types of
German submarine mines would occupy many pages with uninteresting
technical formulae. It is sufficient to say that they carried an
explosive charge (200 to 400 lb. of T.N.T.) sufficient to blow to pieces
vessels of several hundred tons and to seriously damage the largest
warship. They were intended to float a few feet below the surface--being
held down by the mooring rope--but, as there was no means of
compensating for the rise and fall of the tide, many of them often
showed their horns above the surface at low water and were immersed too
deep to be of much use against any but the deepest draught ships at high
tide. A reference to Fig. 24 will make this difficulty clear.

There was scarcely a ship afloat in the zone of operations which did
not, during those years of storm, sight one or more of these hateful
weapons with their horns showing above the surface. Motor launches were
employed to scout for them during the hour before and the hour after low
water. In this way many hundreds were discovered and destroyed almost
as soon as they had been laid. One badly laid mine, which shows on the
surface when the tide ebbs, will often give away a whole field of these
otherwise invisible weapons, and the work of sweeping them up and
destroying them is then rendered comparatively easy.

The effect of strong tides on a moored mine is considerable, and will
render a field quite harmless for several hours out of every
twenty-four. The reason for this is best described with the aid of a
diagram.

It will be seen from the above that the mine will not remain vertically
above its sinker when there is a tide, but will incline at an angle
determined by the strength of the current, which, if considerable, will
press the weapon down much deeper than the keel of any ship (see Fig.
24). When the tide turns the mine will first regain its true
perpendicular position and then incline in the opposite direction,
accommodating itself to the ebb and flow. From this it will be apparent
that in places where there is a strong current or tide a mine-field is
only dangerous to passing ships of shallow or medium draft for a few
hours (during slack water) out of the twenty-four. Between the ebb and
the flow of a tide there is a short period when the water is almost
still. Then the movement begins to set in from the opposite direction
and gradually gains in speed until about one hour before high or low
tide. This period of what is known as "slac