pass through the cordon _while
submerged_ would inevitably collide with one or more of them.

With this latter form of barrage the surface of the sea is quite clear
of mines and is comparatively safe for the unrestricted movement of a
numerous patrol flotilla, which forms part of the system, the under-seas
alone being made dangerous by the mines.

It will be apparent that if a hostile submarine base is enclosed by one
or more of these barrages the under-water craft entering and leaving
that base have the choice of travelling _submerged_ across the danger
zone and thereby risking contact with the mines, or of performing the
passage _on the surface_ and encountering the patrolling ships. In
either case, the result is more likely than not to be the destruction of
the submarine.

In most cases the exact position of the barrage would be unknown to the
hostile submarines, which, even if running on the surface, would dive
immediately on the approach of a patrol ship. The few lucky ones
succeeding in getting safely through the cordon of deep-laid mines, or
passing unnoticed the patrol of surface ships on their outward
journey--as might be the case in fog--would have the same peril to face
on the return to their base, and probably without the aid of thick
weather. This double risk would probably have to be taken by every
submarine in the active flotilla at least once a month, this being
approximately the period they can remain at sea without replenishing
supplies of fuel, torpedoes and food.

The object of the flotillas of shallow-draught patrol vessels operating
in the vicinity of the deep mine barrier is twofold. Primarily their
duty is to prevent the hostile submarines from running the blockade on
the surface and, secondly, to prevent enemy surface craft from emerging
from the base and sweeping clear a passage through the mine-field, or of
laying counter-mines, which, on being exploded, would detonate some of
the blockading deep-laid mines and so destroy a section of the barrier.

From this it will be apparent that a force of hostile submarines hemmed
in in this way would run a double risk of losing a number of vessels on
every occasion on which a sortie was made. This is what actually
occurred to the German under-water flotillas in the years 1917-1919,
and, in combination with the other methods employed by the Allied
navies, was mainly responsible for the failure of the great under-sea
offensive.

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