[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; then, when the time for which the delay was set had expired, they rose to within ten feet of the surface and became a great danger to shipping in places recently swept and reported clear--caused a new form of sweep to be devised and used in waters where these mines were likely to be sown. [Illustration: FIG. 27.--Diagram showing mine mooring being cut by sweep-wire. _A._ Mine-mooring wire. _B._ Hard and cutting face of sweep-wire. The dotted lines _C_ show how the mine floats to the surface by its own buoyancy when the mooring wire holding it down has been cut.] This type of sweep was known as a "bottom sweep," and generally consisted of a chain fitted into the bight of a sweep-wire and dragged along the sea-bed, the idea being to overturn the delayed mine and so upset its mechanism that it would either rise immediately to the surface or else remain for ever harmless at the bottom of the sea. In many cases the heavy chain passing over the horns of the mine would bend and make them useless, so destroying the efficiency of the mine even if it did eventually rise to the correct firing depth.