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150 THE SUBMARINE TORPEDO BOAT
aeroplane would offer an easier target to gunfire from the
submarine than the submarine would in turn afford the
aeroplane.
Another theory of using a contact mine suspended
from an aeroplane by a wire is equally impractical. In
fact the vibrations of wire and the resistance of the mine
to passage through the air would set up such extreme
gyrations as to make it almost impossible to strike any
definite object at all.
The passive defense of bays and port entries may be
successfully maintained in a number of ways. Primarily
of course by mines arranged in rows across the mouth
and entrance of the bay or harbor; although it has
recently been demonstrated that the submarine is capable
of both passing safely under the mines and of cutting
them adrift from their moorings as well as to explode
them by counter mining.
Various means for entanglements have also been sug-
gested; such as stretching heavy fishnets at intervals,
and in some cases by fastening small charges of gun-
cotton to the nets, arranged with batteries and circuit
closers so that they are exploded if a submarine becomes
entangled in the net. Another scheme is to stretch spans
of cable across the channel, supported at intervals by
cork floats and weighted down at the ends to hold them
in place.
After all then, it may be truthfully said that as yet we
have been able to devise no adequate means of defense
against the submarine. Some means have been suggested
for defense of coast or harbor, but at sea so far the only
means of defense seems to be in the superior speed of the
surface craft.