order to encompass
this many kinds of minesweeping gear were devised and given practical
trial during the war. The one most generally used, however, was the
original but vastly improved sweep. This consisted of a special wire
extended between two ships and held submerged by a device known as a
kite. This apparatus is best described diagrammatically (Fig. 25).

There was, however, another type of sweep used for exploratory work and
also for sweeping in shallow water. It was a one-ship sweep (_i.e._
required only one vessel to drag it), and this can also be best
described by a diagram (Fig. 26).

[Illustration: FIG. 25.--Diagram showing the form of apparatus
principally used by British minesweepers. _AA._ Sweeping vessels. _BB._
Sweep-wire. _CC._ Wires holding kites. _DD._ Kites which hold sweep-wire
at correct depth below the surface by their "kite-like" action when
being towed through the water. _E._ Mine and mooring. _F._ Surface of
the sea. _G._ Sea-bed.]

It will be observed that in all cases the object is to drag a submerged
wire through the water at an angle from the ship's course until it
encounters the mooring wire of a mine. When this takes place it is the
purpose of the sweep-wire to cut the mooring wire and allow the buoyant
mine to float up to the surface free of its sinker (see Fig. 27). In
order to effect this various kinds of hard wire with a cutting capacity
were used as sweep-wires, and also numerous mechanical devices, all of
which are more or less of a secret character; but the object remained
the same--to find and cut the mooring wire.

[Illustration: FIG. 26.--Diagrammatic sketch showing principal parts of
a single-ship sweep. _A._ Towing vessel. _B._ Tail wire. _C._ Kite
holding sweep-wires _D_ at correct depth below the surface. _D._ Light
sweep-wires held at an angle by spars _E_ and surface hydroplane floats
_F_. The dotted lines show how either arm of the sweep swing towards the
centre line when exposed to the pull of a mine. This movement of the
hydroplane floats indicates to those on board the sweeping vessel that a
mine has been caught. The mine _H_ slides down the sweep-wire until the
mooring is cut at _G_, and the mine floats freely to the surface.]

The introduction of what became known as "delayed action mines"--weapons
held down on the sea-bed, after being launched, for varying periods of
time, so that sweeping operations might take place above them without
their being discovered;