e German navy being situated on the North Sea
littoral, it was possible for the Allies to lay a vast mine barrier,
stretching from the coast of Norway to the Scottish islands, and another
smaller one across the Straits of Dover; also to concentrate in the
vicinity of these two submarine "trench systems" a very numerous surface
patrolling force, thus enclosing the thousands of square miles of sea
forming what was sometimes boastfully referred to as the "German Ocean"
in an almost impenetrable ring of steel and T.N.T.

Here let us consider the gigantic nature of the task that was
successfully accomplished. The distance from the Norwegian coast to the
Orkney Islands is approximately 600 miles. It was over this vast expanse
of sea, bent at the eastern end so as to rest on the Heligoland Bight,
that the system known as the "Northern Barrages" extended. No exact
statistics of the actual number of mines used is at present available,
but reckoning at the low rate of one mine to every 750 feet of sea, with
five lines stretching from shore to shore, the number required would be
21,000 of these costly and difficult weapons. The number required
annually to maintain such a barrage would also be very heavy, and it is
safe to assume that _considerably_ over 50,000 mines were employed on
the northern barrages alone. From this rough estimate some idea of the
work of designing, manufacturing, testing, laying, renewing and watching
this one field will be obtained.

[Illustration: FIG. 31.--Diagram illustrating a mine barrage, or
deep-laid mine-field. The submarine _A_, diving to avoid a surface
warship, has become entangled in the mooring of a deep-laid mine which
is being dragged down on top of her. These mines are often moored at a
depth of 60 feet below the surface, which can then be patrolled by
surface warships.]

There were, of course, in the actual barrage several mine-fields placed
strategically, and probably a far greater number of weapons than that
given in the above estimate was needed. There were also the smaller
fields lying between the northern barrage and the one across the Straits
of Dover. These were so placed as to catch hostile submarines operating
off the east coast of England, or a surface raiding squadron, such as
those which in the earlier years of the war bombarded certain British
ports.

Finally, when victory had been achieved, there came the cold-blooded
task of clearing these immense areas of sea, not only o