That afternoon the course of the convoy was abruptly changed to due west again, in obedience to a signal from the escorting cruiser. It was quite a simple matter that resulted in the alteration of course. The cruiser found that she was in the track of an unknown vessel that, although invisible, left a tell-tale track by throwing overboard ashes and other debris. A keen-witted _kapitan-leutnant_ of a U-boat would not fail to take advantage of these it "signs and portents", hence the advisability of giving the steamer's track a wide berth.