MINES). Both were fired by electricity, which was applied only
at the moment a hostile ship was within the dangerous zone of a mine. In
the observation mines the moment of applying the electric current was
ascertained by a position-finder, which, tracing a ship's course on a
chart, made an electrical connexion at the moment the ship was over a
mine. These mines were placed so as to be well below the bottom of any
ships afloat and were used in channels which it was desired to leave
open for the entrance of friendly vessels. Contact mines, which are
moored a few feet below the surface of the water, are fired after
certain electrical connexions have been made in a firing room on shore
by the ship itself striking against the mine. These are used in waters
which it is intended to deny to friend and foe. Except in narrow waters
where the whole width of the channel was required for friendly traffic,
contact mines were generally used to limit the width of the channel to
the minimum consistent with the amount of friendly traffic which would
use the port in war. It will be readily understood that by bending this
channel and disclosing its exact position only to special pilots, a very
complete measure of security against surprise would be obtained. In
English ports the practical importance of allowing free ingress for
friendly traffic overruled all other considerations, and the friendly
channels were always straight and coincided with some part of the usual
fairway channel. They were also carefully marked by lightships and
buoys.

A variation of the submarine mine is the Brennan torpedo, purchased by
the British government about 1890. This differs from the torpedo used on
board ship, mainly by the fact that the engine-power which drives it is
on shore and connected with the torpedo by two strong wires. These wires
are wound out of the torpedo by the engine, and by varying the strain on
the two wires very accurate control of the steering can be obtained.
This torpedo shares with the submarine mine the disadvantages that it
must wait for the enemy to venture within its range, and with all other
forms of defence (except contact mines), that it is made useless by fog
or rain. As compared with a mine it has the advantage of being
unaffected by tide or depth, and of forming no obstruction to traffic,
except when actually in action. It was installed at the principal ports
only.

The system of defence hitherto described is thus a main g