"by the mark seven... less a quarter... by the deep six!" chanted the leadsman, as the canvey approached the bar.

"starboard! meet her at that!" ordered the lieutenant-commander, telegraphing for speed to be still further reduced. "any signs of armed resistance?"

"no, sir," replied broadmayne; for now that the alerte had swung through eight points, her quickfirer could be seen from the bridge of the canvey. "the poor bounders have got the wind up badly," he added.

"they'll get it worse, if they don't chuck up the sponge," rejoined raxworthy. "by jove! if they don't strike that jolly roger there'll be trouble. for'ard starboard gun, there! one round at the enemy's bows!"

the shell, a seven-pounder, shrieked as it sped on its errand of destruction. a flash, a cloud of black smoke and a shower of pieces of metal announced that the missile had accomplished its work. practically the whole of the for'ard superstructure of the pirate submarine had vanished.

"black flag's struck, sir!" announced the gunnery-lieutenant.

"they're doing the 'arms up' stunt," supplemented another of the group of officers on the canvey's bridge.

the alerte was losing way rapidly. a solitary figure appeared on the hitherto deserted bridge.

"we—surrender," came the semaphored message.

"wise men," commented raxworthy, as he faced aft to order away the boats containing the prize-crew.

"she's submerging, sir!" exclaimed broadmayne.

the lieutenant-commander turned abruptly. he was about to order every gun able to bear upon the pirate submarine to open fire, when he observed that men were leaping overboard in a state of uncontrollable panic. that altered matters. had the crew of the alerte been at diving stations, he would not have hesitated to hasten her departure by means of half a dozen high-explosive shells. the fact that the pirates were swimming for dear life in a shark-infested sea, compelled him to stay his hand.

"away lifeboat's crews!"

to the shrill trill of the bo'sun's mate's whistle the bluejackets rushed to man the boats. the excitement of the chase had vanished; in its place was the whole-hearted eagerness to save life.

the alerte disappeared with very little noise or commotion. although the water was considerably disturbed, there was hardly any suction. the swimmers, although impeded by their cumbersome cork lifebelts, had little difficulty in getting clear of her as she submerged.

"what's young maynebrace doing?" asked the lieutenant-commander as the loud report of a revolver rang out, followed by three shots in rapid succession.

broadmayne, also attracted by the reports, saw the midshipman in charge of the second cutter standing up in the stern-sheets and firing apparently at some of the swimmers. apparently several of the pirates thought that they were about to be shot as they swam, for they turned and began to strike out away from the rescuing boats.

there was a wild, almost unearthly shriek. one of the wretched men threw up his arms and disappeared. a patch of blood appeared on the surface over the spot where he had vanished. again midshipman maynebrace fired, his objective being the head of an enormous shark, just as the monster turned on its back to seize another victim.

right amidst the straggling crowd of swimmers dashed the two boats, their crews engaged between dealing spanking blows with the blades of their oars upon the water, and hauling the terrified pirates over the gunwales.

cain had revenged himself upon his mutinous crew. only fifteen escaped the jaws of the ferocious tigers of the deep, and these were almost mad with the horror of the scene.

among those who fell victims to the sharks was marchant the gunner. pengelly, wearing only a shirt and trousers, was one of the survivors. his hair had turned white during his desperate swim.

the late second in command of the alerte hardly hoped to pass himself off as one of the ratings of the pirate submarine. he realised that he was far from being popular with the crew. sooner or later they would "give him away." but the attempt was worth trying.

as he came over the side of the canvey he was interrogated by a stern-faced lieutenant, who demanded his name and rating.

"smith, tom—deck-hand," he replied.

the canvey's officer noted the particulars without comment. pengelly went for'ard under arrest, ignorant of the fact that sub-lieutenant gerald broadmayne was watching him from the bridge.

"there's no sign of cain, sir," remarked the sub to the owner. "that fellow just gone for'ard is pengelly. marchant the gunner and barnard the bo'sun don't appear to be present."

"hang it all!" ejaculated raxworthy, "you don't suggest that three of the pirate officers, including the ringleader, are still on board the submarine? pass the word to mr. hamley to send pengelly to the quarter-deck under an armed guard."

the lieutenant on the gangway received the message. consulting the list he had made, he found that no one answering to that name had been received on board. he sent a message to that effect to the captain.

after considerable delay, pengelly was found and brought aft. the moment he saw broadmayne standing behind the lieutenant-commander, he knew that the game was up as far as concealing his identity was concerned.

"where's cain?" demanded raxworthy, without any preliminaries.

pengelly explained what had occurred, spinning an elaborate yarn that he had done his utmost to persuade captain cain to surrender, and trying to excuse himself for having ever set foot on board the alerte.

the lieutenant-commander brought him up with a round turn.

"enough of that!" he said sternly. "where is the gunner of the alerte?"

pengelly shook his head. that was a question that he could not answer. he was still unaware of the fate of mr. marchant.

"and the bo'sun—barnard, i believe, is his name?" continued raxworthy.

again pengelly let his tongue run riot, dwelling on barnard's action in siding with cain and going below with him.

"for what reason?" asked the lieutenant-commander.

"cain will probably try to bring the alerte to the surface when he thinks the coast is clear," replied pengelly readily enough.

"two men cannot do that," interrupted raxworthy.

"there may be more," rejoined the pirate. "i remember two hands at least going below. i did not see them come on deck again. please remember, sir, i've done my best to answer your questions. i deeply regret——"

"remove the prisoner," said raxworthy sternly.

he waited until pengelly had been taken for'ard, then he turned to broadmayne.

"i suppose you are quite certain that the alerte hasn't electrical propelling machinery?" he asked.

"there was none when i was on board, sir," replied the sub.

"i don't suppose four men will be able to disconnect the clutches and turn the propellers sufficiently to make the submarine move," remarked raxworthy, half-seriously, half-jokingly. "she's there right enough. well, i've given cain a fair chance; he wouldn't accept it. what happens now is his funeral, not mine."

raxworthy returned to the bridge. it was now about an hour before sunset. the sheltered bay was as smooth as a millpond. there was nothing to indicate that the elusive pirate submarine lay ten fathoms deep except a small mark-buoy that had been placed over the spot where the alerte had disappeared.

his orders were plain enough—to capture or destroy. he had done his best to carry out the first part of his instructions. cain had foiled him in that direction by submerging. short of powerful salvage craft and plant there was no means of bringing the submarine to the surface and then effecting her capture. the canvey could wireless to gibraltar dockyard for the necessary gear, but days—weeks perhaps—would elapse before the cumbersome salvage lighters could be towed to bahia arenas. there was no help for it but to act upon the second alternative—to destroy.

"there's one consolation," soliloquised the lieutenant-commander, "the poor brutes won't know much about it. it's a quick end."

slowly the canvey turned until her bows pointed nearly end-on to the mark-buoy. on the starboard side of the poop was a squat-looking object somewhat resembling the old-time siege mortar, its wide muzzle grinning upwards at an elevation of forty-five degrees. the weapon—a depth-charge projector—was loaded with a missile set to explode at sixty feet beneath the surface.

"all ready, mr. garnett?" sang out the lieutenant-commander to the gunner who was in charge of the apparatus.

"ay, ay, sir!"

the engine-room telegraph bell clanged. almost immediately the canvey increased speed. the mark-buoy bore abeam, a cable's length to starboard.

crash! went the propelling charge.

like a gigantic salmon-tin, the missile described its parabolic flight—so slowly that observers on the bridge could see the huge canister turning over and over in mid-air.

it struck the water with a resounding thud, flinging up a shower of spray. already the canvey under fifteen degrees of starboard helm was rapidly increasing her distance from the mark-buoy. slowly the intervening seconds passed; so slowly that broadmayne began to think the fuse of the depth-charge had proved defective.

then came a truly stupendous roar. a slender column of water was hurled quite two hundred feet in the air. the hull of the canvey shook under the terrific blast of displaced air. the tranquil waters of the bay were transformed into a mass of agitated waves.

the column of upheaved water fell with a loud hissing noise. for nearly half a minute the turmoil continued. then, in the midst of the maelstrom, appeared a patch of calm, iridescent oil spreading steadily in all directions, while multitudes of fish, killed or stunned by the detonation, floated belly-upwards upon the surface.

"away, diving-party," ordered captain raxworthy.

"with your permission, sir, i would like to accompany the divers, sir," said broadmayne.

"are you qualified?" asked the lieutenant-commander.

"yes, sir," replied the sub. "i did a diving-course at whaley when i paid off from the arcturus, and i've been down to fourteen fathoms."

"very good," was the rejoinder.

broadmayne saluted and went off to make the necessary preparations.

the canvey was equipped with two types of diving dresses, both designed and made by the firm of siebe, gorman & co. one was of the common variety, in which the air is pumped through a pipe from a pump above the surface of the sea. the other was of the self-contained type; the air supply, judiciously combined with oxygen, is contained in cylinders strapped to the back of the diver. thus he is independent of air-tubes, life-lines and other contrivances likely to impede his movements.

the sub chose the latter type of dress. the depth in which the alerte had sunk was between fifty and sixty feet at low water, the maximum distance below the surface at which the self-contained diving-suit can be used without undue risk.

one of the seaman-divers was already being garbed in a similar suit by his attendants; the other man was preparing to don a dress with lifeline and air-tube, the helmet being provided with a telephone by means of which he could engage in conversation with the above-water party in the boat.

the descent was to be made as speedily as possible before more sharks appeared upon the scene of the wreck to feast on the bodies of their less fortunate kind who had been killed by the explosion. nevertheless, broadmayne and his companions were warned to keep a sharp look-out while under the surface. as a rule, a shark will hesitate to attack a diver, but there have been instances in which a terrible submarine struggle has taken place between a diver and the tigers of the deep.

the diving-boats pushed off and anchored fore and aft as close as desirable to the wreck. the diver with the air-tube type of dress was the first to descend, sliding at a steady pace down the shot-rope.

a tug on the life-line gave the attendants warning that the man had reached the bottom. "ready, sir?" asked a petty officer.

"right," replied broadmayne.

the glass plate in the front of his helmet was screwed home. he was now cut off from the outside world as far as the air supply was concerned, and the sensation was not a pleasant one.

unlike the first man to descend, whose helmet had been closed only when he was waist-deep in water, the sub had to be finally equipped while in the boat. assisted by the attendants—for his movements were hampered by the weight of his helmet, chemical-containers, chest and back weights, and leaden-soled boots, the whole amounting to 190 lb.—broadmayne scrambled awkwardly and ponderously over the gunwale, grasped the shot-line used by his predecessor and began the descent.

in spite of the weight of the dress in air, it now had so little weight in water that the sub had no difficulty in retarding the downward movement. even the inconvenience caused by the unaccustomed air supply passed away after a few seconds.

presently his leaden-soled feet touched the bed of the bahia arenas so lightly that he could hardly credit that he was standing on a floor of hard sand. so transparent was the water that he had no difficulty in seeing objects five or six yards off, all grotesquely distorted and exaggerated.

grasping the second of the three distance lines, the sub commenced his submarine walk, following the cord that the first diver had paid out. evidently the man had not erred in his sense of direction, for the line lay motionless on the sandy floor. all around were pieces of jagged steel-plating, copper pipes and other débris from the ill-fated alerte.

presently an enormous dark grey mass loomed up in front. it was the hull of the pirate submarine. the seaman-diver, with bubbles rising from his helmet, was standing by. his job lay outside the hull; broadmayne's and that of the third diver, inside.

in less than two minutes the third member of the party appeared. the first man, turning to reassure himself that his air-tube and life-line were clear of the jagged plates, worked round towards the stern. it was here that the full force of the powerful depth-charge had expended itself. thirty feet or more of the after-portion of the submarine had been completely blown apart, together with most of the propelling machinery. there was not the slightest doubt about the destruction of the after-part of the submarine. it remained to be seen whether the water-tight bulkhead separating the motor-room from the 'midship and fore compartment had withstood the strain.

signing to his similarly-equipped companion to follow him, broadmayne clambered up the sloping side of the considerably-listing vessel. the ease with which he performed this feat rather surprised him.

once again the sub trod the deck—or, rather, what remained of the deck of the alerte. the bridge had disappeared and the whole of the bulwarks and deck aft, leaving bare a full fifty feet of the massively-built submarine hull to where it terminated abruptly in a jagged edge of twisted steel. most of the raised fo'c'sle had been blown away by shell-fire before the alerte submerged, but between the rise of the fo'c'sle and the conning-tower, which was practically intact, the false deck was still in position.

making his way to the forehatch—it was originally the torpedo-hatch—broadmayne tried to open it. being secured from below, the metal cover resisted his efforts. foiled in that direction, the sub retraced his steps to the conning-tower hatchway. as he did so, a dark object above the rail attracted his attention. it was cain's ensign—the skull and cross-bones—still lashed to a boathook. when the alerte submerged, the natural tendency of the ash stave was to float, but the metal hook engaging in one of the shrouds of the housed foremast had held it down. even the explosion of the depth-charge had failed to dislodge it.

drawing his knife, broadmayne cut the emblem of piracy adrift and secured it to his belt. then he resumed his investigations.

the conning-tower was also secured and clipped from the inside. was it possible, he wondered, that cain and his companions were still alive in the apparently intact and air-tight for'ard compartment of the hull? going aft, the sub lowered himself cautiously over the riven edge of the hull-plating, lest a sharp projection should penetrate his inflated dress. then, signing to his fellow-diver to remain, he switched on his submarine electric lamp and crept forward inside the hull.

the first twenty feet or so was greatly encumbered with wreckage, but on passing through the transverse bulkhead, the watertight door of which had been blown inwards, broadmayne found that there was little damage done to the 'midships section.

as a matter of precaution and to save negotiating the debris-strewn motor-room again, the sub unclipped and threw open the conning-tower hatch. then proceeding for'ard he found that the door between the 'midships section and the bow compartment was wide open. it swung freely on its hinges, although the straining the hull had received made it impossible for the usually close-fitting door to close.

in the bow compartment, broadmayne searched diligently for the bodies of cain and his companions, but without success. then he came to the door of the air-lock, by which a man in a diving suit could leave the submarine when the vessel was lying on the bottom. the door was shut. usually six diving suits and twenty-four life-saving helmets were ranged along the bulkhead. the latter were there, jammed against the curved roof under the deck, but four of the self-contained diving-dresses were absent.

prising back the locking-gear of the door of the air-lock the sub entered the compartment. it was, as he expected, empty, but the hinged flap on the outside hull-plating was open.

captain cain had made a bid for life and freedom. whether he had succeeded or had been caught by the explosion before he had got well clear of the ship remains an unsolved problem.

there was no need for further investigation. broadmayne returned to his companions by means of the conning-tower hatchway. together they dropped over the side and found the other diver waiting by the distance cords.

in single file, the man with the air-tube leading as the attendants in the boat slowly heaved in his air-tube and lifeline, the three made their way to the shot-rope.

then came the tedious ascent. to go up quickly and without a pause was not to be thought of. the great risk of being killed by excessive blood-pressure on the brain had to be guarded against. slowly broadmayne was hoisted, kept hanging for several minutes and then hoisted a few feet more, until at length he felt himself being grasped under the arms and assisted into the boat. then his helmet glass removed, he sat and gasped, gratefully inhaling copious draughts of fresh air.

as soon as the other divers were in the boat the anchors were weighed and a course shaped for the canvey, which was steaming slowly in wide circles round the scene of the wreck.

"satisfactory job?" inquired raxworthy laconically.

"after-part blown clean away, sir," replied broadmayne. "all the other compartments are full of water."

"any signs of bodies?"

"no, sir."

"did you see any?" inquired the lieutenant-commander, turning to the seamen-divers.

"no, sir," answered the man who used the air-tube pattern dress. "i went right round the wreck on the outside—starboard side first and then port to the full extent of my life-line. no doubt, sir, the men in her were blown to bits. there was a plate torn right out close to her bows, i noticed. that shows how strong the force of the explosion was."

captain raxworthy nodded.

"then there's no possible doubt about it," he remarked to the officers standing by. "well, our work's done. the alerte's destroyed." he paused and glanced over the side across the tranquil waters of the bay. "i'm rather sorry for that chap cain," he continued. "he evidently was a bit of a sport. i'd like to have met him." before sunset, h.m.s. canvey was steaming to the nor'ard, homeward bound.

pengelly was found guilty and sentenced to a long term of penal servitude. the surviving members of the alerte's ship's company received lighter sentences, but of sufficient severity to deter others who might wish to emulate the misdeeds of the captain and crew of the pirate submarine.

gerald broadmayne, lately promoted to lieutenant in consideration of his services in the operations against the alerte, had to give evidence at the trial. but there were two points upon which he was silent: cain's real name and former rank in the royal navy was one; the other was the incident of the air-lock.

often broadmayne thought of that air-lock, especially when he gazed at the skull and cross-bones bedecked relic of the alerte. it was to him a fascinating and yet unsolved mystery. did cain succeed in his desperate effort to escape? or did the bed of the land-locked bahia arenas hold the secret of the fate of the captain of the pirate submarine until the sea gives up its dead?