Alone in the conning-tower, the manhole of which, communicating with the interior of the hull, was left open, the Hon. Derek stood, his eyes fixed to the object-bowl of the periscope, on which the surrounding surface of the water was reproduced with absolute fidelity, marred only by a vertical and a horizontal line marked in degrees. In the middle part of the image, corresponding with the centre of the field of vision, a specially-constructed lens enlarged the view, enabling the observer to gauge the distance and the direction of the target with the greatest exactitude. Although there were voice-tubes and indicators at hand, the Lieutenant-Commander's attention was directed mainly upon the object-bowl. Consequently he shouted his orders to a petty officer, whose head, as he stood on the short steel ladder, was level with the floor of the conning-tower. "Down to eighteen feet." With an almost imperceptible movement, as the horizontal diving-planes were actuated, R19 slid beneath the waves, the while "pumping" or rising and falling vertically under the constant alteration of the pressure of the water above her. Momentarily the vision in the object-bowl dimmed and again recovered its normal clearness, as clouds of spray enveloped the tips of the exposed periscopes, almost immediately to vanish from the surface of the anti-moisture-treated glass. In the confined space the noise of the well-running electric motors was deafening. The torpedo-men were in the present instance able to "stand easy", but the engine-room artificers and stokers, their moist faces glistening in the glare of the electric lights, were far from idle. The gun-crews, clustered round the hatchway ladders, ready to rush to their posts, were grimly silent, awaiting the order that would give them the chance to "strafe" a Hun gas-bag. Opportunities for "strafing" were few and far between in the British submarine service, not from inclination but from the absence of a suitable target; when a chance did occur the eager men were "all over it". Standing immediately behind the petty officer stationed at the sound-receiving apparatus in a glass-encased compartment Sub-Lieutenant Fordyce noticed the man was listening intently, first at the right-hand disk then at the left. Then, turning his head, he regarded his officer with a puzzled air. Opening the door, Fordyce entered the sound-proof cabinet. "What's wrong now, Chalmers?" he asked. "Something fishy, sir," replied the man, stepping aside. "Will you stand here a minute, sir?" The Sub took up a position between the two concave disks. He could distinctly hear the bass hum of the Zeppelin's aerial propellers, while faintly through the right-hand disk came the thud of a marine "screw". That meant that on the starboard hand, abeam if anything, a vessel was under way. "Very good; carry on, Chalmers," said the Sub as he relinquished the apparatus to the man's charge. "I'll report to the Captain."