were mounted in monitors.

In the matter of blocking the entrance to the ports of Zeebrugge and
Ostend, the fact had to be recognized that effective _permanent_
blocking operations against destroyers and submarines were not
practicable, mainly because of the great rise and fall above low water
at ordinary spring tides, which is 14 feet at Ostend and 13 feet at
Zeebrugge for about half the days in each month. Low water at Ostend
also lasts for one hour. Therefore, even if block-ships were sunk in the
most favourable position the operation of making a passage by cutting
away the upper works of the block-ships was not a difficult matter, and
the Germans are a painstaking people. This passage could be used for
some time on each side of high water by vessels like destroyers drawing
less than 14 feet, or submarines drawing, say, 14 feet. The block would,
therefore, be of a temporary and not a permanent nature, although it
would undoubtedly be a source of considerable inconvenience. At the same
time it was realized that, although permanent blocking was not
practicable, a temporary block would be of use, and that _the moral
effect alone of such an operation would be of great value_. These
considerations, together with the abandonment of the proposed landing on
the Belgian coast, owing to unfavourable military conditions, led to the
decision late in 1917 to undertake blocking operations concurrently with
an attack on the vessels alongside the Mole at Zeebrugge.

In order to carry out the general policy mentioned, the eastern end of
the Straits of Dover had been heavily mined at intervals during the war,
and these mines had proved to be a sufficient deterrent against any
attempt on the part of surface vessels larger than destroyers to pass
through. Owing to the rise of tide enemy destroyers could pass over the
minefields at high water without risk of injury, and they frequently did
so pass. Many attempts had been made to prevent the passage of enemy
submarines by means of obstructions, but without much success; and at
the end of 1916 a "mine net barrage"--i.e. a series of wire nets of wide
mesh carrying mines--was in process of being placed by us right across
the Straits from the South Goodwin Buoy to the West Dyck Bank, a length
of 28 miles, it being arranged that the French would continue the
barrage from this position to the French coast. The construction of the
barrage was much delayed by the difficulty in procuring mooring b