Although valuable work in detecting gun positions was done by sound ranging, yet both sides located guns by watching their flashes. We improved flash-ranging sets of the allies. These were simple in principle. A number of observers at posts commanding good views were equipped with observation telescopes mounted on tripods to watch for the flashes of enemy guns. Whenever two or more of them observed the same flash and reported its direction, the position of the gun could be determined by ordinary triangulation.
However, in operation the system was not so simple, because of the fact that the observers reporting might not have turned their instruments upon the same flash. This difficulty was met by furnishing each observer with an outpost switch set. As soon as he observed the flash through his telescope he closed the switch, and that action turned on a small electric light at the headquarters station, which might be miles away. Then, as soon as he could, he telephoned in the direction of the flash observed. If the operator at the switchboard saw two or three of the lights flash simultaneously, he knew the observers at the front had probably caught the same flash. Lights that came on a little ahead or a little behind the simultaneous lights were disregarded when the observers telephoned reports.
In developing the telescope for this system considerable difficulty was experienced on account of the shortage of the proper optical glass in this country. We were, therefore, obliged to buy our telescopes in France until our supply would be available. These telescopes were expensive mechanisms, and in some of the work of the flash-ranging sections two of them were originally required at each observing station—one to determine the position of a shell burst in elevation and the other its position on the horizontal circle in azimuth. Since the declaration of the armistice an American Engineer officer has designed a telescope eyepiece which enables this work to be done by observing through a single instrument, thus effecting a marked saving in the number of telescopes which might be required in the future.
AMERICAN PARABLOID TYPE ACOUSTIC DETECTOR.
60-INCH OPEN TYPE PORTABLE SEARCHLIGHT.
60-INCH HIGH INTENSITY SEACOAST TYPE SEARCHLIGHT.