submarine and of the great volume of traffic
passing up and down our coasts will assist in a realization of the
varied and difficult problems set to the British Navy.

For instance, the total number of vessels passing Lowestoft during the
month of April, 1917, was 1,837 British and Allied and 208 neutral,
giving a _daily_ average of 62 British and Allied and 7 neutral ships;
and as Admiral Sir Reginald Bacon has mentioned in his book, "The Dover
Patrol, 1915-17" (page 51), an average of between 80 to 100 merchant
vessels passed Dover daily during 1917. A study of these figures gives
some idea of the number of targets offered daily to ordinary submarines
and minelaying submarines in two of the areas off our coasts. When it is
borne in mind that the Germans had similar chances of inflicting heavy
losses on our mercantile marine all round the coasts of the United
Kingdom, and that it was obviously impossible to tell where an
underwater attack would take place, it will be realized that once
submarines reached our coasts, nothing short of an immense number of
small craft could deal satisfactorily with the situation, and afford any
degree of protection to trade. Minelaying by submarines was a
particularly difficult problem with which to deal; the enemy frequently
changed his methods, and such changes when discovered involved
alterations in our own procedure. Thus for some time after the
commencement of minelaying by submarines, the whole of the mines of one
submarine would be laid in a comparatively small area. It was fairly
easy to deal with this method as a dangerous area was proclaimed round
the spot where a mine was discovered, and experience soon showed the
necessary extent of area to proclaim. Later the submarines laid mines in
groups of about six. This necessitated the proclamation of more than one
area, and was naturally a more difficult problem. At a further stage the
submarines scattered their mines in even smaller numbers, and the task
of ensuring a safe channel was still further increased. The most
difficult artifice to deal with, however, was the introduction by the
Germans of a delay action device in their mines, which caused them to
remain at the bottom for varying periods after being laid. The ordinary
mine-sweep, the function of which was to catch the mooring rope of the
mine and drag the mine clear of the channel, was, of course, ineffective
against the mine on the bottom, and there was no guarantee that min