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 of German mines, laid haphazardly, but also of the thousands of British mines laid methodically and away from neutral traffic.
The English Channel barrage differed from the northern line in several important respects. Being so much shorter (31 miles against 680), it could more easily be made perfect. The swift-running tide, however, greatly increased the difficulty of laying effective mine-fields.
The Lighted Barrage
This southern system consisted, on the surface, of a number of vessels specially built to ride out the heaviest gale at anchor. These were moored at intervals across the Straits of Dover, forming two lines from the English to the French coast. The first line extended from Folkestone to Cape Gris Nez, and the second line about seven miles to the westward of these points (see [Fig. 32]). Each vessel was fitted with powerful searchlights for use at night, and the dark spaces of sea between were patrolled by large numbers of armed craft.
From a photo by Stephen Cribb, Southsea
Fig. 32.—Diagram illustrating the Dover lighted barrage. This barrage consisted of two lines of lightships, E and F, from England A to France B. The first line extended from Folkestone C to Cape Gris Nez D. The second line F was situated seven miles westwards of the first line. The small top diagram shows how the two pathways of light, with a numerous patrol between, compelled the U-boats to dive in order to avoid observation and destruction by gun-fire. The lower diagram shows the deep-laid mines arranged to receive the U-boats when they attempted to run the blockade in a submerged condition.