war in 1917, they encouraged work on submarine detection. The U.S. National Research Council, a civilian organization, brought in British and French experts on underwater sound to a meeting with their American counterparts in June 1917. In October 1918, there was a meeting in Paris on "supersonics", a term used for echo-ranging, but the technique was still in research by the end of the war. The first recorded sinking of a submarine by depth charge was U-68, sunk by Q-ship off Kerry, Ireland 22 March 1916. By early 1917, the Royal Navy had also developed indicator loops which consisted of long lengths of cables lain on the seabed to detect the magnetic field of submarines as they passed overhead. At this stage, they were used in conjunction with controlled mines which could be detonated from a shore station once a 'swing' had been detected on the indicator loop galvanometer. Indicator loops used with controlled mining were known as 'guard loops'. By July 1917, depth charges had developed to the extent that settings of between were possible. This design would remain mainly unchanged through the end of World War II. While dipping hydrophones appeared before war's end, the trials were abandoned. Seaplanes and airships were also used to patrol for submarines. A number of successful attacks were made, but the main value of air patrols was in driving the U-boat to submerge, rendering it virtually blind and immobile. However, the most effective anti-submarine measure was the introduction of escorted convoys, which reduced the loss of ships entering the German war zone around the British Isles from 25% to less than 1%. The historian Paul E. Fontenoy summarised the situation as: "[t]he convoy system defeated the German submarine campaign." A major contributing factor was the interception of German submarine radio signals and breaking of their code by Room 40 of the Admiralty. To attack submerged boats, a number of anti-submarine weapons were derived, including the sweep with a contact-fused explosive. Bombs were dropped by aircraft and depth charge attacks were made by ships. Prior to the introduction of dedicated depth charge throwers, charges were manually rolled off the stern of a ship. The Q-ship, a warship disguised as a merchantman, was used to attack surfaced U-boats, while the R1 was the first ASW submarine. 211 of the 360 U-boats were sunk during the war, from a variety of ASW methods: Mines 58