submarines were expected to pass. The watch off the Horn Reef and in the
Heligoland Bight, which had previously been in force, was also
maintained.

A little later the submarine flotilla off the Irish coast was
strengthened, and a regular patrol instituted near the North Channel
between Ireland and Scotland. The next step was the withdrawal of some
"C" Class submarines from coastal work on our east coast to work in the
area between England and Holland near the North Hinder Lightship, a
locality much frequented by enemy submarines on passage. Still later
some submarines were attached to the Portsmouth Command, where, working
under Sir Stanley Colville, they had some striking successes; others
went to the Dover Command. The latter were fitted with occulting lights
on top of the conning-tower, and were moored at night to buoys in the
Dover Net Barrage, in places where enemy submarines were likely to pass,
in order that they might have a chance of torpedoing them. A division of
submarines was also sent to Gibraltar, to operate against enemy cruiser
submarines working in that vicinity or near the Canaries. Successes
against enemy submarines were also obtained in the latter locality.

Finally, the arrival of some United States submarines enabled the areas
in which this form of attack was in force to be still further extended,
after the American personnel had been trained to this form of warfare.
There was a great increase in the number of enemy submarines sunk by
this method of attack during 1917 as compared with previous years; the
number of vessels sunk does not, however, convey a complete appreciation
of the effect of this form of anti-submarine warfare. The great value of
it lay in the feeling of insecurity that it bred in the minds of the
enemy submarine commanders. The moral effect of the constant
apprehension that one is being "stalked" is considerable. Indeed, the
combination of our aircraft and our submarine patrols led to our vessels
reporting, regretfully, that it was very seldom that German submarines
were found on the surface in daylight, and towards the end of 1917 quite
a large proportion of the attacks on merchant ships took place at night.

The work for our own vessels was very arduous indeed. It was only on
rare occasions that it was possible to bring off a successful attack on
a submarine that had been sighted, the low underwater speed of
submarines making it difficult to get into position when the enemy w