Many detection devices had been tried out and proved failures, but the American apparatus was so successful that the British ordered them for their own vessels. Thousands were manufactured, and our sub-chasers sent abroad were equipped with them. In December, 1917, it was estimated that at times two to five U-boats had passed through the English Channel in a day. After July 1, 1918, when patrol ships were equipped with the improved listening devices, only one enemy submarine is known to have passed through the Channel. Blocking the entrances to Zeebrugge and Ostend, the Dover patrol and the better mine defenses are to be credited with the larger part of this. But considerable credit is due to these "listeners," whose ability to locate under-water craft greatly increased the hazards of U-boats, especially in narrow waters. The listeners also proved decidedly effective in high waters, off the French coast, in the Adriatic, the Mediterranean, and wherever they were used. They compelled the U-boats to change their tactics, and remain motionless for hours, fearing that the slightest movement of their propellers would disclose their presence. Our submarine force began listening tests off Pensacola, Fla., in January, 1917, using privately-invented apparatus which gave such promise that an experimental station was established at Nahant, Mass., the General Electric, Submarine Signal, and Western Electric companies coöperating with the Navy Department and Naval Consulting Board. The Consulting Board had created a special Experimental Committee headed by Mr. Lawrence Addicks, and on March 3 held a "Submarine Defense Conference" at New York, which was addressed by Admiral Sims, then president of the Naval War College; Captain J. K. Robison, of the Newport Torpedo Station, and Commander Yates Stirling, Jr., in charge of our submarine base at New London, Conn. Scientists and naval officers engaged in this work held a conference in my office in the Navy Department on May 9, and two days later I created a Special Board on Anti-submarine Devices, with Rear Admiral A. W. Grant as chairman, and representatives of the electrical and signal companies, and the National Research Council as advisory members. Extensive experiments were carried on at our submarine station at New London, as well as at Nahant.