Popular Science Monthly/Volume 19/May 1881/Deep-Sea Investigation
By J. G. BUCHANAN, F. R. S. E.,OF THE CHALLENGER EXPEDITION.
THE first problem of deep-sea investigation is to determine the extent of the ocean, its size, its volume. The superficial extent and limits are determined by the surveyor. In order to map out the bottom of the sea, there is only one method, namely, the direct determination of the depth at as many places as possible. When a ship is "in soundings," the depth is ascertained by the ordinary hand lead-line, which is from twenty to twenty-five fathoms long, and is conventionally marked at stated intervals with bits of leather, white, red, and blue bunting, and knots. The lead is a long, finely-tempered block, generally weighing fourteen pounds, which has a recess at the thick end, and is perforated at the other end for the reception of the line. This instrument is chiefly used while the vessel is in motion. The leadsman swings the lead vigorously, so as to give it momentum enough to carry it well in advance of the ship before it touches the water. It sinks rapidly while the leadsman's position is advancing to the spot where it touched the water. The depth is ascertained by looking at the marks on the line. This method is effective and correct enough for ordinary purposes, in depths of not more than twelve or fifteen fathoms. Accurate soundings may be obtained by reducing the speed of the vessel as much as possible, in depths which do not much exceed thirty or forty fathoms. In ocean-water, where depths of two or three thousand fathoms are met, the vessel must be kept stationary, and heavier weights than are found sufficient for shallow soundings must be employed.
Deep-sea soundings have received much attention during the last thirty years. The first attempt at them appears to have been made by Captain Constantine John Phipps, during his Arctic Expedition in 1773. He sounded a depth of six hundred and eighty-three fathoms with a lead weighing one hundred and fifty pounds, which appears to have sunk about ten feet into the mud. Determinations of the temperature of the sea water and of its density were made at the same time. Captain John Ross employed, during his Arctic voyage of 1818, one of the earliest satisfactory instruments for bringing up a considerable quantity of the bottom mud in deep water, with which he was able to ascertain the temperature at any depth.
A contemporary of Ross, the younger Scoresby, observed that, when in sounding at great depths the ordinary deep-sea line and lead are used, the increasing weight of line, in proportion as more of it is required, renders less certain the determination of the moment when bottom is reached. He has also left the record of the first observation of the effects of the enormous pressure which is acting under the deep waters. The Americans have introduced the method of using fine twine and a heavy weight, both of which may be sacrificed at every sounding, to obviate the inconveniences arising from overweight of rope. The practice of observing the rate at which successive equal lengths of line pass out has been found useful in cases where ordinary observation or feeling does not suffice to indicate when the shot has reached the bottom. Iron wire was first used instead of twine about 1850, by Lieutenant Walsh, of the United States schooner Taney.
When the surveys for telegraphic cables were begun, it became important to ascertain the nature of the ground at the bottom. The apparatus invented by Midshipman John Mercer Brooke, of the United States Navy, in 1854, answered this purpose. It consisted of a cannon-ball with a hole drilled through it. Through this hole passed a straight rod. fitted at its upper end with peculiar disengaging hooks. The weight was slung to these hooks by means of a wire which passed from a ring slipped over the rod under the weight, up on each side of the cannon-ball to the hooks. The sounding-line was attached to eyes in these hooks, and, as long as the lower end of the rod was not resting on anything, the weight was kept securely in its place, and was available for taking out the sounding-line. As soon, however, as bottom was reached, and the rod came to be supported on its lower end, the hooks at the upper end fell forward, and allowed the wire to disengage itself. The weight was thus released, and, on the line being pulled up, the rod came away through the perforation of the shot, and brought with it specimens of the mud in small quill tubes fitted in a recess in the lower end of the rod. This apparatus has been improved by substituting a tube for the rod, and so arranging the attachment of the weight that it shall continue till the hauling in is begun, whereby its mass and momentum are available for forcing the tube as deep into the ground as possible. Captain Shortland devised another modification of the apparatus in 1868, for the soundings between Bombay and Aden. The essential part was the insertion of two butterfly valves in the lower end, and two conical valves opening upward in the middle of the tube, between which a sample of the bottom water is secured, while a specimen of the mud is brought up in the lower segment of the tube. It was used with general satisfaction during the first year of the cruise of the Challenger. The chief objection to it was founded on the smallness of the samples of bottom which it brought up. This machine, the "Hydra," was replaced after the first year by the "Bailey," an apparatus having a larger tube fitted to bring up more considerable samples of mud.
An apparatus which the author has devised for sounding the Scottish lakes, and found to act well, consists of a straight brass tube an inch in diameter, carrying a shoulder about one foot from the lower end. A cylindrical leaden sinker of suitable weight is slipped over the upper end, and rests on the shoulder. The line is made fast to an eye at the top of the tube, and the part of the tube below the shoulder can be unscrewed, and the mud which it has brought up squeezed out. The tubes bury themselves readily in soft mud and clay, and bring up considerable samples.
It is necessary, in making a sounding in deep water, to load the end of the line with such a weight that in the deepest water that may be reasonably expected the velocity of descent shall not be diminished to an excessive extent by the friction of the increasing length of line in passing through the water. Wire has been largely employed for the line, and has great advantages in this respect over hemp. For example, in water of fifteen hundred fathoms a sinker weighing three hundred-weight is twenty minutes in reaching the bottom, with the best hempen sounding-line; while with wire and a sinker of thirty pounds the sounding may be completed in from twenty-five to thirty minutes. Wire, however, is less flexible than hemp, and breaks under the influence of kinks and twists, which do not affect the strength of hemp in any degree. The balance of advantages is in favor of wire, but it is well to have ropes of both kinds.
The anchor used by the author for holding his vessel in place, during his explorations on the west coast of Scotland during the summer of 1878, brought up so many fine specimens from the mud in which it
sank before taking hold on the bottom, that he determined to provide himself with one which should retain the mud. For this purpose he had an anchor made with an open frame, instead of a solid bar connecting the two palms, to which was laced a stout canvas bag, into which any mud sticking to the palm at the moment of its breaking out of the ground would fall (Fig. 1). The instrument proved a useful one for exploring the bottom, particularly when the object was to collect the mud itself rather than the things living on its surface, and was, moreover, efficient as an ordinary kedge-anchor.
Doubts have sometimes been thrown on the trustworthiness of deep soundings with the line and heavy sinker. First, it was asserted that under some great pressure the density of water would become equal to that of lead, and the sinkers would float instead of sinking. This might be the case were water as compressible as air, and if the lead could escape compression; but the amount of pressure that will double the density of air will increase that of water by only one twenty-thousandth part, and it would require the pressure of more than two hundred thousand atmospheres to squeeze water to the density of lead. The deepest water, five thousand fathoms, is not subject to a pressure that can raise its density as much as one-twentieth part. Moreover, the weight of lead is increased by pressure much faster than that of water, so that, however dense the water may be, it would have to encounter a still denser lead. This objection, however, fallacious as it has been shown to be, has been admitted by persons of high authority, of course without sufficient thought.
A more real but exaggerated objection to the trustworthiness of deep soundings is founded on the existence of currents likely to cause deviations in the direction of the line, and to change the position of the ship. There is no doubt concerning surface-currents; they are observed and measured every day, and form an important factor in the navigator's daily reckoning. It has been inferred that they may be complemented by return under-currents which will be harder to deal with because they can not be so easily detected and measured. Soundings taken in the presence of such currents are, it must be admitted, less to be relied upon than those taken in manifestly quiet waters; but the extent of under-currents has been very much exaggerated. By far the greater part of the ocean is, for sounding purposes, practically still water. The surface-currents of any importance are easily recognized, and so also are the under-currents. Just as a physician can, by bringing his experience to bear on the sounds transmitted to him by the stethoscope, divine what is taking place inside the body of his patient, so the experienced seaman can, by observing the behavior of his sounding-line, form a fair diagnosis, of what is taking place in the depths of the sea. When the sinker passes into a belt of under-current, the fact is very soon apparent; but, even with the greatest care, soundings taken under such circumstances are of doubtful value, unless bottom is brought up. In the latter case, we know the depth is not greater than the length of line used, and a correction, suggested by observation and experience, may be applied, which will bring our estimate of the depth very near the truth. It is evident that this can not be satisfactorily done by the sounding-line alone, and it early occurred to those who thought on the subject that the method which promised most success was that which should give the depth in terms of the height of the column of water; in other words, the barometrical measurement of altitudes was extended from the land to the sea. The instruments which have been suggested for this purpose are constructed with a view to record the amount of compression produced on a given mass of a certain elastic substance. From the known law regulating the variation in volume of the substance with variation in the pressure, the maximum pressure to which the instrument has been exposed can be deduced, and from the known density of the water the height of the column of it which would produce that amount of pressure can be calculated; this height represents the depth to which the instrument has been sunk. Perkins, about 1812, constructed a piezometer, or instrument for measuring pressure, consisting of a glass tube sealed at one end, filled with water, and inverted in a, cup of mercury. A steel index placed within the tube rose with the mercury, and was retained by a spring at the highest point reached. Instruments made on this principle were used by him, by Aimé in the Mediterranean, in 1848, and by the United States Coast Survey a few years later. Essentially the same instrument, with certain convenient practical modifications, was used by the author in the Challenger Expedition.
Soundings from vessels in motion may be taken with Massey's machine, in which the friction of the passing water as it sinks causes a screw-fan to make rotations which are registered by an index. Sir William Thomson has proposed the use of a glass tube, sealed at one end, and coated internally with a chemical preparation, the color of which is changed by the action of sea-water. The sea-water forces itself in as the tube sinks, changing the color of the coating to an extent from which the depth may be calculated. Each of these instruments is good for only one sounding.
The author has patented a device by which the depth of compression to which an inclosed mass of air has been subjected is measured by the water which has gained admittance to the instrument. It is represented in Fig. 3. It consists of a glass tube open at both ends, but capable of being closed by a stopper or other means. At some part of the tube a spout is introduced, and the tube is bent over through two right angles immediately above it. When the instrument is to be used, the end is closed, and the line let go; when bottom has been reached it is brought up again, and we find that a certain amount of water has lodged in the lower part of the tube.Fig. 3. & Fig. 4. It is evident that, as the instrument descends and the air in it is compressed, the water forces its way in through an orifice, and past the spout. This spout is so formed that it delivers the water against the walls of the tube, down which it runs, and collects at the bottom. When the motion of ascent begins, the air, by its elasticity, tends to recover its original volume, and expands in the direction of greatest freedom. Now, all the water which has entered has collected below the spout; consequently, in reëxpanding, this water will be left undisturbed.
Assuming that the volume of the mass of air in the instrument varies inversely with the pressure to which it is subjected, we require, in order to be able to construct a scale for our instrument, and so to interpret its results, to know the total volume of the tube, the volume of the part which I call the vestibule, the dimensions and volume of the narrow tube, and of the wide one.
Fig. 4 represents an instrument modified so that it can be used either for great or small depths, according as either end is closed. Mr. Hunt, of the United States Coast Survey, has invented an apparatus consisting of an air-tight bag, made of flexible material, with a long, flexible tube attached to it. The bag, being filled with air, is sunk to the bottom (in a moderate depth of water), while the other end of the flexible tube is connected with a Bourdon's pressure-gauge in the ship or boat, the observation of which gives an exact profile of the bottom as the bag is towed over it.
Bottom temperatures may be measured by common thermometers protected so as to be uninfluenced in coming up through the warmer upper strata of water, by bringing the water to the surface and taking its temperature, or by self-registering thermometers, such as Cavendish's and Six's. A great amount of ingenuity has been displayed in the invention of machines for registering the actual temperature of the water at any given depth, independently of that of the water above it, all of which require some assistance from the observer in bringing about a catastrophe which shall leave its mark on the condition of the instrument.
All the self-registering thermometers are liable to error from the effects of pressure, which may amount to five or six hundred atmospheres on the outside of the instrument, while inside it is never greater than was that of the atmosphere when the tube was sealed up. Attempts to obviate them have been made by placing the thermometers or their bulbs in protecting inclosures, and by the device of leaving the instrument open at one end. Fig. 5. This was adopted by Aimé in some of his experiments, when the effect of pressure on the apparent volume of the liquid was determined independently, and a correction applied accordingly. The author has devised and constructed a mercurial thermometer, or piezometer (Fig. 5) on the same principle, but his object in admitting the water-pressure to the inside of the instrument was to utilize it in shifting the scale of the thermometer as the depths changed. The thing registered in such instruments is always the apparent volume of the liquid, and this varies with the temperature and the pressure. Hence the indications will represent the sum of the effects of the change of temperature and of pressure. If from any independent source we know either of these, we can determine the other. In a sea of uniform temperature throughout its depth, the apparent volume of the liquid would diminish as the pressure increased, and, if the temperature increased with the depth, the apparent volume of the liquid would diminish at a slower rate; but it would be always possible to determine the true temperature as long as it did not increase at so great a rate as to dilate the liquid more than it was compressed by the increasing pressure. For the investigation of seas such as the Mediterranean, this form of instrument is most valuable. No one instrument, however, fulfills all the conditions required of a perfect deep-sea thermometer, and the investigator must use his judgment in selecting the one or more best suited to his particular purpose.
The water from the bottom is usually collected in the so-called "slip" water-bottle. Water from intermediate depths is obtained in an instrument represented in section in Fig. 6. It consists of a cylinder, A, terminated at both ends by similar stopcocks, B, B, which are connected by the rod C. This rod carries, near its upper extremity, a piece of stout sheet-brass, D, ten centimetres long by fifteen broad, soldered to the casting E, which is movable about the axis e.
When intermediate water is to be obtained, the water-bottle is firmly attached to the sounding-line, which carries at its end usually a fifty-six pound or one hundred-weight lead; the stopcocks are then opened, giving them, with the rod C, the position represented in the figure. During the passage of the bottle downward, the water courses freely through it, being considerably assisted by the conical end-pieces K K. When the requisite depth is reached, the line is Fig. 6. checked and is finally hauled in. Under the pressure of the hauling, the flap D falls down into an horizontal position, when it is caught by the movable piece of brass F, which moves round an axis, f, and is supported on the side opposite to E by the rod G, which rests on the spiral spring H. The water rushing past D, when thus in an horizontal position, exercises a sufficient pressure upon the rod to close the stopcocks B, B. When the speed with which the bottle is hauled through the water is increased, the pressure on D becomes so great that it overcomes the tension of the spring H, and E passes the catch F, when the rest of the journey upward is performed with the flap D hanging down, and therefore offering the least possible resistance to the water. When the water-bottle has been brought up, it is only necessary to substitute for the lower funnel a small nozzle, by which the water may be drawn off, and the instrument be made ready for immediate use without having: to detach it. It has been ascertained by experiment that the water obtained by this instrument is an average of the last two fathoms through which it has passed.