d (2) to the top rope of the whole line
by a lanyard, which, when pulled free, exposed the chemical contents of
the canisters in the float to the sea-water. The float was then dragged
along the surface burning furiously.

As there was nothing to materially impede her progress, a submarine
would consequently be unaware that she had passed through a line of nets
and was actually towing a flaming buoy. Even if she became aware of the
tell-tale appendage it would be extremely difficult to clear herself,
owing to the forward hydroplanes becoming entangled in the wire-netting,
before the fast surface ships, waiting in readiness, had spotted the
flaming buoy being towed along and were hot in pursuit.

Once entangled in such a net, the submarine's chance of avoiding
destruction was small. Not only did the indicator buoy proclaim her
every movement to the pursuing surface ships, so that she could not
avoid them by turning, sinking to the bottom or doubling in her tracks,
but it also enabled depth charges to be literally dropped on her decks.

[Illustration: FIG. 12.--Diagram showing a submarine entangled in a
submerged net. The submarine _A_ after passing through a line of nets
emerges with her bows enveloped by one section _B_ which she has carried
out of its wire-rope frame. The flaming buoy _C_, betraying her
movements, is being towed along the surface.]

A considerable measure of success attended the use of this ingenious
device until "Fritz" became shy of waters close inshore, and kept a
careful look-out for possible lines of indicator nets when forced to
pass through narrow channels and waterways. One of the main
disadvantages attending the use of these nets was the impossibility of
laying them--or, when laid, of hauling them inboard again, during even
moderately rough seas. Another difficulty which presented itself when
indicator nets were required to be laid in the open sea was the
screening of the waiting surface ships from observation. Submarines
could not be used on account of their slow speed, and when fast patrol
craft cruised about openly within easy range of the nets "Fritz"
suspected a trap and steered clear. Even this, however, had its uses.

MINE NETS

It was sought to overcome this difficulty by attaching small explosive
mines to the nets instead of indicator floats, so that when a submarine
passed through a line she unavoidably struck one or other of the
attached mines, which instantly exploded.

Th