As soon as the destroyers made contact with an inward-bound convoy, the ocean escort, the cruiser or pre-dreadnought, if an American, abandoned it and started it back home, sometimes with a westbound convoy if one had been assembled in time. British escorts went ahead at full speed into a British port, usually escorted by one or more destroyers. This abandonment sometimes aroused the wrath of the passengers on the inbound convoy. Their protector had dropped them just as they had entered the submarine zone, the very moment its services were really needed! These passengers did not understand, any more than did the people at home, that the purpose of the ocean escort was not to protect them from submarines, but from possible raiders. Inside the danger zone this ocean escort would become part of the convoy itself and require protection from submarines, so that its rather summary departure really made the merchantmen more secure. As the convoy approached the danger zone, after being drilled all the way across the ocean, its very appearance was more taut and business-like. The ships were closed up into a much more compact formation, keeping only such distances apart as were essential for quick manoeuvring. Generally the convoy was formed in a long parallelogram, the distance across the front of which was much longer than the depth or distance along the sides. Usually the formation was a number of groups of four vessels each, in column or "Indian file," at a distance of about five hundred yards from ship to ship, and all groups abreast of each other and about half a mile apart. Thus a convoy of twenty-four vessels, or six groups of four, would have a width of about three miles and 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 manoeuvre 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.