_also fitted with
listening appliances_. Whereas if she remained still and waited for the
enemy to approach, mines might be laid in the meantime across important
fair-ways which it was her duty to guard.

German mine-laying submarines were designated U-C boats, and often these
vessels would employ a ruse in order to lay their mines in safety.
Sometimes a decoy would draw the patrols away on a fruitless chase while
the mines were being launched from the tubes of another U-C boat. In one
case a big armed steamer was attacked with torpedoes while mines were
being laid across the line of advance by which a flotilla of warships
would be likely to come out to her aid from a near-by base.

In these and other ways over 3000 mines were laid off the British coast
in one year. There were also several raids by surface mine-layers, which
succeeded in eluding the network of patrols in the fogs and snows which
prevail in the North Sea during several months out of every twelve. The
two most important of these were the cruises of the _Wolfe_ and the
_Moewe_. The former vessel left Germany during the November fogs of
1916, and, by skirting the Norwegian coast, succeeded in passing the
British patrol flotillas. She carried 500 mines, and after crossing the
North Sea in high latitudes, proceeded down the mid-Atlantic until off
the Cape of Good Hope, where the first mine-field was laid. She then
crossed the Indian Ocean, laying fields off Bombay and Colombo.

It was in these seas that she succeeded in capturing a British
merchantman. Placing a German crew and a cargo of mines aboard, she sent
the prize to lay a field off Aden, while she herself proceeded to New
Zealand. In these far-distant waters another field was laid, and a few
months later the last of her cargo was discharged off Singapore. From
this time onward she became a commerce raider.

[Illustration: FIG. 23.--A typical German mine and sinker. _A._ The
mine-casing containing about 300 lb. of high explosive, and the electric
firing device which is put in force when the horns _B_ are struck and
bent by a passing ship. _B._ Horns, made of lead and easily bent if
touched by a surface ship, but sufficiently rigid to resist blows by
sea-water. _C._ Hydrostatic device, operated by the pressure of the
water at a given depth, rendering the mine safe until submerged. _D._
Slings holding mine to mooring rope _F_. _F._ Mooring rope to reel in
sinker. _G._ Reel of mooring wire, which unwin