In shallow water where there was a fairly smooth bottom it generally happened that a submarine damaged by depth charges elected to sink to the sea-bed and trust to luck. This was also frequently resorted to as a means of eluding pursuit even when the U-boat was not damaged by the first few charges dropped. It was then that the hydrophones carried by the surface ships were again brought into use to ascertain if the submarine was still under way. When no sound was heard those on the surface knew that "Fritz" was lying doggo, or else that he had escaped. If a number of ships were available a few waited over the spot where it was considered the U-boat was lying, while the others scoured the surrounding seas in circles trying to pick up the sound of the runaway's engines if she had escaped in the melee. When nothing further was heard they returned to the scene and set about the work of systematically bombing the surrounding sea-bed. [Illustration: FIG. 10.--Diagram illustrating a depth charge attack on a submerged submarine. _A._ Motor launch, which has dropped a depth charge to destroy a submarine _B_ travelling at a depth of 90 feet below the surface. _C_ is the depth charge sinking as the M.L. steams away from the danger area. _D_ is the point (80 feet below the surface) at which it will explode, and _E_ indicates the danger area for the submarine _B_.] As many as one hundred depth charges were dropped in quite a small area of sea and yet a submarine known to have been lying "doggo" in the locality was not damaged. In cases such as this other means, which will be described in a succeeding chapter, were then resorted to. 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 manoeuvre 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