proved ineffective owing to heavy tides, currents, and bad bottom
conditions, until it was strengthened by Admiral Keyes in 1918. By
employing a large type of mine perfected by the United States Naval
Bureau of Ordnance, it was found possible, however, to reduce by
one-third the number of mines and the amount of wire needed for the
North Sea Barrage. The task was therefore undertaken, and completed
in the summer of 1918. Out of a total of 70,000 mines, 56,570, or
about 80 per cent, were planted by American vessels. The barrage
when completed gave an enemy submarine about one chance in ten of
getting through. According to reliable records, it accomplished
the destruction or serious injury of 17 German submarines, and by
its deterrent effect, must have practically closed the northern
exit to both under-water and surface craft.

[Illustration: OSTEND-ZEEBRUGGE AREA]

_The Attack on Zeebrugge and Ostend_

At the Channel exit of the North Sea, a vigorous blow at the German
submarine nests on the Belgian coast was finally struck on April
22-23, 1918, by the Dover Force under Vice Admiral Roger Keyes, in
one of the most brilliant naval operations of the war. Of the two
Belgian ports, Ostend and Zeebrugge, the latter was much more useful
to the Germans because better protected, less exposed to batteries
on the land front, and connected by a deeper canal with the main
base 8 miles distant at Bruges. It was planned, however, to attack
both ports, with the specific purpose of sinking 5 obsolete cruisers
laden with concrete across the entrances to the canals. The operation
required extensive reconstruction work on the vessels employed, a
thorough course of training for personnel, suitable conditions of
atmosphere, wind, and tide, and execution of complicated movements
in accordance with a time schedule worked out to the minute.

At Ostend the attack failed owing to a sudden shift of wind which
blew the smoke screen laid by motor boats back upon the two block
ships, and so confused their approach that they were stranded and
blown up west of the entrance.

At Zeebrugge, two of the three block ships, the _Iphigenia_ and
the _Intrepid_, got past the heavy guns on the mole, through the
protective nets, and into the canal, where they were sunk athwart
the channel by the explosion of mines laid all along their keels.
To facilitate their entrance, the cruiser _Vindictive_ (Commander
Alfred Carpenter), fitted with a false deck and 18