Both periscopes were "housed", and the boat prepared to dive at an instant's notice. For a full quarter of a mile the course was due south, until, at a flashing-signal from the leading torpedo-boat, the big German starboarded her helm, and steered almost at right angles to her former direction. R19's officers noticed that the rearmost destroyer made no attempt to alter helm until she gained the position where the armed merchantman had turned. Evidently the "gateway" through the mine-field was narrow, and permitted no liberties. As the following destroyer turned she flashed out a signal, to which a distant vessel replied. The next instant the concentrated rays of a dozen search-lights swept the surface of the water. Down dropped R19 to 20 feet. Her periscopes were raised until they projected but 18 inches above the surface. Until the crucial moment, Stockdale chose to keep the escort under observation. Again the Hun vessels turned, this time to port, and were heading straight for the centre of the far-flung line of patrol boats and destroyers. "We're through, I fancy!" exclaimed the Hon. Derek. Then: "Down to forty feet." At that depth the submarine was immune from the danger of being stove in, even by the keel of the heaviest battleship afloat. For the rest of the distance, until the last of the patrolling craft was left astern, the submarine had to depend upon direction by listening to the thresh of the Hun torpedo-boats' propellers. The raider and her escorts were now increasing speed, another indication that the danger of the mine-field was a thing of the past. Before long R19, in her efforts to keep up with her hostile guides, was pushing ahead at fourteen knots--a rate sufficient to raise an ominous swirl upon the placid surface. The while Macquare and the Sub were working out the course for future reference, noting the varying compass bearings and the distance run between alterations of helm. Knowing the exact spot where the second channel began, it would be a relatively simple matter to "plot out" the secret channel on the chart for use on the return run--if R19 were fortunate enough to leave the Baltic. In order to check each other's calculations the Lieutenant made his readings on a magnetic compass, while the Sub used the gyro-compass, which, unaffected by deviation and variation, enabled the navigator to obtain his knowledge of direction without having to take into consideration half a dozen intricate but important influences to which the magnetic instrument is subjected.