Part 6
A sound-ranging section on an active sector of the American front usually consisted of four officers and from eighty to a hundred men, one-half of whom were specially trained in the care of instruments, observation work, and mathematical computations. On a stable sector the personnel of the section could, of course, be considerably reduced.
The principles, methods, and instruments employed by the sound-ranging section of the Engineers for locating active enemy batteries or for ranging the friendly artillery on any objective whose map-location was known were of an extremely technical nature and not easy of comprehension by a lay mind. So for the information of those readers who are technically inclined I have asked the Engineer officer who was in charge of sound-ranging in the A. E. F. to explain in the simplest possible language how the work was done. Here is his explanation. Make the most of it.
The principle employed by the sound-ranging section of the Engineers for locating active enemy batteries or for ranging the friendly artillery on any objective whose map co-ordinates are known is the following: The time of arrival of the sound from an enemy gun (or from the burst of the shells from the friendly artillery) at three surveyed stations inside the friendly lines determines the position of the source of the sound if simple corrections are applied for the temperature of the air and the direction and velocity of the wind. For example, if the three surveyed stations are on an arc of a circle and the sound of the enemy gun arrives at all three stations at the same time, then the gun must be at the centre of the circle. If the sound arrives first at the westernmost station and last at the easternmost, then the gun must lie to the westward of the centre. If the sound arrives earliest at the middle station and later at the flank stations, then the gun must lie between the centre of the circle and the stations. In practice six stations are used to insure greater accuracy, and graphical methods of computation are employed to shorten the time of calculation. Accuracies of fifty yards are regarded as average, and from one to two minutes for calculation are usually needed.
A somewhat different form of sound-ranging is used for the detection of aircraft at night. The apparatus for this aerial sound-ranging consists of large sound-gathering instruments which are used to direct search-lights in the location of approaching airplanes. When a bombing-plane approaches at night the hum of the motor can be heard at a distance of from one to three miles or more, depending upon the direction of the sound and the atmospheric conditions. The direction of sound, however, particularly when it originates in the sky, is illusive to the naked ear and search-lights were obliged to sweep the heavens in the general direction from which the airplane was believed to be approaching, in an endeavor to locate it. By the use of these detectors, however, the sound of an airplane can be detected at a considerably greater distance than by the naked ear, and, what is even more important, its direction can be determined within a very small angle—less than five degrees. In this way the area over which the search-light has to sweep is greatly reduced, and the chances of locating the aerial marauder are enormously increased.
Extensive experiments have been conducted in this country by the Engineer Corps in the development of these aerial sound-detectors. One form consists of four horns, two in a vertical and two in a horizontal plant, with listening-tubes leading from the small ends to the receivers of the observer’s head-set. These horns are mounted so as to permit rotation on a horizontal shaft and turning on a plane-table, the whole being supported on a sort of steel tower which, owing to its height and the fact that it cannot easily be moved, affords a rather conspicuous target for the enemy. The obvious disadvantage of this type is recompensed in a measure, however, by its accuracy and by the fact that it will so magnify a sound that the operators can hear the tick of a watch a hundred and fifty yards away. This apparatus is, however, large and cumbersome, and though excellent for seacoast and fortress defense, is not adapted for use in the field, where extreme mobility is required. For this latter purpose paraboloid sound-reflectors have been developed. These paraboloids are about nine feet in diameter, made in sectors of material similar to beaver board, and look like enormous editions of kettles used for boiling soap. They can be taken down and packed into small space for transportation, and are easily set up; being mounted on Ford chassis, they can go anywhere that a “flivver” can go. The paraboloids, like the horns, are directed by balancing the sound so that it is equally audible in both ears. These instruments have a sensitiveness double that of the unaided ear and by means of them a sound can be located to within three degrees.
When the officer in charge of one of these sound-detectors hears through the receivers of his head-set the rhythmic hum which denotes an approaching airplane—and I might mention, parenthetically, that experienced observers can tell with almost absolute certainty not only the nationality of the approaching machine but even the type and power of its engines—he orders several sound-readings to be taken at definite intervals of time. With these readings as a basis for the calculation, the probable location of the airplane at the end of the next time interval is plotted and the search-light is flashed in that direction just long enough to locate the machine. Quick work is required, however, for the airplane often travels at a hundred miles or more an hour and may abruptly change its course at any moment. Then, the plane once spotted, the beam of the search-light never leaves it, and the waiting crews of the antiaircraft guns get to work. Experiments are now being conducted to enable these listening devices to be used in synchronization with search-lights, so that, when the light is flashed, the airplane will be within the beam and no indication of the presence of the search-light will be given the aviator until he finds himself illuminated as a spot-light follows the movements of a dancer on a darkened stage.
In the autumn of 1917 the National Research Council, at the request of the Chief of Engineers, inaugurated an extensive series of search-light investigations, which, thanks to the enthusiastic co-operation of scientists, manufacturers, and certain government bureaus, resulted in a number of remarkable developments. Eighteen different kinds of search-lights were developed during these experiments, the first being placed in operation in France in October, 1918. This represented an entirely new form of light, more powerful than any heretofore produced by any nation. It weighs about one-eighth as much as the most powerful search-light theretofore produced, costs only about one-third as much, and has about one-quarter the cubage. Other improvements now in progress give assurance that its range will be doubled, its cost still further reduced, and its mobility greatly increased. And, what is of almost equal importance, the designs are now becoming so simplified that production need no longer be confined to highly specialized shops, but may be distributed over the country to all classes of machine manufacturers, thus making it possible to produce a large quantity in a relatively short time. With this new equipment the United States will possess a search-light having an effective range approximately twice that of the best search-light produced before the war, with four times as great a field. Two features of the latest types of lamps are particularly worthy of notice. These are, first, the “dish-pan” type of light, the chief characteristic of which is that it has no lens; and, second, the metal mirror, which is much more easily manufactured, is far less fragile, costs only a third as much, and possesses almost as great reflecting qualities as the glass ones.
The search-light used by the American forces for antiaircraft work is the heavy 60-inch seacoast type—the largest light known—lightened and modified for use in the field, with a range of practically 30,000 feet. As the result of recent experiments it has been found that the visibility at 12,000 feet was 85 per cent, while at 15,000 feet, or nearly 3 miles, it was 43 per cent. In order to obtain these standards of comparison for visibility for search-lights, an aviator was directed to fly back and forth through the beam a certain number of times. If the observers on the ground recorded the full number of passages across the beam, 100 per cent was registered, this occurring regularly at 5,000 feet, and in most cases during tests at 8,000 feet. The percentage of visibility was, in other words, the number of times the airplane was seen to the number of times it crossed the beam.
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When warfare of movement becomes stabilized into position or trench warfare, it is almost certain that, sooner or later, one side or the other will resort to some form of underground attack. To permit of this subterranean warfare, certain conditions are requisite: the lines must be fairly close together, the level of the ground-water must be deep, and the ground itself must not be too hard. These obstacles to successful mining are not insuperable, however, for, preparatory to their assault on Messines Ridge, the British drove a tunnel which was a mile in length, and on the Carso I saw Italian engineers driving their galleries through solid rock. France and England early recognized the importance of this form of warfare and organized their miners accordingly, and, upon our entrance into the war, we too organized and sent to France a mining regiment—the 27th Engineers. It is estimated that by the summer of 1918 there were upward of 40,000 skilled miners on the Western Front, these soldiers of the pick and drill having been brought from the remotest corners of the earth—from the Yukon, the Rand, and the Congo, from Mexico, Australia, and California. In my “Vive la France!” I told, if I remember rightly, of the Cornish miners, known as “kickers,” who lay on their backs, as they do in the tin mines in Cornwall, where the galleries are so low that there is no room to swing a pick, and kicked away the earth by means of a sort of spur attached to their heels.
The officers of the American mining regiment were engineers who had had practical experience in all those far-off regions where men seek their fortunes in the earth. One of them, a young lieutenant, was diamond-mining in the Katanga district of the Congo when word reached him by native runner that the United States had decided to take a hand in the Great War. It took him four months of uninterrupted travel by horse, wagon, rail, and boat to reach the United States and offer his services to the Chief of Engineers. Another of our mining officers was a prisoner of the revolutionists in Mexico when the rumor penetrated to his prison cell that the United States had gone to war. That night he overpowered his guards, scaled the prison wall, made his way on foot across northern Mexico, the journey being relieved from monotony by several hairbreadth escapes from bandit bands, and reached the border in time to join the Engineers and go to France with one of the first contingents.
In former wars military mining was almost wholly confined to siege operations; that is, driving galleries under fortified positions and blowing them up. But the Great War developed an entirely new system of mining tactics, which included frontal and flank attacks, raids, enveloping movements, and other phases of war as fought on the surface of the earth. “Unlike the soldier who fights above ground,” explained a mining officer, “the miner has to be prepared for attacks not only against his front and flanks, but for assaults which may come from overhead or from underneath. In other words, he has four flanks to defend instead of two.”
A typical mining position, such as would be prepared on an active sector of the front, would consist of an upper level having a series of forked galleries, known as “feelers,” with geophone listening-posts at their extremities, and a deeper level, with numerous “fighting branches” projecting from it, to protect the lower flank. Just as the sentries in the trenches strained their eyes to detect any ominous figures in the darkness of No Man’s Land, so the mining sentinels, crouching over their geophones in the headings of dim-lit galleries, strained their ears to catch the faint sounds which gave warning that the enemy was approaching underground. The geophone, which has proved of incalculable value in mining warfare, is an instrument for augmenting small sounds coming through the ground. The American geophone, which is a highly sensitive, extremely simple, and easily portable instrument, is in no sense an electrical device, resembling, rather, the stethoscope used by physicians for testing the lungs. In mining operations two geophones are used, one for each ear, the instruments being so sensitive that the sounds caused by a fly walking on the wooden support of the geophone appear as loud as the tramp of a horse on the floor of a stable. If a sentinel on duty in an underground listening-post caught through his geophone a sound which was more distinct in, say, his right ear than in his left, he gently shifted one of the instruments, inch by inch, until the sound was the same in both ears. Then, by means of a compass, he took the magnetic bearing of a line perpendicular to that passing through the two geophones, which would give the direction from which the sound came. Meanwhile sentries in the other listening-posts were doing the same thing, so that, by the co-ordination of their reports and by triangulation, the enemy’s gallery could be located within a few yards.
If the mining officer was convinced that the enemy was driving a gallery for the purpose of putting a mine under his position, two courses of action would be open to him. He could remain on the defensive and check the enemy’s advance by the use of “camouflets,” this being the name applied to explosive charges which expend their force laterally, thus destroying the enemy’s gallery without causing a crater; or he could resort to strategy and engage the enemy’s attention at one point by exploding camouflets or by working noisily, and under cover of this diversion drive a fighting gallery toward his flank elsewhere. If, instead of being content to remain on the defensive, the officer in charge of mining operations decided to assume the offensive, he would engage the enemy’s attention at one point, either by exploding camouflets or by working noisily, and at the same time drive a fighting gallery toward his adversary’s flank. In this latter case the most profound silence had, of course, to be enforced in the fighting branch if the enemy’s geophones were not to give warning of its approach. No talking was permitted, the men wore felt-soled shoes and worked with trowels instead of picks, and the earth was carried out in cars with rubber tires. So silently were the operations in the fighting branches conducted that they would frequently break into the enemy galleries without the slightest warning, whereupon would ensue a struggle fought scores of feet beneath the surface of the earth, by combatants armed with picks, pistols, bombs, and knives, and illuminated only by flickering miners’ lamps—a battle so weird and strange in its character and setting that it seemed like the creation of a motion-picture writer’s brain.
One of the essentials for the success of a mining operation is the concealment of the spoil—_i. e._, the excavated earth—which, if piled in a heap at the entrance to the workings, would almost certainly be photographed by aerial observers, thus informing the enemy, as unmistakably as though it were announced on a placard, that a mining gallery was being driven. The French, in order to hide the spoil from their mining operations, conceived the ingenious plan of digging a shallow trench, usually only a few inches deep, and lining it with black paper, so that when photographed from an airplane it produced the effect of the black shadow cast by a trench of customary depth. They would then distribute the spoil from their subterranean galleries along the sides of this false trench, so that it appeared in the photograph to have been thrown up from it.
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Dugouts have become such a commonplace in the past four years that few, save the miners themselves, gave much thought to or had more than the haziest ideas of the time, skill, and labor required in their construction. Take yourself, for example. You have read about dugouts and seen pictures of dugouts and have probably had relatives or friends living in dugouts. How long, then, think you, would it take a force of skilled miners to complete a front-line dugout large enough to accommodate a half-platoon? (For your information I might explain that such a dugout is 35 feet long, 9 feet wide, and 6 feet high, with 17 feet of overhead cover.) Using all the men that could be employed, and working from nightfall until dawn, it would require at least three months to complete such a dugout. If in the rear area, where the men could be worked continuously in shifts, it could be completed in about thirty days.
[Illustration: NEW TYPE OF SEARCH-LIGHT USED IN THE AMERICAN ARMY.
The steel tower is collapsible and light and, being mounted on a motor-truck, is extremely mobile.]
[Illustration: CAMOUFLAGING A DIVISIONAL HEADQUARTERS IN THE TOUL SECTOR.
Constructing the screen.
The screen as it appeared upon completion.]
A recent and little-advertised development of trench warfare was the introduction of “mobile charges.” These consisted of packages of high explosive in ten, twenty, or thirty pound sizes, which were used by assaulting troops for destroying dugouts, much as depth bombs were used by the navy to destroy submarines. With the increasing use of mobile charges it became necessary to design dugouts which would be proof against them. In this work, which was carried on by the Mining School, extensive use was made of dogs, experiments having shown that explosions which will rupture the lung-tissues of a dog will similarly affect those of a human being. Thanks to the knowledge thus obtained at the cost of canine lives, a type of dugout construction was perfected which afforded the occupants comparative immunity from mobile charges and hand-grenades. An ingenious receptacle for this latter form of enemy visiting-card was the “bomb-pit,” which was a sort of small cistern, built at the foot of the dugout stairs, into which a hand-grenade would fall and explode harmlessly.
Though it has no direct relation to the work of the American Mining School, I might mention, as an illustration of the part played by miners in the great conflict, that when the British in 1917 blew off the entire top of Messines Ridge prior to their assault on that position, 19 mines, containing a total of 950,000 pounds of ammonal—equivalent to 1,580,000 pounds of dynamite—were exploded simultaneously. A single one of these mines contained 95,000 pounds of ammonal and made a crater 186 feet wide and 125 feet deep.
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Though few activities of the Engineers were more important than the work of the Camouflage Section, and though certainly none was more picturesque or interesting, it is with some diffidence that I introduce the subject, for I am perfectly aware that American readers have been, to make use of a British colloquialism, “jolly well fed-up” on everything pertaining to camouflage. The point is, however, that they have been largely “fed-up” on misinformation. They have read hundreds of magazine articles and newspaper stories about fake trees and papier-maché horses and the like, but of the real work of the Camouflage Corps—which, as an American general remarked, was “as practical as machine-guns and as necessary as ammunition”—they have heretofore been permitted, for quite obvious reasons, to know next to nothing. Certain camouflage operations on the American Front were of such vital importance to the success of our armies that, far from acquainting the public with them, they were veiled in the profoundest mystery.
Military camouflage is a development of the Great War and has, therefore, no history and little literature. It differs from the purely scientific work of engineering, which has few variants and in which nearly all problems can be worked out by formula, in that it has countless variants of light, color, and position, and each problem of concealment is an individual one. Upon the entry of the United States into the war, much study was devoted to French and British camouflage methods, both in the factory and in the field. The British, it was found, did nothing without the most careful scientific investigation, which included aerophotography of all materials, while the more careless and temperamental French relied rather on their innate artistic sense of form and color. By combining the best features of both systems and strongly tincturing them with American energy, ingenuity, and manufacturing methods, our Camouflage Service soon came to be recognized as the best equipped and most efficient in the Allied Armies. At Dijon, in the Department of the Haute-Marne, we established a huge plant, known as the Central Camouflage Factory, where a hundred soldiers and some nine hundred Frenchwomen were employed in the production of materials, while at the Army Camouflage School of Fort St. Menge, near Langres, practical instruction was given in the use of these materials in the field.
When the war ended, the American Camouflage Service consisted of a battalion of the 40th Engineers—which was on the point of being expanded into a regiment—under the command of Lieutenant-Colonel H. S. Bennison, with Evarts Tracy, one of the foremost architects in America, as major. Captain Homer Saint-Gaudens, a son of the famous sculptor, was in charge of the camouflage work of the Second Army, and Captain John Root, whose father was architect of the Colombian Exposition, was in charge of all camouflage work for the army artillery, he being largely responsible for the remarkable developments in this branch of warfare. The director of the Camouflage School at Fort St. Menge was Lieutenant Wilford S. Conrow, the noted portrait-painter. Another officer of the battalion, Lieutenant Harry Thrasher, a graduate of the Ecole des Beaux-Arts and a winner of the Prix de Rome, was killed while doing camouflage work at Fismes, as was Sergeant Everett Herter, a son of Albert Herter, the artist. Because of the exacting nature of its requirements, the Camouflage Service had, perhaps, a more highly educated enlisted personnel than any other organization in the army. Among the men wearing the uniforms of privates in the corps was the landscape-architect who laid out the grounds of the San Diego Exposition, the stage-manager for Maude Adams, the head property-man of the Universal Film Company, and Louis Tiffany’s chief designer. One of the instructors at the school was a successful osteopath who in his younger days had been a scene-painter; another was a sculptor whose statues may be seen in many American museums and parks.
Figures are, as a rule, dry reading, but they provide the best means I know of giving some idea of the magnitude of our camofleurs’ operations. During the summer of 1918 the Camouflage Section used materials _per month_ as follows:
12,000 fish-nets. 50,000 pounds of wire. 700,000 gallons of paint. 2,160,000 square yards of poultry-netting and approximately 1,000 acres of burlap.