One of the most picturesque and interesting developments of the war—and, because of the secrecy which surrounded it, one of the least known—is the work of the flash and sound ranging section of the Engineer Corps. For the benefit of the uninitiated—and most people are uninitiated, so far as this phase of warfare is concerned—I might explain that flash-ranging means the location of an enemy gun or battery by the detection of the flash, and sound-ranging by the location of the sound. Flash-reading, as it is called, is carried on by means of two or more observers provided with powerful telescopes, who are stationed at known distances apart. By “spotting” the flashes of an enemy battery and reporting them, together with the exact direction at which they occur, and by using these readings as a basis, a simple calculation in triangulation will give the location of the gun. So highly has this flash-ranging been developed that a gun can now be located within five yards, when the “core” of the flash can be seen. In principle the process is extremely simple, but in practice it is complicated by the fact that the observers may not train their telescopes on the same flash, in which case the gun position calculated at headquarters from their telephoned reports will be in error. This difficulty is met, however, by providing each observer with an outpost switch-set, by means of which he can flash a miniature light at the headquarters station at the instant he makes an observation, just as a light glows on a hotel telephone switchboard when a guest up-stairs rings for ice-water. When several of the observers flash their light simultaneously, it is assumed that they all probably caught the same flash, and their observations are then plotted. In other words, the line of sight of each observer is prolonged on a map until they intersect, the point of intersection corresponding with the location of the German gun. Flash-ranging was also found to be of great value in checking the ranges of our own guns. If we were firing at a hidden target, a shell was timed so that it would burst when at the top of its trajectory. Observers would “spot” this burst, and if it was reported as being at the spot where calculations showed that it should occur, the gunners knew that they had the correct range.