f 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.

[Illustration: MINESWEEPING GEAR ON A TRAWLER

_From a photo by Stephen Cribb, Southsea_]

[Illustration: 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.]

By this means the only avenues by which hostile submarines could hope to
pass on the surface through the barrage at night were the dark lanes of
water between the lightships. It was these points which were closely
guarded by strong patrol flotillas, whose duty it was to attack
submarines attempting to get through and, with the aid of guns and depth
charges, to force them to dive below the surface.

Here certain destruction awaited them on the submerged mine-fields. If,
however, one line of defence was safely passed by a hostile submarine,
there was another to be negotiated seven miles farther on, and once a
submarine got between the two lines her chances of escape were indeed
small, for whichever way she turned the surface would be covered with
fast patrol