a depth of one. Most of the destroyers were stationed on the narrow sides, for it was only on the side, or the beam, that the submarines could attack with much likelihood of succeeding. It was usually necessary for a destroyer to be stationed in the rear of a convoy, for, though the speed of nearly all convoys was faster than that of a submarine when submerged, the latter while running on the surface could follow a convoy at night with a fair chance of torpedoing a vessel at early daylight and escaping to the rear if unhampered by the presence of a rear-guard destroyer. It was generally impracticable and dangerous for the submarine to wait ahead, submerge, and launch its torpedoes as the convoy passed over it. The extent to which purely mechanical details protected merchant ships is not understood, and this inability to attack successfully from the front illustrates this point. The submarine launches its torpedoes from tubes in the bow or stern; it has no tubes on the beam. If it did possess such side tubes, it could lie in wait ahead and shoot its broadsides at the convoy as it passed over the spot where it was concealed. Its length in that case would be parallel to that of the merchant ships, and thus it would have a comparatively small part of its area exposed to the danger of ramming. The mere fact that its torpedo tubes are placed in the bow and stern makes it necessary for the submarine, if it wishes to attack in the fashion described, to turn almost at right angles to the course of the convoy, and to manœuvre into a favourable position from which to discharge its missile—a procedure so altogether hazardous that it almost never attempts it. With certain reservations, which it is hardly necessary to explain in detail at this point, it may be taken at least as a general rule that the sides of the convoy not only furnish the U-boats much the best chance to torpedo ships, but also subject them to the least danger; and this is the reason why, in the recent war, the destroyers were usually concentrated at these points.
I have already compared the convoy system to a great aggregation of railroads. This comparison holds good of its operation after it had entered the infested zone. Indeed the very terminology of our railroad men was used. Every convoy nearly followed one of two main routes, known at convoy headquarters as the two "trunk lines." The trunk line which reached the west coast of England usually passed
