet, and very few beyond 18,000 feet.
The ordinary deep-sea lead, which is a prismatic block about two feet in length, and from 80 to 120 pounds in weight, has a simple provision for bringing up material from the bottom, which is called "arming" that is, the lower end, which is slightly cupped, is covered with a thick coating of soft tallow. If it 5 reaches the bottom, mud, shells, gravel, 21125 ooze, or sand, sticks to the tallow, and, when drawn up, affords a sample of the nature of the ground. As the interest in the bottom of the sea increased, there was a more eager curiosity to scrutinize the particles thus procured for chemical and microscopical examination, and it became desirable to devise means of bringing up larger amounts of matter. Many contrivances Lave been made for this purpose. Sir John Ross, in 1818, invented a machine for this purpose, called the "deep-sea clamm." A large pair of forceps were kept asunder by a bolt, and the instrument was so contrived that, on the bolt striking the ground, a heavy iron weight slipped down a spindle and closed the forceps, which retained within them a considerable quantity of the bottom, whether sand, mud, or small stones. By this arrangement Sir John Ross brought up six pounds of soft mud from a depth of 6,300 feet.
In the year 1854, J. M. Brooke, passed midshipman in the United States Navy, contrived the arrangement known as "Brooke's Deep-Sea Sounding-Apparatus," of which all the more recent contrivances have been to a great extent modifications and improvements, his fundamental principle being the detachment of a weight when the bottom is struck. The weight is a 64-pound shot (E, Fig. 1), cast with a hole through it. An iron rod (A B) passes through this hole, with an opening or chamber at the lower end "armed" with tallow. When the instrument strikes, the end of the rod is driven into the material of the bottom, which fills the chamber. At the same time a pair of hinged arms (D) at the top, which were upright in the descent, fall down and release the cord (C), which sustains the ball by a leather collar below. As the loops of the sling are relieved from the teeth of the arms, the rod slips through the hole in the shot, and comes up alone with its enclosed sample of sediment. The difficulty with this machine was the washing out of the material in the ascent. This was remedied by Commander Dayman, by adapting a valve, opening inward, to the terminal chamber of the rod.
In 1860 the assistant engineer of H.M.S. Bulldog contrived a dredging-lead that combined the principle of Ross's clamm with the disengaging weight of Brooke. It is an ingenious and well-known machine, though hardly as simple as could be desired. Prof. Thomson thus describes it:
|Otho Friedrich Muller's Dredge a. d. 1750.|
"A pair of scoops (A) close upon one another scissors-wise on a hinge, and have two pairs of appendages (B), which stand to the opening and closing of the scoops in the relation of scissor-handles. This apparatus is permanently attached to the sounding-line by the rope (F), which in the figure is represented as hanging loose, and which is fixed to the spindle on which the cups turn. Attached to the same spindle is the rope (D), which ends above an iron ring. E represents a pair of tumbler-hooks, fastened likewise to the end of the sounding-line; C a heavy leaden or iron weight, with a hole through it wide enough to allow the rope (D) with its loop and ring to pass freely; and B a strong India-rubber band, which passes round the handles of the scoops. In the figure the instrument is represented as it is sent down and before it reaches the bottom. The weight (C) and the scoops (A) are now suspended by the rope (D), whose ring is caught by the tumbler-hooks (E). The elastic ring (B) is in a state of tension, ready to draw together the scoop-handles and close the scoops, but it is antagonized by the weight (C), which, pressing down into a space between the handles, keeps them asunder. The moment the scoops are driven into the ground by the weight, the tension on the rope (D) is relaxed, the tumblers fall and release the ring, and the weight falls and allows the elastic band to close the scoops and keep them closed upon whatever they may contain; the rope (D) slips through the weight, and the closed scoops are drawn up by the rope (F)."
The attempt has been often made to measure the amount of vertical descent by self-registering machinery. Massey's sounding-machine is the best for this purpose, and operates upon a principle of screw-motion as it falls through the water. As represented in Fig. 3, two thimbles (F F) pass through the two ends of the heavy oval brass shield (A A). To the upper of these the sounding-line is attached, and to the lower the weight at about a yard from the machine. The screw-motion is communicated by a set of four brass vanes or rings (B), which are soldered obliquely to an axis in such a position that, as the machine descends, the axis revolves by the pressure of the water against the vanes. C represents the dial-plate as seen when the slide (D) is withdrawn. The revolving axis communicates its motion to the indices, which are so adjusted that the index on the right-hand dial passes through a division for every fathom of vertical descent whether quick or slow, and makes an entire revolution for 15 fathoms; while the left-hand index passes through a division on the circle for 15 fathoms, and makes an entire revolution during a descent of 225 fathoms. This instrument answers very well for accurate work in moderately deep water; but at extreme depths it has an uncertainty which seems to be shared by all contrivances involving metal wheelwork.
The main theatre of sounding operations has been the Atlantic Ocean, which, from its relation to the leading commercial nations, and for intercontinental telegraphic purposes, has been more carefully surveyed than any other great body of water. Open from pole to pole, participating in all conditions of climate, communicating freely with other seas, and covering 30,000,000 square miles, it is believed to represent general oceanic conditions, and to contain depths nearly, if not quite, as great as the other ocean-basins of the world, although but little is known, it is true, in this respect, of the Indian, Antarctic, and Pacific Seas. The general result of its soundings would indicate that the average depth of the Atlantic bed is not much more than 1 2,000 feet, and there seem to be few depressions deeper than 15,000 or 20,000 feet, a little more than the height of Mont Blanc. Dr. Thomson sums up the general results of the Atlantic soundings as follows: "In the Arctic Sea there is deep water, reaching to 9,000 feet to the west and south-west of Spitzbergen. Extending from the coast of Norway, and including Iceland, the Faroe Islands, Shetland and Orkney, Great Britain and Ireland, and the bed of the North Sea to the coast of France, there is a wide plateau, on which the depth rarely reaches 3,000 feet; but to the west of Iceland and communicating doubtless with the deep water in the Spitzbergen Sea, a trough 500 miles wide, and, in some places, nearly 12,000 feet deep, curves along the east coast of Greenland. This is the path of one of the great Arctic return-currents.
Fig. 6. Ball's Dredge. After sloping gradually to a depth of 3,000 feet to the westward of the coast of Ireland in latitude 52, the bottom suddenly dips to 10,000 feet at the rate of about 15 to 19 feet in the 100; and from this point to within about 200 miles of the coast of Newfoundland, when it begins to shoal again, there is a vast undulating submarine plain, averaging about 12,000 feet in depth below the surface the 'telegraph plateau.'
"A valley about 500 miles wide, and with a mean depth of 15,000 feet, stretches from off the southwest coast of Ireland, along the coast of Europe, dipping into the Bay of Biscay, past the Strait of Gibraltar, and along the west coast of Africa. Opposite the Cape de Verde Islands, it ems to merge into a slightly deeper trough, which occupies the axis of the South Atlantic, and passes into the Antarctic Sea. A nearly similar valley curves around the coast of North America, about 12,000 feet in depth, off Newfoundland and Labrador, and becoming considerably deeper to the south-ward, where it follows the outline of the coast of the States, and the Bahamas and Windward Islands, and finally joins the central trough of the South Atlantic off the coast of Brazil, with a depth of 15,000 feet."
Until within a hundred years but little was known of the living inhabitants of the deep sea, except the few objects that adhered to lead-lines, or were taken accidentally by fishermen in trawls and oyster-dredges; and, as odd things of no market value were generally thrown away, the knowledge from this source increased but slowly. The first dredge used by a naturalist to collect specimens from the sea-bottom was employed by Otho Friedrich Müller, who published a quaint book about it in 1779. His dredge was a square-mouthed bag (Fig. 4), and he does not appear to have used it beyond a depth of 180 feet. The dredges now used by naturalists are modifications of the oyster-dredge, which is described as a light frame of iron, about five feet long by a foot or so in width at the mouth, with a scraper like a narrow hoe on one side, and a suspending apparatus attached to the rope on the other. From the frame is suspended a bag, about two feet in depth, of wide netting or hempen cord. The naturalists' dredge has a scraper on each side, the bag is deeper, and the meshes so fine as to allow only the water to pass through.
Fig. 5 represents the dredge devised by Dr. Ball, of Dublin, and which scraped the surface so perfectly that, when drawn along a drawing-room floor, it would pick up the pence that had been scattered before it. Dr. Thomson states that the most convenient size for dredging from a small boat, at a less depth thaii 600 feet, is a frame 18 inches long and five inches in width. The scrapers are three inches wide, and are so set that the distance across between their edges is 7½ inches.
The dredge used for deep-sea work was larger, the frame being four feet six inches in length, and six inches wide at the throat or narrowest part. The weight of the frame was 225 lbs., but Dr. Thomson thinks it was too large and heavy. The dredge-bag was double, the outer being of strong twine netting lined within with "bread-bag," a light, open kind of canvas.
It was found by experience that very often, when nothing of interest was brought up within the dredge, many echinoderms, corals, and sponges, came to the surface, sticking to the outside of the bag, and even to the first few fathoms of the dredge-rope. This suggested the attachment of swabs, used for washing the decks, to the dredge. The tangled hemp turned out to be very efficient, picking up great numbers of objects that would not be otherwise secured. The bag took the mollusks, which, from their shelly forms, could not be otherwise obtained, while the echinoderms, corals, and sponges—bulky objects that could not readily enter the bag—were more easily caught by the swabs, although, unfortunately, it mutilated them, and brought them up in fragments. So important was this expedient, that a long iron bar was attached to the bottom of the dredge-bag, to which the hempen bundles were suspended, as shown in Fig. 6.
The arrangements for sounding and dredging from the Porcupine are fully described and illustrated in Prof. Thomson's work. The vessel was a 382-ton gunboat, with a steam-engine of 12 horse-power, stationed amidships, with drums of different sizes, from which lines were led fore and aft for working either at the bow or stern. Two powerful derricks were rigged for sounding and dredging, one over the stern and one over the port-bow. The block through which the sounding-line or dredging-rope passed was not attached directly to the derrick, but to a rope which passed through an eye at the end of the spar, and was fixed to a "bit," a piece of timber going through the deck. On a bight of this rope between the block and the "bit" was a piece of apparatus shown in Fig. 7, and called the "accumulator." This consisted of 30 or 40 strong India-rubber springs, working together, and its use was to yield by stretching, when, from any cause, as the pitching of the ship, there was an unusual strain upon the line. The dredge-rope of the Porcupine was of Russian hemp, 2½ inches in circumference, with a breaking strain of 2½ tons, and was 18,000 feet, or nearly 3½ miles long. A row of about 20 large iron pins, about 2½ feet in length, projected over one side of the quarter-deck, rising obliquely from the top of the bulwark. Upon these the rope was continuously coiled, as shown in the figure, which also represents the dredge in position for descent.
The method of dredging at a great distance is thus graphically described by Prof. Thomson, as it was performed in the Bay of Biscay, July 22, 1869. The depth was first accurately ascertained by sounding
and found to be 14,610 feet. "At 4.45 p. m. the dredge was let go, the vessel drifting slowly before a moderate breeze from the north-west. The 3,000 fathoms of rope were all out at 5.50 p. m. The diagram (Fig. 8) will give an idea of the various relative positions of the dredge and the vessel according to the plan of dredging adopted by Captain Calvert, which worked admirably, and which appears, in fact,
to be the only mode that would answer for great depths. It represents the position of the vessel when the dredge is let go, and the dotted line (A B) the line of descent of the dredge rendered oblique by the tension of the rope. While the dredge is going down, the vessel drifts gradually to leeward; and, when the whole (say) 18,000 feet of rope are out, C W and D might represent respectively the relative positions of the vessel, the weight attached 3,000 feet from the dredge, and the dredge itself. The vessel now steams slowly to windward, occupying successively the positions E, F, G, H. The weight, to which the water offers but little resistance, sinks from W to W’, and the dredge and bag sink more slowly from D to B. The vessel is now allowed to drift back before the wind, from H toward C. The tension of the motion of the vessel, instead of acting immediately on the dredge, now drags forward the weight (W’), so that the dredging is carried on from the weight, and not directly from the vessel. The dredge is thus quietly pulled along, with its lip scraping the bottom in the attitude which it assumes from the centre of weight of its iron frame and arms. If, on the other hand, the weights were hung close to the dredge, and the dredge were dragged directly from the vessel, owing to the great weight and spring of the rope, the arms would be continually lifted up, and the lip of the dredge prevented from scraping. In very deep dredging this operation of stealing up to windward until the dredge-rope is nearly perpendicular, after drifting for half an hour or so to leeward, is usually repeated three or four times.
"At 8.50 p. m. we began to haul in. The donkey-engine delivered the rope at the rate of rather more than a foot per second without a single check. A few minutes before one a. m. the weights appeared, and, a little after one in the morning, eight hours after it was cast over, the dredge was safely hauled on deck, having in the interval accomplished a journey of upward of eight statute miles. The dredge contained 1½ cwt. of very characteristic pale-gray Atlantic ooze." The total weight brought up by the engine was:
|Weight of rope,||reduced to 1 in water||1,375||lbs.|
|Dredge and bag,||275|
|Ooze brought up,||||168|
As an abundant and characteristic invertebrate life is now shown to exist at such great depths, it is inferred to extend to all depths; and thus the whole ocean-bed becomes in future the domain of the inquisitive naturalist. But, as Dr. Thomson remarks, little more can be said, for his work is all before him: "A grand new field of inquiry has been opened up, but its culture is terribly laborious. Every haul of the dredge brings to light new and unfamiliar forms—forms which link themselves strangely with the inhabitants of past periods in the earth's history; but as yet we have not the data for generalizing the deep-sea fauna, and speculating on its geological and biological relations; for, notwithstanding all our strength and will, the area of the bottom of the deep sea which has been fairly dredged may still be reckoned by the square yard."