ECHO-SOUNDING

Evening Post, Volume CXX, Issue 123, 20 November 1935, Page 21

ECHO-SOUNDING

REVEALING DEPTH

HOW MODERN SHIPS WORK

PASSING OF THE LEAD

A British salvage company has undertaken the task of locating the Lusitania —torpedoed off the southern coast of Ireland on May 7, 1915, with a loss of more than 1000 lives—and of blasting her open to recover what may be valuable within her, says a writer in the "New York Times." She lies at a depth which may be anywhere from 350 to 400 feet. To find her the salvagers are resorting to the echo-method —an invention which originated in the United States more than thirty years ago in an effort to make navigation safe and which has since been so highly developed that every important transatlantic liner and every warship carries it. The principle of the invention is simple. Sound travels in water at a fairly constant of 4900 ft a second. Strike a bell or fire a charge of explosive under water and the sound will travel in all directions. Some of the sound waves obviously will be reflected by the bottom. Hence there will be an echo. By timing the interval between the emission of the sound and the arrival of the echo it is easy enough to estimate the depth of the water. This broad principle was suggested as early as 1807 by Arago, but it took over a century to apply it practically. Dozens of experimenters failed, including Lieutenant M. F. Maury, who made some unsuccessful tests in 1855. RESULT OF EXPERIMENTS. Curiously enough, success came as the result.of an attempt to utilise the high conductivity of water for sound to make navigation safe in fog. The famous experiments conducted by Tyndall in England and Professor Joseph Henry in this country had showii conclusively that the air trans-> mits the blasts of fog horns and the explosion of bombs and the shrilling of whistles erratically. There are zones of silence in which nothing is heard. Hence the British rule: "Never trust i-nplicitly an air sound signal that you can't see." In water there are no lapses. The sound is always heard, because water does not vary much in density and because it is unaffected below the surface by wind. Attracted by the possibilities, Arthur J. Mundy, of Boston, began to experiment thirty-odd years ago. He found that if a small tank filled with water were placed below the water line, with | the steel of the ship constituting one wall of the tank, it was easy to receive the sounds of a bell struck some distance awas'. While this was a great advance it was not enough. There was no way of determining the exact location of the bell. Mundy having retired from the company that had engaged him to perfect the invention, Josiah B. Millet was commissioned to proceed. By the simple expedient of mounting two of Mundy's tanks on the ship's bottom, at opposite sides of the keel, the problem of determining the direction from which the sound of a bell came was solved. Thus was the double hydrophone invented. By listening first to one hydrophone and then to the other an officer could compare intensities and tell on which side of his ship the warning bell lay. He could head straight for the bell simply by swinging his ship this I way and that until the bell sounds were equally loud. If the sound waxed as he proceeded he knew that he was nearing the bell; if it waned he knew that he was drawing away. A SAFE COURSE. By modifying the construction of the hydrophones so that they resembled a turtle's ear and by connecting them with telephones to the bridge or chart room, Millet made it possible for the navigating officer to steer a .safe course through fog even off a dangerous shore.' In 1904 the Kaiser Wilhelm II of the North German Lloyd was equipped with the improved hydrophones—the first of a long list of liners. She created something like a sensation when, in 1906, she steamed past the Weser Lightship at ten knots into her harbour while other vessels were unable to proceed in a dense fog. The loss of the Titanic in 1912 did much to stimulate interest in submarine signalling.' By that time scores of ships had been equipped with the necessary apparatus, and the more dangerous coasts and important harbours had their bell stations—usually lightships. Something of the efficiency of submarine signalling by sound may be gathered when it is recalled that in 1909 the Baltic, coming from Europe, picked up the Nantucket Shoals bell, laid her course for New York, and proceeded eighty miles when she received a wireless message that the Republic was in distress. The Republic had also picked up the Nantucket bell and with its aid given her position. It was entirely by means of the bell that the Baltic was able to find the Republic after twelve hours of zig-zagging. If a lightship were to send out a radio signal and an audible underwater signal simultaneously, the master of a ship would have no difficulty in calculating his position in a fog, argues Mr. Millet. The radio waves arrive with a speed of light, which means almost instantaneously; the underwater sound signals a little later. The difference in time makes it possible to calculate the distance of the lightship accurately. What is more, it is hot even necessary to have hydrophones on board if the underwater signal comes from a powerful oscillator. NO MEAN TASK. Striking a bell hard so that it will send out a sound loud enough to be picked up by hydrophones from three to ten miles away is no mean technical task. The reason is the virtual incompressibility of water. The late Reginald A. Fessenden, who had been engaged as a technical expert by the Boston company which saw possibilities in submarine signalling, struck out along new lines about 1912. Out of his work came echo-sounding devices carried by every large ship. Fessenden applied his knowledge of radio and telephony to advantage. He devised an oscillator, that is, a diaphragm which could be electromagnetically vibrated to generate a musical hum of any pitch and of almost any intensity. He introduced amplifiers in the hydrophone circuit, like those to be found in any radio set. Thus he could enormously magnify the feeblest sound picked up. Of course the sounds were changed into electric impulses, just as they are in any telephone. Thus powerfully amplified, the impulses could operate a telephone receiver on the bridge or | cause an indicator to swing on a dial. j With this apparatus Fessenden found 1 that he could send out sound waves which were echoed back by icebergs and by the bottom'of the sea. By calculating the time that it took for the echo to reach him he could determine distances. Thus originated what in this country is called the fathometer. There are two systems of echosounding. One is the sonic and the other the super-sonic. In the sonic audible sounds are produced and in the super-sonic inaudible ones. An audible sound is one Ihut may run as high, as 20,000 sdbratious a

second (10,000 cycles)—something much higher than the finest flageolet tone of a violm and barely audible to the most sensitive ear. To produce a sound of low pitch a bell may be struck. For notes, of a medium pitch a Fessenden oscillator is necessary. THE OTHER SYSTEM. In super-sonic systems an oscillator is also used. But the note consists of vibrations which run from 30,000 to 40,000 cycles (60,000 to 80,000 vibrations a second), and which cannot be heard by any human ear. The British Admiralty drops as low as 16,000 cycles a second and thus manages to combine some of the advantages of both sonic and super-sonic methods. The super-sonic system, at least in the French Langevan-Chilowsky form, is good for only 750 fathoms. This is no hardship if it is necessary merely to determine navigable depths. On the other hand, charting the ocean bottom, whether shallow or deep, requires a much more responsive and delicate method of picking up echoes than the masters of merchant ships require. Perhaps the most sensitive of these scientific echo-sounders is that devised for the United States Coast and Geodetic Survey by Dr. Herbert G. Dorsey. His fathometer takes twenty soundings a second at depths of 9 to 120 feet, and the readings will not be more than one inch out of the way. At a cruising speed of ten miles an hour a sounding is therefore obtained for every ten inches of the bottom. Supersonic vibrations are utilised. The time that it takes a sound to travel from the hull to bottom at a depth of only twelve feet and return—a total distance of but twenty-four feet—is one five-thousandth of a second. An older deep-water fathometer of Dr. Dorsey"s sounds depths from 15 to 3000 fathoms. Echo-sounding will ultimately displace the measuring depths by heaving a lead overboard, hauling it in and then calculating the depth by measuring the length of line paid out. At best the old way is slow. It takes about ten minutes. Nowadays the captain of a ship (the Manhattan, Washington, Europa, for instance) looks at a dial and at once reads off the depth beneath his keel Just as he reads time by the clock. Both the oscillator and the amplifier are under his control. When he wants to know his depth he has but to throw a switch. Immediately a red light travels round a dial and flashes out at the moment the echo is received. He sees twenty-four flashes a minute. So far as sounding is concerned, the fathometer is perhaps the finest contribution to increased safety in navigation made in the last twentyfive years.