All the foregoing sounds very thorough and hopeful, but in fairness it must be said that submarine hunting is a heart-breaking task. The reader may have noticed that the method of depth-charge attack presupposes the surface vessel to have attained a position almost directly over the top of her enemy, a man[oe]uvre extremely difficult of achievement even with the most efficient hydrophone. Heavy seas, snow and fog have also to be taken into consideration, to say nothing of darkness, the presence of a second submarine, a surf-beaten rock or sandbank and the confusing sounds of passing merchant ships, making a difficult task more difficult, as will be seen when we come to the actual fighting.
CHAPTER VI
SOME CURIOUS WEAPONS OF ANTI-SUBMARINE WARFARE
Although modern war has shown that there exists no certain antidote for the submarine, it nevertheless brought into being many curious weapons of attack and defence. It is the purpose of this chapter to describe some of the anti-submarine devices used with more or less successful results during the protracted naval operations against the Central Powers.
Indicator Nets
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.
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 (½ 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. 1]2).