Popular Science Monthly/Volume 44/January 1894/How the Sea Is Sounded
|HOW THE SEA IS SOUNDED.|
IT was not until long after astronomers had begun to sound out the realms of space and to measure the distances and weigh the masses of the planets that the longing which has always existed in the human mind to know more of the mysteries of the sea began to be gratified. Indeed, the deep sea remained unfathomed and mysterious until after the second half of the present century had dawned upon the world; and the contemplative mariner of fifty years ago, as he looked upon the heaving bosom of the ocean and wondered at its mysteries, had nothing but myths and legends to sustain his meditation.
Under the stimulus created by the achievements in investigating the earth, the air, and the heavens attempts had already been Fig. 1.—Modern Sounding Cylinder. made to fathom the ocean both by sound and pressure, but in what sailors call "blue water" every trial was a failure repeated.
In 1856, Maury writes: "The most ingenious and beautiful contrivances for deep-sea sounding 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 explosions took place many feet below the surface, echo was silent and the sea gave out no answer. Ericsson and others constructed deep-sea leads having a column of air in them which, by compression, would show the pressure of the water to which they might be subjected, and therefore the depth. This plan was found to answer well for ordinary purposes, but in the depths of "blue water," where the pressure is equal to several hundred atmospheres, the trial was more than these instruments could stand."
Lieutenant Maury planned and constructed an ingenious sea sounding apparatus in which there was attached to the lead, upon the principle of the screw propeller, a small piece of clockwork for registering the number of revolutions made by the little screw during its descent; and it having been ascertained by experiment Fig. 2.—The Sounding Cylinder with Shot attached. 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 the dial, and the instrument was complete. It worked well in moderate depths, but failed in the deep sea on account of the difficulty of getting it down if the line used were large enough to give the requisite strength for hauling up.
Such was the state of the development of the appliances for measuring the depths of the sea in the middle of the present century, when the idea of using a heavy weight attached to a simple hempen cord was proposed. The plan of stretching a line under the strain of a weight at its lower end from the surface to the bottom underlies the method which is now universally employed for sounding the depths of the sea. In shoal water there is cast from the vessel a plummet in the form of an elongated truncated cone attached to a hempen cord which has been previously divided into feet or fathoms. The line is allowed to run out through the hands of a man who detects, by the sense of touch, the instant when the lead reaches the bottom, and reads the depth by noting the division of the line which corresponds with the surface of the water. By filling a small cavity in the base of the lead with tallow, a quantity of the sand or gravel or mud upon which the lead strikes becomes imbedded in the tallow and gives an indication of the character of the bottom soil.
The rough surface of a rope presents an obstacle to its free passage through the water, and therefore as the depths increase it is necessary to employ heavier weights to carry the line swiftly in a straight course to the bottom, and, moreover, stronger rope to bear the increased weight of the sinker. In great depths the size of the rope which is necessary is such as to present considerable surface to the action of submarine currents, which carry the line more and more out of the vertical direction in proportion to the duration of the passage of the sinker to the bottom, and render the results less and less accurate. Moreover, as the weight of the submerged portion of the rope in addition to the weight of the sinker soon becomes so heavy that a man can not lift it, and therefore can not assure himself by the sense of touch when the lead has reached the bottom; and as the weight of the submerged parts is sufficient at great depths to cause the unwinding of the reel, the Fig. 3.—The Sounding Cylinder as it is hoisted from the Bottom after the Sinker has lifted off. line may continue to pass out long after the sinker has reached bottom, and the length unwound may thus bear no relation to the depth to be measured. In addition to these sources of error there is another arising from the drift of the vessel during the period of several hours which is required to effect a deep-sea sounding with rope.
These causes, tending to carry the line off in the direction of the subsurface currents in an ever-increasing complication of loops and bends, and impeding more and more the velocity of the fall of the plummet until it sinks into the oozy soil without communicating to the surface any evidence of its arrival at the bottom, explain the reports of the vast depths of the sea that astonished the public mind less than half a century ago. Lieutenant Berryman, of the United States brig Dolphin, reported an 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 a depth of forty-six thousand feet; and Lieutenant J. P. Parker, of the United States frigate Congress, in attempting to sound the same region, let go his plummet and saw fifty thousand feet of line run out after it as though the bottom had not been reached. The deepest spot in the South Atlantic is not more than twenty thousand feet beneath the rolling waves that sealed its mysteries fifty years ago; and the deepest spot yet discovered in the world not more than twenty-eight thousand feet.
By the use of wire for sounding great depths many of the difficulties and uncertainties which characterize the use of rope are obviated, for the wire, being light in weight and of small cross-section, is not greatly affected by submarine currents, but allows the sinker to pass swiftly to the bottom. While the apparatus for sounding the sea consisted of a weight secured to the end of a hempen cord which was paid out from a simple reel on the deck of a vessel, no reliability could be attained in the measurement of depths, because the cord employed was necessarily so large as to become a controlling element in the weight of the system. But when the project for the Atlantic telegraph cable made it necessary to obtain accurate measurements of the depth of the ocean, Midshipman Brooke, of the United States Navy, took the first great step in providing means for trustworthy deep-sea sounding by inventing an implement in which the sinker, enveloping a tube secured to the sounding
line, was detached on striking the bottom and left behind when the tube was drawn up.The modern form of deep-sea sounding cylinder, which is the result of the experience of Commander Sigsbee, of the United States Navy, during his great work in developing the orography of the Gulf of Mexico, is provided with valves at the upper and lower ends which open upward, and during the descent allow
the water to pass freely through the cylinder so that it experiences a minimum of resistance. On striking the bottom, the slackening of the sounding line, which is secured to the ring shown at the upper end in the accompanying illustration, causes the trigger to spring back and release the sling that supports the detachable weight. As the lower end of the sounding cylinder sinks into the bottom a specimen of the soil forces itself through the lower valve and lodges in the interior of the cylinder. When the cylinder is hauled up the valves at the top and bottom are closed by their own weight and the pressure of the water, and the specimen is sealed until its arrival at the surface, when it is removed for examination by unscrewing the upper and lower halves of the cylinder.
In 1872 Sir William Thomson (Lord Kelvin) succeeded in adapting piano-forte wire to successful use as a sounding line in his navigational sounding machine, and a few years afterward Commander Sigsbee, besides contributing by his inventive genius most of the smaller instruments and implements used in modern deep-sea research, achieved the crowning triumph of the art in his elaborate deep-sea sounding machine, by which, while relieving the delicate sounding wire from the sudden strains to which it would otherwise be exposed by the pitching of the ship while lying to for the purpose of sounding, the profoundest depths are measured with celerity and exactness.
In this machine the wire passes outboard from the reeling drum over a guide pulley mounted on a crosshead that works between two upright guide frames. Each of the guide frames incloses a spiral spring called an accumulator, which is connected with the guide pulley by means of a rope that passes over a pulley at the top of the guide frame. If the ship is suddenly borne upon the top of a wave while the sinker is going down, instead of causing a jerking strain upon the sounding line, the stress is communicated to the guide pulley, which moves downward under the additional load and extends the accumulator springs; and, likewise, when the ship suddenly sinks into the trough of a wave, the tendency to slack the sounding line is counteracted by a rise in the guide pulley brought about by the normal tendency of the accumulator springs to contract.
A ship regularly engaged in deep-sea sounding usually has the sounding machine mounted at the after end, and when about to sound is brought to a standstill with the stern to the sea. The stray line with the sounding rod and sinker attached is over the guide pulley and carefully lowered to the water's edge, the register is set to zero, and the deep-sea thermometer is clamped to the sounding line; a seaman is stationed at the friction line which controls the velocity with which the wire is unreeled, another at the brake, and a third on the grating outside to handle the sinker and instruments and to guide the wire as it passes overboard; a machinist is at the hoisting engine, and the recorder takes a position for reading the register. When the sinker is let go, the vessel is manœuvred so as to keep the wire vertical, and the friction line is adjusted so as to allow it to descend from seventy to one hundred fathoms per minute. The instant the sinker strikes bottom,
which is unmistakably indicated by the sudden release of the wire from strain, the reel is stopped by the friction line and brake; the recorder notes the number of turns of the reel indicated by the register and determines the depth; the cranks are shipped and sufficient wire is hove in by hand to allow the end of the sounding rod to clear the bottom. Steam is then admitted to the cylinder of the hoisting engine, and the wire is reeled in slowly at first but finally at the rate of one hundred to one hundred and fifty fathoms per minute. The last ten fathoms are reeled in by hand, then the thermometer is read and the specimen of the bottom soil brought up in the sounding cylinder is examined.
In an hour this messenger of man's ingenuity makes its excursion through five miles of watery waste to the abysmal regions of perfect repose and brings to the light of day the soil with which the rain of shells of minute infusorial organisms from the upper
waters has been for ages mantling the ocean's floor. Here and there a giant peak rising from these sunless depths lifts his head to see the sky, and the dredge and trawl tell us that all along his rugged sides, and on the hills and plains below, and even in the inky blackness and the freezing cold of the deepest valleys, there is life.