The log gave the "distance run", the reading being checked by independent calculations based upon the revolutions of the propellers. For directing, R19 had to depend solely upon a compass course, since the mist made it impossible to pick up shore bearings, even if these were visible at night. After an hour of high-tensioned suspense the Sub made his way aft to the Petty Officer at the log indicator. "How goes it?" he whispered. "Close on, sir," was the equally cautious reply. "I thought so," agreed Fordyce. "Good enough." "We're over Position A, sir," he reported. "Or thereabouts," added the Hon. Derek under his breath. "Starboard eight, Quartermaster." Round swung the deeply-submersed hull of the submarine to settle on her new course--the awkward turn in the channel through the mine-field. Already the netted area that had all but finished R19's career on her outward voyage was left astern. After a comparatively short run the order was given to port helm. R19 had negotiated the awkward bend in the cleared channel, and a straight run northwards would see her beyond the limits of the mine-field. Suddenly, at a distance of about forty fathoms astern, and slightly on the starboard quarter, a column of water leapt two hundred feet or more into the misty air, accompanied by a roar like the concentrated peals of a dozen thunderclaps. The next instant Fordyce, almost capsized by the rush of displaced air, was knee-deep in water. Rolling sluggishly, R19 shook herself clear of the turbulent swirl while the Quartermaster promptly steadied her on her helm. The Lieutenant-Commander glanced at his subordinate officer. "Narrow squeak that, Fordyce," he remarked. "What's Chalmers doing? By Jove, we cut that corner pretty closely!" The Sub made his way to the after-end of the raised platform, where the petty officer was standing as rigidly as a statue. "All right there, Chalmers?" "All right, sir," replied the imperturbable petty officer. "Only that blessed log's gone to blazes." He held up a coiled length of log-line as a visible corroboration to his statement. The cause of the explosion was now revealed. In turning, R19 had passed perilously close to an anchored mine, while the log, towing astern, described a wider arc than that of the submarine, and also caused a reduction of speed of the revolving metal cylinder. The depth to which the log sinks varies inversely with the speed of the towing vessel. So in "cutting the corner" the trailing log descended sufficiently to come in contact with one of the horns of the submerged mine. It was a blessing in disguise, for the explosion gave the Hon. Derek a clue as to his position, and a slight alteration of helm was sufficient to bring R19 approximately in the centre of the channel. Owing to the fog, none of the hostile patrol-boats risked making a dash between the mines, although away to the south'ard there were audible evidences of activity.