I. Airplanes and dirigibles for the location and destruction of submarines. (_a_) By bomb attacks. (_b_) By directing surface boats to the attack of sub-vessels. II. Offensive appliances for use of surface vessels: Sound detectors. Submerged mines operated from shore stations. Deck guns. Under-water guns. Aerial torpedoes. Searchlights. Echo devices. Magnetic devices for locating and destroying submarines. III. Channel and open-sea nets. IV. Submarine vs. submarine. _Defensive Devices:_ I. To be installed on surface vessels to baffle and elude submarines: Sound detectors (spoken of above). Blinding searchlights. Blinding apparatus. II. To offset torpedo attack: Nets. Plates. Magnets. Bombs. Discs. III. Unsinkable ships. IV. Tactics to elude the submarine: Convoying a merchant fleet. Zigzag course. Smoke screen. Cargo submarine. High speed. In considering the practicability or value of these devices, we must first consider the capabilities of the submarine and the proper tactics for her commander to pursue. In a paper read before the Institution of Naval Architects in London, in 1905, I described, illustrated by diagrams, the proper method to be pursued in attacking a surface ship, in which I contended that the commander of a submarine, on sighting an enemy, should always keep the hull of his own boat below the horizon in its relation to the enemy vessel, and try to intercept the approaching vessel by taking frequent observations of her course and speed. When the two vessels approach sufficiently near to make it possible for the larger surface vessel to observe the smaller submarine (the comparative range of visibility being proportionate to the exposed surfaces of the two vessels above the horizon), the submarine should then entirely submerge, with her telescopic periscope withdrawn below the surface of the water to avoid the making of a "wake"--which looks like a white streak on the water. When the commander wishes to make an observation he should first bring his submarine to rest and then extend the periscope above the surface for a brief instant only, and thus avoid the chance of being seen. Earlier in the war it was common to detect the submarine by her wake, but now, since the fitting of merchantmen with guns, the above tactics are usually pursued, and the first intimation the crew has of the presence of the submarine is the shock of the explosion caused by the torpedo "striking home." [Illustration: GERMANY'S U-9 AND SOME OF HER SISTER SUBMARINES] [Illustration: AEROPLANE AND SUBMARINE (Drawing by T. E. Lake.)