Among the most important of these were the immense meshes of wire known as "indicator nets," which were used to entangle a submarine and then to proclaim her movements to surface ships waiting to attack 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 and (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.