Sound-ranging was carried on along much the same lines as flash-ranging, except that the readings were made by instruments instead of by observers. Large guns may be camouflaged so that their detection, either by aerial observation or by the flash of the gun when fired, is extremely difficult, but there is no known way to conceal the location of the gun from sound-ranging instruments, suitably placed and properly operated. This method became so highly developed that it was reported that during the latter months of the war over 80 per cent of the work of locating gun positions on the British Front was done by sound-ranging. The instruments used for this work are of a highly technical nature and for their successful operation require a skilled personnel. Recording instruments, so delicate that their use heretofore had not been dreamed of outside of experimental laboratories and then only in the hands of men carefully trained in their operation, were set up on the firing-line and operated successfully under battle conditions, even when the air was quivering from heavy bombardments and the earth was shaking from the deluge of steel. The sound-receivers, or detecting instruments, are located well to the front, whereas the recording instruments are several miles in the rear. A sound disturbance due to the firing of a gun somewhere behind the enemy lines is transmitted through several miles of wire to the recording instrument in the rear, and the sound records received almost simultaneously from several detecting instruments are traced on a sensitized ribbon, or tape of photographic paper, or on a ribbon of smoked paper, depending on the type of instrument used. The intervals of time elapsing between the arrival of the various sound disturbances is used as a basis for determining the origin of sound which produced the records. By this means over 100 new German gun positions were located in a single day on the British Front. In fact, before the assault on Messines Ridge the British sound-rangers had located practically every German battery, so that the British gunners had their exact range when the attack was launched. When the Armistice put an end to hostilities there were in operation along the American Front some twelve complete American sound-ranging sections, each covering a front of approximately five miles. 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.