"Some wheeze, this, Meredith!" he exclaimed gleefully. "With luck we may spot little Fritz. I don't think it's much use following the directions given in this signal. There'll be a swarm of destroyers and all that sort of fry buzzing around already, and if the skipper of the U-boat is up to snuff he'll have altered course to the south'ard. We'll just stand on and keep our wits on the alert. If he's legging it to the south'ard he'll cut athwart our course. I'll try what luck we can get with the hydrophone first." The M.L.'s engines were stopped, and the boat rolled heavily in the oily swell. Over her starboard side a weird contraption of wires was lowered, the wires terminating in submerged metal plates, while inboard they led to a complicated device known as a hydrophone. In the wireless-room a man sat with receivers clipped to his ears. He was not listening to wireless messages, but for the sound of a U-boat's propellers. "Anything doing?" inquired Meredith for the twentieth time, as the minutes slowly passed. This time the listener did not shake his head. "Fancy I hear something, sir," he reported. "Would you like to listen?" Kenneth took the proffered ear-pieces and clipped them to his head. Very faintly he could hear the characteristic thud of a marine motor. "Evidently she's knocking around," he observed, as he handed the apparatus to the operator. "All right; carry on." Slowly the man revolved a handle until the thudding sound reached a maximum intensity. A glance at the compass showed that the hydrophones were pointing east by south. Still turning the handle, he noted that the volume of sound gradually decreased until a certain point; then it began to increase again, reaching a state of maximum intensity in a bearing south by east. That was all the operator required. Experience had taught him that the source of emission of the sound came from a direction midway between the two maxima, while a further test revealed the fact that the U-boat was moving in a southerly direction. "If only this blessed fog would lift!" exclaimed Wakefield when his Sub communicated the result of the hydrophone test. "Get the gear inboard, Meredith. See that the ammunition is brought up and the gun cleared for action. Now for a game of blind man's buff." "None of our submarines are about here, I suppose?" asked Meredith. "Not within seventy miles," replied the skipper. "So if we do have the luck to run across a submarine, we'll go for the brute bald-headed." "And if Fritz can't dive?" "Then, of course, we'll have to try our best to tickle his ribs with a shell while he's on the surface. Tricky work, but we'll keep him fully occupied with our little pea-shooter"; and Wakefield indicated the six-pounder, by the side of which the gun-layer was standing ready and alert to train the weapon upon its objective.