tack with guns and
depth charges.

[Illustration: FIG. 11.--Diagram showing principal features of a line of
submerged indicator nets. _AA._ Two sections (100 feet in breadth) of
thin wire-netting with a very wide mesh. _B._ Framework of wire rope
holding each section of net in place by means of metal clips _C_. _C._
Metal clips which expand and release netting from rope frame when a pull
of more than 100 lb. is exerted upon them. _D._ Line of invisible glass
balls, or hollow floats, attached to a surface wire _E_, supporting by
wires _F_, the nets which hang down from the surface vertically in long
lines (1/2 to 1 mile in length and 50 feet deep). _G._ Heavy iron
weights or sinkers holding down the nets by their weight when hanging in
water. _H._ Wooden floats, attached to each section of net by wires _I_.
_J._ Canisters of chemical which give off flame and smoke when exposed
to sea-water. _K._ Lanyard attached to surface wire _E_. When a section
of net is pulled out of its wire frame by a submarine passing through
the line the float is dragged along the surface by the wire _I_. The
lanyard is held back by being attached to surface wire _E_, and pulls a
plug out of the canister _J_, exposing the chemical inside to the
sea-water (see Fig. 12).]

These nets were made of specially light but strong wire, with a mesh of
several feet. They were joined together in lengths of 100 feet by metal
clips which opened when a certain strain was exerted on any particular
section. Their depth was usually about 50 feet, and they were laid in
lengths varying from a few hundred yards to two miles. Weights at the
lower end and invisible glass floats along the top held them suspended
vertically from the surface. The floats were kept in place by a wire
hawser running along the top of the nets, and to this were attached, at
intervals, wooden buoys containing tin cases filled with a chemical
compound which, when brought into contact with sea-water, emitted dense
smoke by day and flame by night.

The 100-feet sections were linked together, and to the top and bottom
ropes, by the metal clips. These clips opened when a submarine headed
into that part of the line. The result was that a section of net
enveloped the bow of the under-water craft, was detached from the line
and carried along, dragging its _indicator float_ on the surface behind.

The indicator float, containing the chemical, was attached (1) to the
section of net by a short wire an