Submarine Signaling and Maritime Safety
A SHIP approaches port. Thick weather prevails. A dense fog, or it may be fiercely driving snow, obscures the vision. Everything that might serve to guide is hidden by the baffling mantle that cloaks the ocean’s face. Soundings give but uncertain indication ; the clanging of bell buoys, the hoarse booming of fog horns, are voices that convey little assurance amid the anxieties of such perilous gropings. Perhaps on the shore a powerful steam siren makes robust hailing, to be heard many miles away. Yet the atmosphere may be furrowed or pitted with treacherous troughs and cavities ; areas in density, temperature, or movement so different from its main body that the tremendous blasts may remain unheard even close at hand.
Indifferent, however, to warnings of that sort, the ship keeps steadily on. The pilot, with chart spread before him, listens intently, his ear held against a simple rod of wood. Sounds indeed guide him, sounds sent from shore ; but a more trusty messenger than the erratic air is that which brings the signals; the faithful water bears them unerringly and with invariable persistence. Noting the difference in the intervals, the pilot finds immediately the position of the ship upon the chart, and the location is true within the vessel’s length. So the good craft keeps confidently on through the murk, and finds her path surely and accurately into port, her course as true as were the sun shining and every landmark plain in sight.
Little knowledge of maritime conditions is needed to perceive the value of anything that makes possible so fortunate a landfall, which means the prevention of disasters otherwise innumerable. With safe approach to shore made certain, seafaring loses the greater part of its terror. It is all so new, and yet so clear and so beautifully simple, that the brief story of how it becomes possible cannot fail to be of wide interest.
“ Acoustic triangulation” is the name appropriately given to the principle that makes this thing a reality, and Mr. Arthur J. Mundy, a Boston gentleman, devised and elaborated the system. Mr. Mundy had been studying the problem of submarine signaling in association with the eminent physical scientist, Professor Elisha Gray, and together they had invented a remarkably successful apparatus for the purpose, — ringing a bell under water by electrical connections. The question was how to make this invention effective in the simplest possible way, so that it could become at once of service to the navigators of any craft, whether large or small, without the necessity either of using elaborate special instruments or of following instructions and rules more or less complicated. It occurred to Mr. Mundy that, since surveyors are enabled to fix very definitely the location of any point where they may chance to be, by determining its relation to the position of three other points in sight, whose location is known with exactness, a like result might be achieved in determining the situation of a vessel by means of sound signals transmitted from three different stations located at certain known points. Accordingly the problem was worked out upon the basis of the following theorem : “ The fixed mathematical relation of time intervals subsisting between simultaneously sounded, signals received at any unknown point from three triangularly disposed signaling stations established at known, distances from one another, determines the angles between these stations and the, point, of observation.”
This being the case, it is clear that, with sound signals transmitted from stations located off the entrance of a harbor, the navigator of a vessol approaching the harbor can determine his position at any unknown point at which such signals can he heard. While sound travels in the air at the rate of 1100 feet in a second, its velocity is increased to 4712 feet in a second when it is transmitted by water. The simple working of this principle is illustrated in the accompanying diagrams.
In Figure No. 1. two bells are located, at A and B respectively, separated by the distance that sound will travel in ten seconds, which, under water, is ten times 4712 feet. Sound radiates from its source in an expanding circle, like the ripple made by casting a pebble into the water. The circular lines in the diagram represent these expanding circles of sound at the end of each second, as they spread out from the two bells. If these bells are rung simultaneously, they would be heard simultaneously at any point on the straight dotted line in the centre, numbered 10 ; as may be seen by counting the circles. It is likewise evident that any point on dotted line 12 is two seconds nearer to bell A than to bell B. For reasons that will appear, however, the diagram is drawn on a basis of an interval of ten seconds between the sounding of the two bells. The interval between the sounding of the bells on dotted line 12 is therefore twelve seconds, as may be seen. At the point on this dotted line that intersects with a line drawn directly between bells A and B, the sound of bell A will be heard four seconds after it strikes. There being an interval of ten seconds between the striking of the two bells, the ringing of bell B will occur just six seconds after the sound of bell A is heard. It takes six seconds for the sound of bell B to reach that point, and therefore the interval between the sound of the two bells will be twelve seconds. It may be seen that this interval of twelve seconds will occur at any point on dotted line 12. For instance, take the point marked by the asterisk where this dotted line intersects with the six-second circle from bell A and the eight-second circle from bell B. At this point, therefore, the sound of bell A is heard six seconds after it is rung ; four seconds thereafter bell B is rung ; eight seconds after that it is heard, making the interval still twelve seconds. In the same way each one of these hyperbolically curved dotted lines represents a line of equal sound-intervals between the two bells, and the length of the interval expressed in seconds is represented by the numbers at the ends of the lines.
Therefore the observer who knows the distance between the two bells and the intervals at which they are sounded can determine upon which of these dotted lines he is located. He cannot tell, however, his exact position from this, for he may be at any point on the line denoted by the interval. A third bell is necessary to reveal his position on the line. These three bells should sound at intervals of just ten seconds between each, making a cycle of thirty seconds.
The diagram shown in Figure No. 2 depicts such an arrangement of bells placed at equal distances from one another, and therefore representing an equilateral triangle. The bells here are marked One, Two, and Three. The lines of equal sound-intervals are, of course, the essential things, and are given here as continuous lines, without the circular lines that denote the sound-distances from the respective bells. The distances between the bells have been reduced one half, — five seconds instead of ten, — and the sound-interval curves represent half-second intervals. The diagram is formed by combining three diagrams with lines of equal sound - intervals as in Figure No. 1, partly overlapping and forming the shield-shaped triangle with intersecting lines.
If we stand at exactly the centre of the triangle, at the circle marked 10, we shall hear the bells sounded at intervals of precisely ten seconds, making a total of thirty seconds. Let us move to the second point of intersection directly over the circle. Here there will be an interval of nine seconds between bells One and Two, ten seconds between bells Two and Three, and eleven seconds between bells Three and One ; again a total of thirty seconds for the complete cycle. The marginal numbers at each line represent the intervals as heard anywhere along said line. Consequently the intervals between the bell-sounds will denote the point of intersection within the triangle where we may be. A location without the triangle and within hearing of the three bells may be determined in a similar way. While equilateral triangles are preferable, it is possible that geographical conditions might make some other form more convenient in certain instances, in which event the location of the signals could be adapted to the purpose by different geometrical arrangements.
In practical operation the submerged sound-signals would be installed in the manner represented in Figure No. 2, making a distance of something like four miles apart. A series of careful experiments shows that sound can thus be transmitted unerringly, controlled by electric connections, and heard on shipboard at a distance of two miles, simply by pressing one end of a wooden rod against the skin of the vessel anywhere below the water line, and holding the other end firmly against the ear. With a special telephone - receiver, however, invented for the purpose by Professor Gray, the signals can be heard more than five miles away. This instrument may be attached to the outside skin of the ship under water, near the keel and on either side of the bow, like a pair of ears, with wire connections made to the pilot house therefrom, or it may be dropped over the side like a sounding-line when there is any occasion to use it. Such a simple device naturally makes the system universally applicable, from the biggest ocean liner down to the smallest craft that floats.
Signal No. One would be located at the entrance of a harbor, with Nos. Two and Three off shore, to the right and left. With a special cable laid to each, the signals would be automatically sounded once in thirty seconds, at intervals of ten seconds between each. To identify each signal, No. One would declare itself by one stroke, No. Two by two strokes, and No. Three by three strokes, sounded in quick succession, as in the clicking of a telegraph instrument. These signal stations would be accurately located on the coast chart. The curves of equal sound - intervals could either be printed on the chart, or better still, to the avoidance of confusion with other markings on the chart, they could be printed or engraved on some translucent substance like glass or celluloid, and laid upon the chart.
A ship approaches shore in thick weather, either by day or night. The navigator listens. Coming within range of the signals and hearing No. One, he presses a button on a little recorder invented for the system by Mr. Mundy. He repeats the operation on hearing NoTwo, and again at the sound of No. Three. The record gives a certain series of intervals — say the following : —
Between One and Two, 9 seconds ;
Between Two and Three, 8 seconds;
Between Three and One, 13 seconds ;
Total 30 seconds.
Finding the marginal numbers with these figures and following the corresponding lines, their point of intersection will accurately represent the position of the ship. The sum of the three intervals always amounts to thirty seconds. In this way the correctness of the observation is proved. Should the footing show a different result, there would be some error in observation, to correct which an additional one would be taken. As a safeguard against error, the recorder is so arranged that two or three cycles of observations can be made and their average obtained.
It might be that the best course to port would be along the straight line from No. One, passing midway between Two and Three. Should the vessel find itself anywhere upon this line, signals Two and Three would be heard at an interval of exactly ten seconds. This observation could be made at a distance of at least eight miles from the harbor mouth. The course would then be followed by keeping the interval at ten seconds. Immediately on hearing No. One, the position on this line could be exactly located. On passing close to any submerged signal its vibrations would be actually felt on board, as well as heard.
Not the least merit of the system is the fact that it is not necessary for the navigator to understand the underlying scientific principles. All that has to be done is to listen for the signals and record the intervals by means of the simple instrument, whereupon the position of the vessel can be immediately located on the chart.
With this system in operation a vessel can find its way in thick weather all along the coast as well as into port. Another invaluable use is that of a warning at points of danger. With such a triangle of sound-signals located near Sable Island, Cape Cod, Nantucket Shoals, the Goodwin Sands, the Scilly Islands, the Needles, and other dreaded places that have terrible records as ocean graveyards, unspeakable losses of life and of property would hereafter be averted.
Sylvester Baxter.