great Allied mine barriers across the
entrances and exits to and from the North Sea were completed and the
losses among the U and U-C boats became heavy. A rapid abatement in the
submarine offensive soon became apparent, and utter failure was only a
matter of time.

FOOTNOTES:

[8] The question of water pressures and many other problems of submarine
engineering relating to under-water fighting are fully treated in
_Submarine Engineering of To-day_, by the Author.

[9] A few of the 7000 were British mines no longer required in the
positions in which they had been laid.

CHAPTER XII

THE MYSTERIES OF MINESWEEPING EXPLAINED

THE task which confronted the naval minesweeping organisations in the
years succeeding 4th August 1914 was an appalling one. Any square yard
of sea around the 1500 miles of coast-line of the British Isles might be
mined at any moment of any day or night. There were, in addition, the
widely scattered fields laid by surface raiders like the _Wolfe_ and the
_Moewe_, which, as described in a previous chapter, extended their
operations to the uttermost ends of the earth. A wonderfully efficient
patrol of the danger zones had its effect in reducing the number of
submarine mine-layers available to the enemy and in rendering both
difficult and hazardous the successful execution of their work, but
neither a predominant and subsequently victorious fleet, nor an equally
skilful and alert patrol, could guarantee the immunity of any
considerable area of sea from mines.

The Germans laid many thousands of these deadly and invisible weapons in
the 140,000 square miles of sea around the British Isles _alone_ in the
face of over 2000 warships. To search for these patches of death in the
wastes of water may well be likened to exploring for the proverbial
"needle in a haystack." Yet the sweepers, whose sole duty it was to
fill this breach in the gigantic system of Allied naval defence,
explored daily and almost hourly, for over four years, the vast ocean
depths, discovering and destroying some 7000 German mines, with a loss
of 200 vessels of their number. The result of this silent victory over
one of the greatest perils that ever threatened the Sea Empire was that
some 5000 food, munition and troop ships were able to enter and leave
the ports of the United Kingdom _weekly_ with a remarkably small
percentage of loss from a peril which might easily have proved
disastrous to the entire Allied cause.

Thi