ach,
working in combination, they used to ascertain its position by taking
cross bearings of the directions of the sound as given by their
respective hydrophones, and gradually closed in on it. When the
position of the submarine was definitely located, an attendant vessel
was signalled to, which did its best to drop depth charges on the
submarine, or, if it came to the surface, attacked it with gunfire.
But it was, of course, possible for the enemy, who also carried his
hydrophones, to slip away; and to successfully trap him by the above
device was an event of rare occurrence. Like the trawlers, the
drifters carry guns and depth charges.

The trawlers and drifters manned by the men who used to fish with
these vessels before the war compose the greater portion of the
Harwich auxiliary force. Shortly after the opening of the war the
Admiralty took over a number of ordinary paddle passenger steamers for
the purpose of mine-sweeping, of which several belong to the Harwich
mine-sweeping unit. These are commanded by R.N.R. captains; carry
six-pounder or twelve-pounder guns, and depth charges. Being of
relatively high speed--some of them attaining a sweeping speed of ten
knots--they can cover a good deal of ground, and being of shallow
draught they are well adapted for mine-sweeping in the Harwich area.
For the tidal range in this portion of the North Sea is about eleven
feet; consequently the paddle steamer, drawing considerably less than
eleven feet, is enabled at high water to engage in sweeping without
incurring much risk of striking a German mine, provided that the area
has been searched at low water and no mines are visible on the
surface. These paddle steamers, which in time of peace had carried
thousands of pleasure-seekers on summer holidays, at once proved very
successful in the work of war. In the year 1917 alone they destroyed
approximately four hundred enemy mines in the immediate approaches to
Harwich. On several occasions the vessels of this section had narrow
escapes; one was twice mined, and one sank in fifty seconds after
striking a mine.

And lastly we come to that interesting class in this heterogeneous
force--the motor launches--the compact M.L. boats and other power
boats of various types, most of which were privately owned pleasure
craft before the war. Handy, rapid, of light draught, these have
proved of great service, especially in enclosed and shallow waters.
They are employed for patrol work, also