But the operation of beating off an occasional surface raider, which necessarily fights in the open, is quite a different procedure from that of protecting an aggregation of vessels from enemies that discharge torpedoes under the water. As part protection against such insidious attacks both the merchant ships and the escorting men-of-war of to-day had in this war to keep up a perpetual zigzagging. This zigzag, indeed, was in itself an efficacious method of protection. As already said, the submarine was forced to attain an advantageous position before it could discharge its torpedo; it was its favourite practice to approach to within a few hundred yards in order to hit its victim in a vital spot. This mere fact shows that zigzagging in itself was one of the best methods of avoiding destruction. Before this became the general rule, the task of torpedoing a vessel was comparatively easy. All it was necessary for the submarine to do was to bring the vessel's masts in line; that is, to get directly ahead of her, submerge with the small periscope showing only occasionally, and to fire the torpedo at short range as the ship passed by. Except in the case of very slow vessels, she could of course do this only when she was not far from the course of her advancing prey when she first sighted her. If, however, the vessel was zigzagging, this pretty game was usually defeated; the submarine never knew in what direction to go in order to get within torpedoing distance, and she could not go far because her speed under water is so slow. The same conditions apply to a zigzagging convoy. This explained why, as soon as the merchant vessel or convoy entered the submarine zone, or as soon as a submarine was sighted, it began zigzagging, first on one side and then on the other, and always irregularly, its course comprising a disjointed line, which made it a mere chance whether the submarine could get into a position from which to fire with any certainty of obtaining results. A vessel sailing alone could manoeuvre in this way without much difficulty, but it is apparent that twenty or thirty vessels, sailing in close formation, would not find the operation a simple one. It was necessary for them to sail in close and regular formation in order to make it possible to manoeuvre them and screen them with destroyers, so it is evident that the closer the formation the fewer the destroyers that would be needed to protect it. These circumstances make the modern convoy quite a different affair from the happy-go-lucky proceeding of the Napoleonic era.