cently, serious doubts have been raised concerning the
future usefulness of nets. Reports have been published that German
submarines have been fitted up with a wire and cable cutting
appliance which would make it possible for them to break through
nets at will, supposing, of course, that they had been caught by the
nets in such a way that no vital parts of the underwater craft had
been seriously damaged. A sketch of this wire cutting device was
made by the captain of a merchantman, who, while in a small boat
after his ship had been torpedoed, had come close enough to the
attacking submarine to make the necessary observations. The sketch
showed an arrangement consisting of a number of strands of heavy
steel hawsers which were stretched from bow to stern, passing
through the conning tower and to which were attached a series of
heavy circular knives a foot in diameter and placed about a yard
apart. Even as early as January, 1915, Mr. Simon Lake, the famous
American submarine engineer and inventor, published an article in
the _Scientific American_ in which he dwelt at length on means by
which a submarine could escape mines and nets. One of the
illustrations, accompanying this article, showed a device enabling
submarines travelling on the bottom of the sea to lift a net with a
pair of projecting arms and thus pass unharmed under it.

[Illustration: (C) International Film Service, Inc.

_Submarine Built for Spain in the Cape Cod Canal._]

Many other devices to trap, sink or capture submarines have been
invented. A large number of these, of course, have been found
impracticable. Others, however, have been used with success. Few
details of any of these have been allowed to become known.

The most dangerous power of submarines, is their ability to approach
very closely to their object of attack without making their presence
known to their prey. This naturally suggested that a way be found to
detect the presence of submarines early enough to make it possible
to stave off an attack or even to assume the offensive against the
underwater boat. A recent invention, the perfection of which is due
to the work of Mr. William Dubilier, an American electrical
engineer, and of Professor Tissot, a member of the French Academy of
Science, is the microphone. Few details are known about this
instrument except that it records sound waves at as great a distance
as fifty-five miles. This would permit in most cases the calling of
patrol boats