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ON THE APPARATUS EMPLOYED IN DEEP-SEA
Br Wm. LANT CARPENTER, B.A., B.Sc., (a Member of the
THE object of this article is not to give an account of the remarkable results obtained in the late deep-sea dredging expeditions, since an abstract of them has been given to the public in the Proceedings of the Royal Institution, and in other scientific journals, and will shortly appear more in detail in the Proceedings of the Royal Society. The aim of the present writer is to endeavour to give the readers of the Popular Science Review some idea of the means employed to obtain these results, by describing the apparatus used, its mode of working, the precautions taken to avoid sources of error, &c.
During the time that the expedition was under the charge of Mr. J. Gwyn Jeffreys, F.R.S., from the latter end of May to the middle of July, the writer was entrusted with the chemical researches to be made, as well as with a portion of the physical investigations, and he had ample opportunities of observing the method of conducting the deep-sea dredging. It may be stated broadly that three sets of enquiries were instituted—(1) an investigation into the temperature of the sea at great depths, with a view to ascertain the extent and direction of submarine currents supposed to exist; (2) an enquiry into the existence and distribution of animal life at these depths; (3) an examination of the sea-water at various depths for its physical and chemical properties, such as its specific gravity, the amount and nature of the gases dissolved in it, and the organic matter contained in it.
It will be convenient to describe—first, the separate parts of apparatus with which the vessel was furnished for the prosecution of these enquiries, and, subsequently, the way in which the whole was used.
H.M.S. Porcupine, paddle steamer, 380 tons, is one of the surveying vessels of her Majesty's navy, and has long been commanded by Staff-Commander E. K. Calver. To the extensive and varied experience gained by him in surveying, is to be attributed in great measure the unexpected success which he attained in this new work, many of the expedients which his ingenuity suggested being of the utmost value.
The sounding apparatus employed in deep water was the form adopted in H.M.S. Hydra, Captain Shortland, in sounding across the Indian Ocean, preparatory to the laying of a submarine cable. In order to obtain the depth correctly, it is necessary that the weight employed should be sufficiently heavy to make the sounding line run out with great rapidity. If this were not the case, and the line were payed out slowly, it would be liable to be deflected by currents, and by the drift of the vessel at the surface, especially in a breeze, during the progress of the operation, so that a greater depth would be indicated on the line than really existed. It is obvious, therefore, that in proportion as the depth is greater, and more line is employed, a heavier weight is required to counteract the increased friction of the water upon the line. In order to avoid the risk of breaking the line while hauling it in, and consequent loss of line and instruments, the following contrivance is employed, by which the weight becomes detached on striking the bottom. It consists of a cylindrical rod, about one inch in diameter, with a ring at the top to which the line is attached. A ring of iron slides loosely on the rod, and above this are cast-iron cylindrical weights, 1 cwt. each, perforated so that the rod passes freely through them. The ring and weights are kept in their place by a wire passing from the bottom ring to a small spring-catch above. As long as there is any strain on this apparatus, i. e. as long as it is descending vertically through the water, the weights are kept in their places, but when it strikes the bottom, the wire becomes slack, and is thrown off the spring, [releasing the weights, so that when the line is hauled in again, the cylindrical rod withdrawn from the weights is the only part of the apparatus that returns, bringing with it a small portion of the sea bottom, retained in its lower part by an arrangement of valves which it is unnecessary to describe. The sounding line employed with this apparatus was specially made for this expedition, and though it was only 0-8 inch in circumference, its breaking strain was 12 cwt. It was marked at intervals of 50 fathoms, and kept for use wound on a large drum.
The apparatus employed for temperature determinations was the result of much forethought and of many experiments made by the Physical Committee of the Royal Society. The experience of former temperature-soundings tended to show that ordinary self-registering thermometers were liable, among other errors, to indicate too high a temperature, from the pressure of the sea-water at great depths (which amounts to one ton per square inch for every 800 fathoms) slightly compressing the hulb and forcing the mercury, or spirit, too high up in the column. Many expedients were tried to protect the thermometers from this source of error, and ultimately the plan suggested by Professor W. A. Miller, Treasurer E. S., was adopted. A figure of the instrument, and a detailed description of it, will be found in the Proceedings of the Royal Society, June 17, 1869. It consists of the ordinary form of Six's self-registering thermometer, the bulb of which is surrounded by an outer bulb, hermetically sealed, and the intervening space partly filled with spirits of wine. A careful series of experiments made on shore with the assistance of a hydraulic press, showed that the pressure only acted upon the outer bulb, the inner indicating the true temperature, and that the difference in indications between this and an ordinary thermometer increased regularly with the pressure. These differences were carefully noted at intervals up to three tons to the square inch, and it was exceedingly interesting to find that a set of independent experiments made at sea, when the protected and ordinary thermometers were simultaneously employed, gave precisely the same differences between the two instruments when the pressure was calculated at the rate of one ton per square inch for every 800 fathoms of depth. In addition to the precautions against pressure, these instruments, to the construction of which Mr. Casella paid the utmost attention, were furnished with registering indices of a peculiar kind, which fitted so tightly in the tubes that they could not be displaced by the shaking to which they were occasionally subjected, notwithstanding the most careful handling, and which could only be set by powerful magnets, with grooves cut in the end of their poles, so as to partially surround the thermometer tube. The whole instrument was protected by a copper cylinder open at each end. The expedition was provided with six of these instruments, but the two which were originally employed were found to work so entirely satisfactorily that they were used for every observation, and it was calculated that they travelled vertically up and down in the sea more than a hundred miles during the three months that they were in use.
The apparatus for collecting samples of sea-water from various depths was of very simple construction. It consisted of a cylindrical brass tube (the interior being coated with varnish, to avoid the action of the sea-water upon it), provided at each end with an accurately ground valve opening upwards. As long as this was descending vertically, the water passed freely through it, but when the motion was reversed the pressure of the superincumbent water kept the valves closed, and the water which last passed into the bottle was retained there. That "bottom water" really was brought up, the
Wattr-bottle as seen at A externally, and at B'in section; drawn to
writer had ample evidence, as from the greatest depths it-was cloudy with suspended mud, minute Globigerinas, and other Foramenifera.
The dredges with which the expedition was provided were the ordinary form of "naturalist's dredge" introduced by Ball and Forbes. They were made of wrought iron, the scrapers being pitched at a very low angle, and the arms being maveable. One of the arms was in two pieces, terminating in rings, which were fastened together by a few turns of spun yarn, so that, when the dredge got fouled by rocks, as occasion^ ally happened in depths less than 500 fathoms, the strain upon the dredge broke the "stop," the dredge changed its position, and freed itself. The. loss of several dredges was avoided bj this simple precaution. For very deep dredging, the weightjn the frame was increased to about 2cwt. by the addition of plates of iron. The bag was double, the outer being a close net of sounding-line, the inner a piece of " bread bag," a somewhat open canvas, and was so arranged as to be readily detached from the frame, and washed, to avoid the possibility of mixing the results of one dredging with another. The animals &c. brought up in the dredge were readily separated from the mass of ooze or mud in which they were embedded by a series of graduated sieves, placed one within another, the sieve of largest diameter and smallest mesh being at the bott jm of the series. When the whole was agitated in a tub of water, the impalpable mud speedily passed through all the sieves, and the animals were deposited upon them according to their size.
The dredge-rope was the best Chatham "hawser-laid " rope that could be made; two sizes were employed—2 inches and 2£ inches in circumference respectively. There was an admirable arrangement for stowing it on deck, by which its manipulation was rendered very easy, which is represented in the full-page plate. A number of pins, about 18 inches long, terminating in white balls, projected inwards from the bulwarks, and on these pins the rope was loosely coiled. In this way, 3,000 fathoms of rope (nearly 3£ statute miles), weighing nearly 2£ tons, were placed within easy reach on the deck.
In order to haul in the great lengths of sounding-line and dredge-rope, as well as the dredge with its contents, a small donkey-engine was provided and placed on deck between the paddle-boxes. The experience of the 1868 expedition, in H.M.S. Lightning, had shown that a double-cylinder engine was almost essential, so that by the alternate action of the two pistons the application of the power might be uninterrupted. The engine on board the Porcttpine fully answered the expectations formed of it, and in almost every case, whether it was sounding-line from a moderate depth, or the dredge and its contents with 3,000 fathoms of rope attached, the hauling-in was performed at the rate of one foot per second, or 600 fathoms per hour. Occasionally, in fine-weather soundings, a drum of greater speed was attached to the engine.
A very important adjunct to the dredging and sounding apparatus remains to be described. It is figured in the following cut, and its position on board the vessel is clearly seen in the full-page plate. It was called the "accumulator," and the twofold object of the contrivance was—to indicate any sudden and undue strain upon the dredge-rope or sounding-line, and to relieve it in a measure—automatically, as it were. The dredging was carried on at both ends of the vessel, a similar contrivance to that seen in the stern being fixed near the bow.