Waters suspected of containing hostile submarines are swept, very much as they would be for mines, by pairs of destroyers or steam trawlers, dragging an arrangement of strong cables between them. Sometimes this is festooned with explosives to blow in the side of any undersea boat it may touch. Usually the vessels engaged in this work use a large net. When they feel the weight of a catch, it is said that they let go the ends and leave it to the submarine’s own twin propellers to entangle themselves thoroughly. An undersea boat so entrapped is helpless to do anything but either sink or else empty her tanks and try to rise and surrender. A submarine in trouble usually sends up notification in the form of large quantities of escaping oil and gas. Inventors have been busy devising new kinds of traps, snares, and exaggerated lobster-pots to be placed in the waters about the British Isles. How many German submarines have poked their noses into these devices probably not even the British Admiralty could tell, if it was so minded, but the traps are said to have been put down very plentifully and most of the published designs are extremely ingenious. Individual torpedo-nets for ships have rather gone out of fashion, but the most effective way of keeping submarines out of a harbor is to close its entrance with booms and nettings. The principal naval bases on both sides are undoubtedly so protected. It has been persistently reported that the immunity of British transports crossing the channel is due to a double line of booms, nets and mines stretching from one shore to the other, and enclosing a broad, safe channel outside which the “U-boats” roam hungrily. There would seem to be no great difficulty in building such a barrier, but it would be extremely difficult to keep intact in heavy weather and for that reason most of our naval officers are skeptical of its existence. Microphones which have been placed under water off the coasts of France, Great Britain, and Ireland have succeeded in detecting the presence of submarines at a distance of fifty-five miles. This device has been perfected by the joint labors of an American electrical engineer, Mr. William Dubilier, and Professor Tissot of the French Academy of Science. These two gentlemen, experimenting with microphones and a submarine placed at their disposal by the French government, “discovered in the course of the tests that the underwater craft were sources of sound waves of exceedingly high frequency, quite distinctive from any other subaqueous sounds. While the cause of the high-pitched sound is known to the inventors, it cannot be divulged since it would then be possible for German submarine constructors to eliminate the source of the tell-tale sound waves, and thus render void the purpose of the detector installation.”[25]