Mining beneath the trenches was a quite hazardous feature of the Great War, and even today there are thought to be five fully charged lost mines containing 166,000 pounds of explosives remaining under old French battlefields. There is even a volunteer group of civil engineers dedicated to exploring and mapping old mines beneath the former Western Front.
Devices using sound were also employed for the first time during World War I to detect the direction of hostile aircraft. At first the problem seemed insolvable, and truly effective countermeasures—especially at night—were not developed until the invention of radar for guns, searchlights, and planes in World War II. However, in 1916, bombing attacks on the front line of the Western Front along the Maricourt Plateau on the Somme were becoming a serious nuisance. Attacks by zeppelins on mainland England had begun the year before, and the government was worried about their effect on civilian morale. Experiments with early acoustic and lighting devices met with varying degrees of success. Then a research team at the Ministry of Inventions and Munitions in London was established to look at all aspects of air defense. The team was headed by a mathematician, A.V. Hill, who devised a pattern-sound locator using pairs of wooden conical horns mounted on a frame. An observer using a stethoscope balanced the sound of engines in each ear to obtain a bearing to direct searchlights onto enemy aircraft.
Other scientists followed different lines of inquiry. Professor Thomas Mather of the London City and Guild Engineering College tested an idea inspired by French engineers. It involved building a circular bowl, or dish, of curved concrete wall structures to magnify and trap the sound from approaching aircraft. The War Office was initially interested but subsequently withdrew its support and there the matter rested until now-Major William S. Tucker, who earlier had invented a successful hot-wire microphone for artillery calibration, obtained War Office funding to research the development of long-range detection of aircraft for the protection of the English coastline against air attack. After reviewing all the known information on the subject and examining several experimental mirrors, Tucker oversaw the construction of several different shapes of acoustic mirrors along the Kentish coast. All involved monolithic concrete structures varying in size from 15 to 200 feet, with a listener equipped with a microphone standing in front of a concrete bowl-shaped dish to pick up the sound of approaching aircraft. Results of the tests were hardly impressive; aircraft were detected only up to 10 miles away, and detection was subject to weather and wind noise.
Attempts to use searchlights on the battlefield to illuminate enemy positions were quickly abandoned since they attracted enemy gunfire. Lights were mounted experimentally on kite balloons for observation, but the wobbling of the platforms made them useless. Using a searchlight beam to pick up enemy planes at night was largely ineffective, although the result was blamed on limitations of equipment and poor cooperation with antiaircraft gunners. In England, searchlight units were used more successfully against the slower-moving zeppelins, but proved useless against planes. Eventually, projectors were given a 13-foot-long arm that enabled the operator to see the target in the beam without being blinded. The Germans were wary of using searchlights to defend their homeland, quickly realizing that the lights led enemy bombers to the very area they were attempting to defend.
In the autumn of 1915. Frederick Lanchester, a successful British car and aircraft designer, came up with an idea to flood the skies in eastern and southern England with a belt of light against which hostile aircraft could be seen by gunners and fighter planes while the ground below was left in darkness. The lights were to be mounted on towers up to 100 feet high, with their beams inclined upward to reflect off the clouds. There was to be one tower for each three miles of area to be lit. Representatives from the American firm Edison and Swan were consulted, and a full-scale experiment was carried out in Wiltshire in July 1917. The results were not successful, partly due to the unpredictable English weather and the increasing the altitude at which planes were beginning to fly.
All that was known scientifically about the characteristics of sound and light was used in World War I to advantage the combatants, even though the application for warlike use was in its infancy. To some extent, the experience gained was useful when World War II broke out. Flash spotting and sound ranging continued to be used in all theaters without much change in technique. With improvements in wireless and communication technology, “overhearing” became radio interception, and the resultant code-breaking and intelligence analysis contributed enormously to the advance knowledge of enemy intentions.
This article first appeared on the Warfare History Network.
Image: Wikimedia Commons