the development of a chemical pellet trigger as the Type B. These were effective at a distance of around . Perhaps the best early concept arose in a 1913 RN Torpedo School report, describing a device intended for countermining, a "dropping mine". At Admiral John Jellicoe's request, the standard Mark II mine was fitted with a hydrostatic pistol (developed in 1914 by Thomas Firth & Sons of Sheffield) preset for firing, to be launched from a stern platform. Weighing , and effective at , the "cruiser mine" was also a potential hazard to the dropping ship. First World War During the First World War, submarines were a major threat. They operated in the Baltic, North Sea, Black Sea and Mediterranean as well as the North Atlantic. Previously, they had been limited to relatively calm and protected waters. The vessels used to combat them were a range of small, fast surface ships using guns and good luck. They mainly relied on the fact a submarine of the day was often on the surface for a range of reasons, such as charging batteries or crossing long distances. The first approach to protect warships was chainlink nets strung from the sides of battleships, as defense against torpedoes. Nets were also deployed across the mouth of a harbour or naval base to stop submarines entering or to stop torpedoes of the Whitehead type fired against ships. British warships were fitted with a ram with which to sink submarines, and U-15 was thus sunk in August 1914. During June 1915, the Royal Navy began operational trials of the Type D depth charge, with a charge of TNT (amatol, as TNT supplies became critical) and a hydrostatic pistol, firing at either , and believed to be effective at a distance of ; the Type D*, with a charge, was offered for smaller ships. In July 1915, the British Admiralty set up the Board of Invention and Research (BIR) to evaluate suggestions from the public as well as carrying out their own investigations. Some 14,000 suggestions were received about combating submarines. In December 1916, the RN set up its own Anti-Submarine Division (ASD), from which came the term "Asdic", but relations with the BIR were poor. After 1917, most ASW work was carried out by the ASD. In the U.S., a Naval Consulting Board was set up in 1915 to evaluate ideas. After American entry into the