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 pa.s.sing 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."

"By Jove! if she does come up," thought Barcroft. "It will be Third Single to Perdition for a set of skulking pirates. The fog is lifting, too. I can distinguish the wave-crests nearly a cable's length ahead. We'll be into another patch in another minute, though, worse luck."

Suddenly the watchers on the destroyer's bridge caught sight of a short series of flashes slightly on the port bow, and perhaps at a distance of a mile.

In a trice Tressidar brought his binoculars to bear upon the glimmer of light, thanking Heaven as he did so that a rift in the fog enabled him to spot the presence of the hunted Hun. The powerful night-gla.s.ses revealed the outlines of a conning-tower and twin periscopes just emerging from the waves. Then as quickly as it appeared the light vanished. It was enough. The lieutenant-commander could still discern the patch of phosph.o.r.escence that encircled the partly submerged U-boat.

"Starboard ten!" ordered Tressidar, at the same time telegraphing for full speed ahead both engines.

Before the destroyer could work up to her maximum speed her knife-like bows rasped and bit deeply into the hull of the doomed unterseeboot. An almost imperceptible jar as the quivering vessel glided over her prey, a smother of agitated water on either hand, and the deed was done. Another of the modern pirate craft had been dispatched to its last home.

"Voices ahead, sir," shouted the look-out man. "Land ahead! By smoke! We've done it."

The engine-room telegraph bell clanged shrilly. As the propeller blades bit the water with reversed action the "Antipas" began to lose way. It was too late.

With a shock that threw almost every officer and man to the deck the destroyer charged bows on to a ledge of rocks. Her forefoot lifted almost clear of the water, while to the accompaniment of the hiss of escaping vapour from a fractured main steam-pipe, the "Antipas"

buckled amidships.

"Clear lower deck! All hands fall in facing outboard!" ordered the skipper.

From the mess-deck the "watch below," already roused by the impact of the destroyer with the ill-starred U-boat, came tumbling out, forming up in orderly silence to await further commands. Out of the steam-laden stokehold and engine-room staggered black-faced, partly-clad men, many suffering from the effects of terrible scalds, while others, too badly injured to help themselves, were a.s.sisted by their heroic comrades. Risking a hideous death in the partly-flooded engine-room the devoted "ratings" performed acts of valour that, although unseen and unheard of, represent the acme of courage. Fresh from the overheated stokehold and engine-room the survivors of the "Black Squad" found themselves faced with the immediate prospect of involuntary immersion in the chill waters of the North Sea.

"Ahoy!" shouted a seaman at the skipper's instigation. "Where are we?"

"'Ard aground," replied a voice through the darkness.

In spite of their hazardous position several of the crew laughed, and tried to switch on a husky cough to hide their levity from their officers. The unknown's reply was certainly brief and to the point, but hardly the sort of answer that Tressidar required.

"Silence there!" he ordered.

Then a boyish voice penetrated the night air.

"You're on Black Ghyll reef," it announced. "Do you require any a.s.sistance?"

"Not at present," replied the lieutenant-commander. "You might stand by, though, in case we do."

The after part of the "Antipas" was now a couple of feet beneath the water, and had settled on the sandy bottom of the bay. With the falling tide--it was just after high-water springs when the destroyer grounded--there was no immediate necessity to abandon ship. Nevertheless it was imperative that the injured men should be taken ash.o.r.e, and a.s.sistance obtained as quickly as possible if there should be any possible chance of salving the wreck.

"Clear away the whaler!" was the next order.

The boat was manned and rowed cautiously towards the sh.o.r.e. Although the sea was calm the men were in total ignorance of the nature of the coast. Lacking local knowledge they were not even at all certain whether a landing might be effected. On either side rose the jagged points of vicious-looking rocks, while looming against the misty starlight could be discerned a range of frowning cliffs with no apparent break in the line of continuity.

"Thank G.o.d that there ain't a stiffish onsh.o.r.e breeze," muttered the c.o.xswain of the whaler. "'Tain't 'arf a rotten crib."

"Boat ahoy!" came the same boyish hail from the invisible strand.

"Starboard a bit.... You're close on the Double Fang. I'll tell you when to turn.... Now, straight in. It's all sand here."

The whaler's forefoot grounded on the soft sh.o.r.e. The c.o.xswain, producing a small handlantern from the stern sheets flashed it upon the group of figures gathered at the water's edge four Boy Scouts.

"Crikey!" e.j.a.c.u.l.a.t.ed the c.o.xswain admiringly. "You're game'uns. Wot are you doing here at this time o' night?"

"We're coast-watching," replied the patrol leader. "We had just collared a spy when your vessel ran ash.o.r.e. There's a chief petty officer of coastguard up the top of the cliff."

The lad did not think it necessary to explain that the petty officer had rather wisely declined to risk his neck by clambering down the precipitous face of the rugged wall of rock. At his age he lacked the steady head and sureness of foot that were essential for such feats of agility.

"Landing's easy enough when you knows 'ow," remarked the c.o.xswain.

"I've been sent ash.o.r.e to find out. Look 'ere, we've a dozen or more badly injured hands aboard, an' we wants to get 'em off. Any chance of carrying 'em up those cliffs?"

The lad shook his head.

"Not up the cliffs," he explained. "There's a path up the valley. It leads to Scarby."

"Any doctors there?"

"None nearer than Tongby. We'll send a couple of Scouts there, if you like."

"P'raps you'd better," agreed the man. "It's a tidy 'andful for our Pills--our doctor, that is. All right, chummy, you might stand by and give us a hail when we come ash.o.r.e again. 'Tis a rum crib, swelp me, if it ain't--but it might be worse."

The whaler backed from the sh.o.r.e, to return presently with a heavy load of wounded men and other members of the destroyer's crew told off to carry the injured to the nearest house.

Guided by the patrol leader the grim procession set out on its journey of pain. The fearfully scalded men, temporarily bandaged by the R.N.R. Surgeon Probationer borne on the destroyer's books as doctor, groaned and uttered involuntary cries of agony as, in spite of the care of the bearers, they were jolted along the narrow, uneven path.

Presently the scout came to a sudden halt. "There's a man lying at the foot of the Cliffs," he exclaimed. "Why, it's Pattercough, the man we were looking for when we captured the second spy."

A seaman bent over the body.

"Dead as a bloomin' doornail," he announced. "He's broke 'is bloomin' neck an' saved the 'angman a job, I'll allow that is, if he's the spy you says he was. Lead on, matey. The dead must look after themselves while this affair's under way."