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.