Behind him stood the helmsman at the steam-steering gear, his eyes fixed upon the cryptic movements of the operator's hands, as the latter transmitted the course to the quartermaster. The principle of the microphone signalling apparatus is simple enough. In the vessel's hold and as far beneath the waterline as possible, are two metal tanks each filled with water and containing two sensitive instruments that readily pick up sounds transmitted through the medium afforded by the sea. One tank is placed on the starboard the other on the port side, and both are connected by wires with the receiver in the chart-room. Supposing the operator hears the thud of a distant propeller, and the sound is more distinct from the port side he knows that the submerged vessel is somewhere in that direction. Conversely, the sound being greater in the right-hand receiver he is able to locate the object emitting the sound as being on the starboard side of the ship. When the volume of sound passing through both receivers is equal the operator knows that the vessel's bows are pointing practically "dead on" to the unseen but audible peril. "That's all very fine," remarked Kirkwood. "But supposing that man has a cold in one ear. How is he to guard against being misled by the inequalities of hearing? I've heard of a fellow being deaf in one ear and not knowing it for months." "The inventors have taken that into consideration," replied Tressidar. "That's why both ears are connected with the receiver on one side only of the vessel at a time. As he turns that switch from side to side both ears are listening to the sounds from the port and starboard tanks alternately. What's that?" he added, addressing the operator. "Three cables ahead? This won't do; she's gaining on us." The skipper quitted the chart-room, followed by the three airmen. Coming from the lighted compartment; they were momentarily dazzled by the transition from artificial illumination to murky, pitch-black night. "Increase speed to fifteen knots," ordered Tressidar. "Where there's water for that strafed U-boat there's enough for us.... Overhauling her? All right; twelve knots, then." "Those fellows have plenty of nerve," remarked Barcroft, "or else they've no nerves at all. Suppose fog doesn't make the slightest difference to them when they are submerged, but to us it appears otherwise. What is that U-boat doing, I should like to know, plugging along at twelve knots and in the direction of the British coast?" "Keeping a pressing appointment, perhaps," said the A.P. with a laugh. "Many a true word spoken in jest, old bird," rejoined the flight-sub. "It is----" "A little less talking there, if you please," interrupted Tressidar curtly. The three airmen took the hint. It was only on very rare occasions that the genial lieutenant-commander "choked any one off." It was an indication of the mental strain upon the skipper of the "Antipas."