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Sounding - Encyclopedia

GEOGRAPHICAL NAMES

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SOUNDING (for derivation see Sound above), the term used for measuring the depth of water (and so, figuratively, of anything). The process of ascertaining the depth of the sea has been practised from very early times for purposes of navigation, but it is only since the introduction of submarine telegraphy that extensive efforts have been made to obtain a complete knowledge of the contour of the ocean-bed (see Ocean).

The operation of sounding is readily performed in shallow water by letting down a weight or "lead" attached to a cord, which is marked off into fathoms by pieces of leather, rag and twine. The bottom of the weight usually presents a hollow, which is filled with tallow, so that a portion of the material from the bottom may be brought up and give an indication of its nature as well as an assurance that it has really been touched.

For depths over 20 fathoms sounding machines are often employed, and for deep soundings they are practically indispensable. In them wire, the use of which for this purpose was introduced by Sir William Thomson (Lord Kelvin), has entirely superseded hemp gear. Its smooth surface and minute section, reducing friction to a minimum, give a rapidity of descent of about ioo fathoms per minute, and this velocity is not materially diminished even at great depths. Reeling in may be accomplished at nearly the same rate. Soundings are thus obtained with a degree of accuracy not formerly possible. The apparatus is light, compact and automatic in its action. Soundings with wire can be carried out at night with the same facility as in daytime, and in almost any circumstances of wind and weather short of a strong gale, against which the ship could not steam or face the sea. A sounding of 1000 fathoms may be obtained in twenty-five minutes from the time the weight is lowered to the time the order is given to put the ship on her course, or in half that time if sounding from astern and going ahead on getting bottom; 2000 fathoms will require forty-five minutes and 3000 fathoms seventy-five minutes. Beyond that depth, much greater caution being required, the time occupied is correspondingly increased, and reeling in must then be done very deliberately. A sounding of 5269 fathoms was obtained near the island of Guam by the U. S. cable-surveying ship "Nero." Soundings at such depths may occupy as long as five or six hours.

Among the sounding machines in general use the Lucas carries nearly 6000 fathoms of 20-gauge wire, and is fitted with two brakes - one a screw brake for holding the reel when required, the other an automatic brake for stopping the reel when the weights strike the bottom. A guider for the purpose of winding the wire uniformly on to the reel is also attached, and is worked by a small handle. After leaving the reel the wire passes over a registering wheel, the dial of which indicates the amount of wire run out. Similar machines of smaller size are supplied for use in boats. The large machine is represented in fig. 1.

Heaving in is accomplished by means of a hemp "swifter" or dri ing belt, which conveys the motion of the drum of a donkey engine to the drum carrying the wire of the sounding machine. It being impracticable to regulate the speed of the engine by hand according to the heave of the ship, in order to obviate the sudden and excessive strains on the wire so caused, an ingenious mechanical arrangement has been fitted by which frictional disks, geared by cog-wheels and capable of adjustment are interposed on the axle connecting the grooved wheel actuated by the hemp swifter and the revolving drum carrying the wire. By this arrangement the latter can be controlled as desired, both in speed and direction of motion, by means of a lever regulating the strap on the frictional disks, which may be set by experiment to act at any given tension of the wire. As the tension approaches this limit, the velocity of revolution of the drum is automatically checked; and if the tension further increases, the motion of the drum is actually reversed, thus causing the wire to run out, until the tension is relieved sufficiently to allow the frictional disks again to. act in the direction of heaving in. The drum may be stopped instantly by moving the lever in the proper direction to throw the apparatus out of gear.

Galvanized-steel wire of 20 -gauge and 21 -gauge is supplied on drums in lengths of 5000 fathoms. The 20 -gauge wire when new has a breaking strain of 240 lb, and the smaller wire 190 lb. The large machines will hold sufficient quantity of the larger wire for the deepest soundings; there is therefore no longer any necessity for the smaller wire, and its use is not recommended. The zinc wears off to a considerable extent with constant use; it is necessary to pass the wire through an oily wad whenever soundings are suspended for a time, and the surface layers on the drum should be kept well coated with oil and covered over with oily waste. A fortnight's continuous use is about the limit to the trustworthiness of any piece of wire; no amount of care will prevent it from becoming brittle; and directly it can be snapped by twisting in the hand, it should be condemned and passed on to the boats' machines. A magnifying glass will assist in examining its condition. Taut and even winding on the reel from the drum is most important; otherwise, when heaving up after a sounding, the strain forces each layer as it comes in to sink down amongst the previous layers loosely reeled on, with the result that at the next sounding slack turns will suddenly develop on running out, to the great risk of the wire. The wire is liable to cut grooves in the interior of the swivelling frame; a file must constantly be applied to smooth these down, or they will rip the splices. A roller of hard steel, underneath which wire passes, and which placed in rear of the swivelling frame, obviates this to a great extent.

Splices are made about 5 ft. in length, one wire being laid round the other in a long spiral of about one turn per inch. A seizing of fine wire is laid over each end and for 2 or 3 in. up the splice, no end being allowed to project, and solder is then applied the whole length of the splice. Three more seizings should be placed at intervals. Splices are the weakest parts of the wire, and their multiplication is to be avoided. They should be frequently examined and their position noted, so that in heaving in they may be eased round the wheel with the guider nearly in the centre, to avoid tearing.

Under 1000 fathoms a lead of 30 to 40 lb weight can be recovered, and no detaching rod is necessary. At a little risk to the wire, when sounding froth astern up to that depth, the ship may go ahead directly bottom is struck, increasing speed as the wire comes in; the great saving in time thus effected will often justify the increased risk of parting the wire. For greater depths the "Driver rod" is the best detaching apparatus for slipping the sinkers; its construction is easier than that of the "Baillie rod," and with a piece of gas piping cut to the proper length the ship's blacksmith can make one in a day. Both rods are fitted with tubes to bring up a specimen of the bottom, and the same sinkers fit them both.

The "Driver rod" is shown in fig. 2. ABC is a tube about 2 ft. in length, fitted at the top with a flap valve D, working on a hinge at E. The lower part of the tube C screws on and off, and contains a double flap valve to retain the bottom specimen. The sinkers WW, each 25 lb in weight, conical in form, and pierced with a cylindrical hole through which the Driver rod passes loosely, are slung by wire or cod line secured to a flat ring or grummet shown at L and passing over the stud G. A stud K on each side of the tube fits loosely into the slot H in the lower part of the slipping lever MH. The weight of the apparatus being taken by the sounding wire, the sinkers remain suspended; but on striking the bottom, the wire slackens, and the weight of the sinkers drags the slipping lever down till the stud K bears against the upper part of the slot H. By this action the point M of the slipping lever is brought to bear against the upper end of the standard EF, being -thereby forced outward sufficiently to ensure that the weight acting at the point G will tilt the slipping lever right over, and thus disengage the sling. The tube being then drawn up, the sinkers are left behind. In descending, the valves at top and bottom, opening upwards, allow the water to pass through freely; but on drawing up they are closed, thus retaining the plug of mud with which the tube is filled. For water under 2000 fathoms two conical weights are sufficient. In deeper water a third cylindrical weight of 20 lb should be put between them. It is important to interpose a piece of hemp line, some 10 fathoms long, between the end of the wire (into which a thimble is seized) and the lead or rod. This tends to prevent the wire from kinking on the lead striking the bottom. A piece of sheet lead, about 2 lb in weight, wrapped round the hemp just below the junction, keeps the wire taut while the hemp slacks. Small brass screw stoppers, fitted with a hempen tail to secure to a cleat, hold the wire during the sounding if necessary to repair splices or clear slack turns. In heaving in the springs are replaced with a spring balance, by which the amount of strain is seen and the deck engine worked accordingly. A system of signals is required by day and by night, by which the officer superintending the sounding can control the helm, main engines and deck engine.

The machine is placed on a projecting platform on the forecastle. An endless hemp swifter, led through blocks with large sheaves, connects the sounding machine and deck engine, and when heaving in is kept taut by a snatch block set up with a jigger. As the wire runs out, the regulating screw of the brake must be gradually screwed up, so as to increase the power of the brake in proportion to the amount of wire out. The regulating screw is marked for each 500 fathoms. In fairly smooth water the brake will at once act when the weight strikes the bottom and the reel stops. Under 3000 fathoms one spring only is sufficient, but beyond that depth two springs are required. If the ship is pitching heavily, the automatic brake must be assisted by the screw brake to ensure the reel not overrunning. The marks on the regulating screw are only intended as a guide; the real test is that the brake is just on the balance, so as to act when the strain lessens, which may be known by the swivelling frame being just lifted off the stop. As the wire weighs 72 lb for each 500 fathoms, the 500fathoms mark on the screw should be at the position in which the screw has to be to sustain a weight of 72 lb; the moo-fathoms mark, 15 lb; and so on. This can be tested and the marks verified.

Handling the Ship. - Sounding from forward enables the ship to be handled with greater ease to keep the wire up and down, and especially so in a tide-way; but in very heavy weather soundings may be obtained from a machine mounted over the stern, when it would be quite impossible to work on the forecastle. The spanker must be set with the sheet to windward, unless a strong weather tide renders it undesirable; the ship's head must be kept in a direction which is the resultant of the direction and force of the wind and current; and this is arrived at by altering the course while sounding, point by point, until the wire can be kept up and down by moving the engines slowly ahead as necessary. It should seldom, or never, be necessary to move the engines astern.

The temperature of the water is usually taken at intervals of 1 00 fathoms down to a depth of moo fathoms, and at closer intervals in the first too fathoms. If a second wire machine is available, the observations may be made from aft whilst the sounding is being taken forward. A 30-lb sinker is attached to the end of the wire, and the registering thermometers are secured to the wire by the metal clips at the back of the cases, at the required intervals. To avoid heavy loss, not more than four thermometers should be on the wire at one time. When sounding a thermometer is usually attached to the line a short distance above the lead.

The primary object of the machine called the "submarine sentry" is to supply an automatic warning of the approach of a ship to shallow water: it has been instrumental in discovering many unsuspected banks in imperfectly surveyed waters. By means of a FIG. 2. - Driver Rod.

single stout wire the sinker, an inverted kite, called the "sentry ," can be towed steadily for any length of time, at any required vertical depth down to 40 fathoms with the red kite and 30 fathoms with the black kite; should it strike the bottom, through the water shallowing to less than the set depth, it will at once free itself and rise to the surface, simultaneously sounding an alarm on board, and thus giving instant FIG. 3. - The Submarine Sentry.

warning. The vertical depth at which the sentry sets itself when a given length of wire is paid out is not changed by any variation of speed between 5 and 13 knots, and is read off on the graduated dialplate on the winch. One set of graduations on the dial indicates the amount of wire out; the other two sets refer to the red and black kites respectively, and show the depth at which the sentry is towing. By this machine single soundings down to 40 fathoms can be taken at any time while the ship is under way. The sentry being let down slowly, the gong will indicate when the bottom is touched, and the dial corresponding to the kite used will show at once the vertical depth at the place where the sentry struck.

By removing the kite and substituting a lead, with atmospheric sounder or other automatic depth gauge, flying single soundings up to IOO fathoms can be obtained in the ordinary manner without stopping the ship. The winch is secured to the deck a short distance from the stern; the towing wire passes from the drum under a roller fairlead at the foot of the winch, thence through an iron block with sheave of large diameter, suspended from a short davit on the stern rail and secured to the sling of the sentry. The dial being set to zero with the sentry at the water's edge, the ship's speed is reduced to 8 or 9 knots, and the wire paid out freely until the kite is fairly in the water, when the brake should be applied steadily and without jerking, veering slowly until the required depth is attained, when the pawl is put on the rachet wheel and the speed increased to 12 knots if desired when using the black kite or pp knots with the red kite.

The kite in its position when being towed is indicated in fig. 3. The point of the catch C, passing through a thimble M in the short leg of the sling, is slipped into the hole at the top of trigger T, which is hinged at K and kept in its place by the spring S attached to the hook H. On the trigger striking the bottom the catch is released, the short leg of the sling slips off, and the sentry, which then rises to the surface, is left towing by the long leg. The winch is fitted with two handles for heaving in the wire; one gives great power and slow speed, and the other, acting on the drum spindle direct, winds in quickly. The wire supplied with the machine has a steady breaking strain of about moo lb. Using the black kite at a speed of 7 knots, the strain on the wire is about 150 lb, and at 10 knots about 300 lb. The red kite increases the strain largely. (A. M. F. *)