"Can't we play at a bit of work now?" I suggested. "Supposing we were at sea, and you saw what you thought to be the wake of the periscope of a U-boat a few miles away. What would you do?" My companion laughed. "Well," he said, "if I had the old _Xerxes_ down there on the other end of the string, I should simply report the bearing and approximate distance of the periscope over the telephone, and let her do the rest." "And what would 'the rest' consist of?" I asked. "Principally of turning tail and running at top speed for the nearest protected waters," was the reply, "and incidentally 'broad-casting' a wireless giving position of the U-boat and the direction it was moving in." "But supposing it was a destroyer we had 'on the string'?" I persisted; "and that you had no other present interest in the world beyond the finding of one of these little V-shaped ripples. The _modus operandi_ would vary a bit in that case, wouldn't it?" "Radically," he admitted. "I would give the destroyer what I figured was the shortest possible course to bring her into the vicinity of the U-boat. As long as the wake of the periscope was visible, I would correct that course from time to time by ordering so many degrees to port or to starboard, as the case might be. As soon as the periscope disappeared--which it would do, of course, just as soon as the eye at the bottom of it saw the 'kite'--I would merely make a guess at the submarine's most likely course, and steer the destroyer to converge with that. Our success or failure would then hinge upon whether or not I could get my eye on the submarine where it lurked or was making off under water. In that event--provided only there was enough light left to work with--it would be long odds against that U-boat ever seeing Wilhelmshaven again. Just as you guide a horse by turning it to left or right at the tug of a rein, so, by giving the destroyer a course, now to one side, now to the other, until it was headed straight over its prey, I would guide the craft at the other end of the telephone-wire to a point from which a depth-charge could be dropped with telling effect. If the conditions were favourable, I might even be able to form a rough estimate of the distance of the U-boat beneath the surface, to help in setting the hydrostat of the charge to explode at the proper depth. If the first shot fails to do the business, we have only to double back and let off another. Nothing but the coming of night or of a storm is likely to save that U-boat once we've spotted it." "Is it difficult to pick up a submarine under water?" I asked.