rease in the buoyancy of the boat,
due to the loss of weight as each mine was discharged into the sea, had
to be instantly and automatically compensated by the admission of
quantities of sea-water of equal weight into special tanks, hitherto
empty, situated below the mine-tubes. If this had been neglected the
submarine would have come quickly to the surface, stern uppermost, owing
to the lightening of the hull by the expulsion therefrom of some fifteen
weapons weighing many hundreds of pounds each.

When the mine was clear of the submarine it sank to the bottom, owing to
the weight of the sinker or anchor. After a short immersion, however, a
special device enabled the top half, containing the charge of explosive
and the contact firing horns, to part company with the heavy lower half,
composed of the iron sinker and the reel of mooring wire. The explosive
section then floated up towards the surface, unwinding the wire from the
sinker.

Each mine being set, before discharge, to a certain prearranged depth
(obtained by the captain of the U-C boat either by sounding wires or
from special charts showing the depth of water in feet), the weapon
could not rise quite up to the surface, being checked in its ascent,
when ten feet from the top, by the mooring wire refusing to unwind
farther.

This may sound a little involved, but a careful study of the
accompanying diagrams will make the various movements of the mine and
its sinker, after leaving the submarine, quite clear to the lay reader.

There were also other types of mines employed. Some were fitted with an
automatic device which was actuated by the pressure of the water at a
set depth. These weapons could be expelled from submarines without the
necessity of knowing and adjusting the depth at which they were to float
below the surface. A mine of this pattern rose up, after discharge from
the tube, until the pressure of water on its casing was reduced to 4 1/2
lb. per square inch (the pressure which obtains at a depth of ten feet
below the surface[8]), and there the weapon stopped, waiting patiently
for its prey.

Another kind of mine was of the floating variety--tabooed by The Hague
Convention--which drifted along under the surface with no moorings to
hold it in one position.

Now that the reader is familiar with the mines themselves and the actual
methods of laying them, we can pass on to a brief review of the German
mine-laying policy during the Great War.

The sub