to do so, I will spend a few moments in describing sound-ranging.
I have spoken of the excellent system of observation along the front that was carried on by our aeroplanes and captive balloons. But night, cloud and camouflage could do much to evade it and it was therefore desirable to get some means of observation that did not depend on light. Sound was chosen. When the sound of a heavy gun was heard at an observing post a microphone automatically telegraphed it to a head station so that the time of the arrival of the sound at the observing post was known and registered at the head station on a slip of paper moving at a definite rate. Two such records coming from different observing posts gave the difference between the time of the sound arriving at the two observing posts and—as the velocity of sound is known—this gave the difference of the distance of the gun from the two stations and enabled the observers at the head stations to draw on their maps by their instruments a curve on which the gun must be situated. A similar message 44
