THE

V

SOUND-RANGING IN THE AMERICAN

EXPEDITIONARY FORCES

AUGUSTUS TROWBRIDGE

HE following picture is not an imaginary one, but rather one of a very common occurrence throughout the entire period of the war on the long battle front which stretched from the Alps to the sea.

It is a dark, cloudy night and enemy shells begin to fall near an important point in the trenches or on battalion or regimental headquarters. There is a hurried report to our artillery and in a few minutes our own guns begin to reply with shells which rend the air or whine as they pass overhead toward some invisible mark five miles distant through the black night. Presently the enemy's fire begins to falter and then ceases and the infantryman, whose life may have been saved and whose comfort and efficiency certainly has been protected, may wonder how the artillery knew just where to direct its fire.

He knows how it is done in clear weather; how the artillery maintains advanced lookout posts from which observations are made on the flash of the nearer enemy guns; that there are other and more elaborate lookouts on high ground or in trees or towers on the forward edge of woods from which accurate triangulation on the more distant hostile batteries may be made; but he knows that these cannot be the means employed in rain, mist or fog and he probably ends by dismissing the question with the thought that it was only a case of good luck.

The chances are that even his officers have no clear idea of

how the enemy guns are quickly located in rain, fog or mist; for it has been the policy of the general staff to keep very secret the details of a scientific service in which the Allies possess a very decided superiority over the Germans.

Let us look for a moment at what may be happening on the same night on the other side of No Man's Land. A German battery begins to shell an important cross-roads in the Allied back area in order to prevent the bringing up of munitions or fresh troops. Presently shells begin to drop from somewhere in France, but at first these are not close enough to make it sure that they are trying to "find" the German battery; then six or eight rounds fall to the left and behind the battery; then there follows a short pause and another series of rounds falls to the right and in front of the battery; another pause and the next group of rounds has crept closer and this goes on until the battery has become the center of a steady rain of projectiles which makes it impossible for the crews to serve their guns.

The German battery commander knows that the Allies are directing their counter-battery artillery fire by a means which he himself does not have at his disposal and he knows what that means is, for his Intelligence has published a number of pamphlets which describe it. He also has a set of instructions as to what measures to adopt if he suspects that the Allies are employing sound-ranging against him in order to render it less effective, but these measures are unavailing against the most improved form of apparatus operated by the Allies.

It is the purpose of the following article to explain in nontechnical terms how the Allies applied certain acoustic principles to determine the position of enemy guns when the more usual and simpler visual means of observation failed because of bad weather or because the German batteries were hidden from view. It is in no sense the purpose to magnify the importance of American scientific achievement in the war, for the present writer, who as an American scientist has every interest in seeing that all due credit comes to his profession,

nevertheless feels that so far as the war in France is concerned, American science contributed far less along original lines than the general public has imagined. This is no slur on American science, for it nobly did its part toward bringing the war to a close, but it did it along lines already laid down by our Allies and it did it all the more effectively for that reason.

The Allies counted much on American ingenuity in bettering the existing scientific services and in devising new applications of science to accomplish new purposes, but both they and we fully realized the paramount importance of first establishing services as good as their own before attempting either to make radical improvements or to establish new services. At the signing of the armistice experiments were under way in America, many of which were nearing completion, which might have added new and valuable scientific services to the number already functioning in France, but the fact remains that at the cessation of hostilities all that had been done was the establishment of American scientific units which were modeled on those of our allies. The most important of the applications of pure science which were a wholly new product of land warfare were: the use of cloud and shell gas, the extremely brilliant application of chemistry in the construction of gas-masks, airplane photography, the scientific aids to accuracy in gunnery and bombing from airplanes, sound-ranging, search-light and listening devices for anti-aircraft defense, directional wireless, and camouflage. Practically all of the absolutely new applications of physical science to warfare on land are contained in this rather short list. These, of all the great number of inventions which have been proposed, it has been possible and necessary to establish on an engineering basis and to organize into services for all the armies of the Allies.

The effect of these few new applications of science on the character of the warfare on the western front was very farreaching. Airplane photography, for example, not only completely revolutionized military map-making but also profoundly modified the methods of the army Intelligence and made nces

sary the establishment of a large force for compiling and comparing data from photographs and for disseminating information to all the various interested services. The profound effects which were produced by the gas warfare were patent to every man on the front and the same was true to a less degree of the camouflage and some of the other services mentioned, but the very existence of one of these, sound-ranging, was not suspected by most of the troops engaged. This was not because it was in its way less important than the others or because it was working less effectively but rather because it was the policy of the Allies to shroud this particular scientific activity in the most complete secrecy. For this reason, even now not only the general public but also the majority of those who were over there knows very little of the methods and achievements of the sound-ranging service. As these methods possess a considerable scientific interest and as these achievements have been very creditable it is quite fitting that some account of them should be included in this volume.

HISTORY

After the first Battle of the Marne the operations on the western front soon took on the character of siege warfare; the artillery of both of the belligerents was augmented, especially as regards the larger calibers and the batteries took up wellorganized positions carefully concealed for the most part from visual observation by the enemy.

The possibilities of visual observation had been vastly improved by the use of the airplane in war, but these were somewhat restricted both by the practice of camouflage and by the generally unfavorable atmospheric conditions on the western front. Experiments were therefore undertaken by the French in the autumn of 1914 with the object of ascertaining whether the location of hostile guns by means of sound waves might prove feasible. It was probably not expected that a high degree of accuracy would be attainable because of the disturbing effect of wind and temperature irregularities, but the desir

ability of even a fairly accurate method of location and ranging which should not be interfered with by rain or fog and against which the practised camouflage should be unavailing was so obvious that the first successful attempts by the French in 1914 led quickly to the establishment of a ranging service.

Instruments of four very different types for recording the arrival of the sound of the enemy gun were tried out on various parts of the French front. By 1916 the majority of the French sound-ranging sections were equipped with standardized apparatus of one type. A school for the instruction of the personnel of the sections was also established in the back area.

The standard type of apparatus adopted by the French had the advantage of simplicity and the further advantage that it was for the most part an assembly of well-known commercial apparatus which had been in use in the field telegraph service of the French Army. These were real advantages since the French were obliged to use men in the ranging service who were often unfitted for more active service or for some of the other more highly technical services. There were, however, certain very serious defects in the French apparatus which prevented its adoption first by the British and later by the Americans for neither of whom were the advantages just mentioned so important as they were for the French.

By the end of 1915 the British had organized a sound-ranging service which employed a photographic recording instrument devised by a British subject, resident in France, Mr. Lucien Bull, and which had been tried out with success on the French front but which had not been officially adopted by the French. In the hands of Mr. Bull and Major Bragg, in technical charge of sound-ranging in the British Army, the original apparatus was perfected so as to combine reliability with ample sensitiveness and an extremely quick recovery so that sound ranging could be carried on without confusion during periods of relatively great artillery activity.

Shortly after the entry of the United States into the warin June, 1917-a French scientific commission arrived in