as sighted in the vicinity of one of the mine barriers
already described she was attacked and forced to submerge herself in
order to escape destruction from the guns of the pursuing surface
flotilla. From that moment her fate was sealed. By cautious
manoeuvring and using to full advantage their great superiority of
speed (20-40 knots against 6-10 knots) the surface ships were able to
head their quarry into the mine-field. Usually the submarine dived deep
in order to throw her pursuers off the track, and all unconscious of the
deep-laid mines in thousands she plunged to her doom--a heavy rumble,
followed by an upheaval of the surface, and the chase was over.

This method, when carried out on the vast and scientifically sound
principle described in a previous chapter, offers the best possible
antidote to the submarine. Its employment in the Great European War
placed the seal of complete success on the Allied anti-submarine
offensive. It should, however, be remembered that comparatively narrow
seas and a restricted zone of major operations made possible of
accomplishment with some hundreds of thousands of mines (average cost,
L400) what would in many cases and in many seas have been quite
impracticable with as many millions of these difficult weapons.

* * * * *

The employment of submarines against submarines also forms a method of
under-sea warfare which gives considerable scope for both daring and
resource. It is of course quite impossible for one of these vessels when
totally submerged to fight another in the same blind condition. But with
just the small periscopic tube--or eye of the submarine--projecting
above the surface, one craft can scout and watch for another to rise to
the surface, thinking no enemy is near, in order to replenish her air
supply for breathing or for recharging the electric storage batteries
which supply the current for submerged propulsion.

When such a position obtains the submarine which comes unknowingly to
the surface stands a grave danger of being torpedoed by her opponent.
This actually occurred to at least one German U-boat during the Great
War.

One or more submarines can also be employed around a slow-moving decoy
ship. In this case they would have the advantage of being invisible
until the actual moment of attack. The result of such a manoeuvre
would be either a gun duel on the surface or the torpedoing of the
attacking submarine by one or other vess