degrees to port, and started in a new direction. And so on for hours, now a few minutes to the right, now a few minutes to the left, and now again straight ahead, while all the time the destroyers were cutting through the water, every eye of the skilled lookouts in each crew fixed upon the surface for the first glimpse of a periscope. This zigzagging was carried out according to comprehensive plans which enabled the convoy to zigzag for hours at a time without signals, the courses and the time on each course being designated in the particular plan ordered, all ships' clocks being set exactly alike by time signal. Probably I have made it clear why these zigzagging evolutions constituted such a protective measure. All the time the convoy was sailing in the danger zone it was assumed that a submarine was present, looking for a chance to torpedo. Even though the officers might know that there was no submarine within three hundred miles, this was never taken for granted ; the discipline of the whole convoy system rested upon the theory that the submarine was there, waiting only the favourable moment to start the work of destruction. But a submarine, as already said, could not strike without the most thorough preparation. It must get within three or four hundred yards or the torpedo would stand little chance of hitting the mark in a vital spot. The commander almost never shot blindly into the convoy, on the chance of hitting some ship; he carefully selected his victim; his calculation had to include its speed, the speed of his own boat and that of his torpedo; above all, he had to be sure of the direction in which his intended quarry was steaming; and in this calculation the direction of the merchantman formed perhaps the most important element. But if the ships were constantly changing their direction, it is apparent that the submarine could make no calculations which would have much practical value.
In the afternoon the Aubrietia, the British mystery ship which was sailing thirty miles ahead of the convoy, reported that she had sighted a submarine. Two or three destroyers dashed for the indicated area, searched it thoroughly, found no traces of the hidden boat, and returned to the convoy. The next morning six British destroyers and one cruiser arrived from Devonport. Up to this time the convoy had been following the great "trunk line" which
