howing dimmed
bow lights whilst doing so.

Two "dark period routes" were laid down, one for vessels bound up
Channel, and another for vessels bound down Channel, and these routes
were some five miles apart in order to minimize the danger of collision,
ships being directed not to use their navigation lights except for
certain portions of the route, during which they crossed the route of
transports and store ships bound between certain southern British ports
(Portsmouth, Southampton and Devonport) and French ports.

Routes were similarly laid down for ships to follow when navigating to
or from the Bristol Channel, and for ships navigating the Irish Sea.

Any system of convoy was at this time out of the question, as neither
the cruisers to marshal the convoy to the submarine area, nor the
destroyers to screen it when there, were available.

There was one very important factor in the situation, viz., the
comparative rate at which the Germans could produce submarines and at
which we could build vessels suitable for anti-submarine warfare and for
defence of commerce. The varying estimates gave cause for grave anxiety.
Our average output of _destroyers_ was four to five per month. Indeed,
this is putting the figure high; and, of course, we suffered losses. The
French and Italians were not producing any vessels of this type, whilst
the Japanese were, in the early part of 1917, not able to spare any for
work in European waters, although later in the year they lent twelve
destroyers, which gave valuable assistance in the Mediterranean. The
United States of America were not then in the war. Consequently measures
for the defence of the Allied trade against the new menace depended on
our own production.

Our _submarines_ were being produced at an average rate of about two per
month only, and--apart from motor launches, which were only of use in
the finest weather and near the coast--the only other vessels suitable
for anti-submarine work that were building at the time, besides some
sloops and P-boats, were trawlers, which, whilst useful for protection
patrol, were too slow for most of the escort work or for offensive
duties. The Germans' estimate of their own submarine production was
about twelve per month, although this figure was never realized, the
average being nearer eight. But each submarine was capable of sinking
many merchant ships, thus necessitating the employment of a very large
number of our destroyers; and therein