“The principle of the location of guns by sound,” writes an artillery officer, “is simple enough.  Suppose there are two observers in the British lines, one at each end of a long line.  Bisect this base, and from the middle point draw a line at right angles to the base and towards the German lines.  Now, if a hostile gun fires from a position on this line, the sound will reach both observers simultaneously.  If the gun fires from a position to the right of the line, the sound will reach the right-hand observer first, and vice versa.  Then, by measuring exactly the time-interval between the arrival of the sound at each observation post, the bearing to the gun can be calculated.”

“Until quite recently the Germans used four human observers, who timed the sound intervals with stop watches.  The British used six microphones of a special type, connected electrically with a photographic-recording apparatus.  Instead of stop watches, therefore, we used a timing device capable of recording the most minute time-intervals with perfect precision.  The whole system was immeasurably superior to the German, and at least twenty times as accurate, for the British system was absolutely automatic.  It recorded the arrival of the sound at the various microphones instantaneously on a permanent record; while the German system, apart from its crude method of measuring time, was subject to the combined errors of four human ‘microphones.’  The British system requires only one forward observer, placed well ahead of the base, and all he has to do is to press a button and start the apparatus before the sound reaches the microphones.

“The photographic record is ready for the computer in from six to ten seconds, and the gun position can be found and plotted in three or four minutes.

“Sound ranging also can be used for ranging our own guns with great accuracy.  When a record has been obtained of a hostile gun, all that need be done is to record the burst of our own shell and give corrections to our battery until the record of our shell-burst is identical with that of the hostile gun.  The shell must then be on the target.

“The system works equally well by day or by night, in rain or in fog.  Its one enemy is a wind which blows towards the hostile gun and prevents the sound reaching the recording apparatus.  It can detect a gun as easily if it is in a wood or in a building as if it were on a hill-top.

“Simple as it appears, however, it is not so easy as one might think to make a practical ally of sound ranging.  We have succeeded.  The Germans failed.  Towards the end of the war at least ninety per cent, of the German artillery was marked down accurately by these means; and the staff employed on sound-ranging and flash-spotting (the last a kindred method depending on a mixture of observation and mathematics) had grown from four in 1914 to four thousand five hundred in 1918.

“Casualties have been heavy, and the work arduous.  But those responsible for it have, at any rate, ‘done their bit.’”