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.

Into almost every operation carried out on or under the sea there enters
the tide difficulty, and in all mining and minesweeping operations it is
one of the most important factors to be considered. The effect of the
tide on mine-laying has been dealt with in a previous chapter, and the
same difficulties in reverse order are experienced when sweeping the sea
for these invisible and dangerous weapons. It has already been shown
that a vessel may sometimes pass safely over a mine at high water which
would touch her sides or keel and explode if she passed over it at low
water when the mine was nearer to the surface. All minesweeping vessels,
therefore, need to be of comparatively shallow draught in order to
reduce the risk of touching mines, but against this is the fact that
shallow-draught ships, even if powerfully engined, have but little grip
on the water and experience an undue loss of speed when towing a heavy
sweep-wire. Such vessels can seldom operate in even moderately heavy
weather owing to their rolling and pitching propensities. Therefore a
vessel of medium--bordering on shallow--draught, with a fairly broad
beam, is the best type. Here, again, is a difficulty. Minesweeping is a
type of defensive warfare requiring a vast number of ships successfully
to carry on against an enemy well provided with surface and submarine
mine-l