"I feel," said Lawless, addressing himself to Sub-lieutenant Trent as they stood together on the deck of the Knat, "as pleased as a pig with two tails."

He glanced around him appreciatively, lingering over each well-remembered object, from the quick-firer on the "bandstand" aft to the battered ventilating cowls each side of the for'a'd funnel.

"Even the old shipboard stink is fragrant in my nostrils," he went on, "and the awful profanity issuing from the galley yonder is like sweet music unto my ears."

"H'm," grunted Trent, "you don't seem to have been very happy in the air service."

The look of joyful appreciation faded from the Lieutenant's eyes.

"I've lost one thousand five hundred and five pounds since I left the Knat," he answered. "Or as good as lost it."

"Eh!" ejaculated the junior officer.

Whereupon Lawless confided to him the story of his uncle's unfortunate bet and the sad episode of the German spy who, after being rescued from the derelict steamer, had borrowed five pounds.

"When you've stopped braying, you jackass, I'll get below," snapped the Lieutenant, and, Trent having mastered his mirth, Lawless departed to his cabin.

The Knat was on her way to the Clyde, where, with two other destroyers, the Arrow and the Kite, she was to escort the recently launched battleship Mars round to Portsmouth. It was evening when the Knat arrived in the Clyde and took up moorings near the leviathan battleship, the existence of which was supposed to be known only to the Admiralty and those who had been engaged in her construction. As she was not to leave till noon on the following day, Lawless decided to take the opportunity of a run ashore, and accordingly, having arrayed himself in mufti, stepped into the boat and waved farewell to his envious junior officer.

Now, on its way to the shore, the boat had to pass close by a very dirty coal-tramp, bearing on her stern the legend, "Black Diamond, Newcastle-on-Tyne." As they pulled near the stern in order to reach the quay, Lawless heard a certain order which made him prick up his ears and indulge in a few moments' very earnest thought. After a little reflection, however, he smiled at his suspicions, and dismissed the subject from his mind.

It was somewhat late when, after a visit to the only music-hall which the district boasted, the Lieutenant made his way down a side street to a certain little Continental restaurant which used to be patronised almost exclusively by foreigners.

His attention was attracted by two men attired in the uniform of Mercantile Marine officers who sat at a table opposite. They excited his curiosity, for, although they appeared to speak English perfectly, they were in uniform, and a British sea officer, like his naval confrÈre, hates above all things to be seen ashore in "regimentals."

So impressed was Lawless that when the two officers left the restaurant he followed at a respectful distance till they reached the quay alongside which the Black Diamond was lying. Here another thing struck him as being decidedly peculiar; for, instead of having an aged and rheumatic watchman aboard, as is the usual custom of British ships when in a home port, there was a man walking up and down in front of the gangway as if on sentry go. Then, more astonishing still, as the two officers went on board this man saluted them—an unheard-of ceremony on a British merchant vessel.

With a growing uneasiness concerning the real character of the Black Diamond, Lawless hailed a picket-boat, and was taken aboard the Knat. Here he detailed his adventures and expounded his suspicions to Trent, who, however, was not inclined to treat them very seriously.

"If you feel nervous about them, why don't you report to the captain of the Mars?" he asked.

"And probably be regarded as a lunatic for my pains!" retorted Lawless.

He remained in thought for some moments, then rose.

"P'r'aps you're right," he remarked drowsily. "Very likely they're only harmless Dagos after all. I'm going to turn in, so clear out!"

And a few moments later Lieutenant Lawless was sleeping.

On the following morning Lawless noticed that the Black Diamond left her berth and steamed down the river about an hour before the Mars and her escort were due to leave.

"Well, your nightmare's ship's gone now," remarked Trent. "One of the port officials told me she had cleared for Valencia with coals."

"Coals!" echoed Lawless. "Rum sort of place to ship coals for the Mediterranean! They could get it cheaper and quicker at Cardiff and Newport, besides saving about four days on the double trip."

Soon afterwards came the signal to get under way. The Mars was to proceed at about ten knots, with the Arrow and Kite astern on each side, and the Knat in front to act as scout and clear the way for her.

Slowly and majestically the marine monster—the latest triumph of death-dealing mechanism—swung down the river, her convoy of destroyers looking no larger than midges against her huge bulk. Evening was drawing in as they came abreast of the white light on Cambrae Island, and, although he had been keeping a careful look-out all the afternoon, Lawless had seen nothing of the mysterious Black Diamond.

"It's strange!" he mused. "I can't understand it."

An hour or two later, when darkness had set in, Lawless left Trent in charge and went below to snatch a little rest. He had hardly been in his bunk ten minutes, when a messenger knocked at the cabin door and informed him that he was wanted on the bridge.

"What's the trouble?" asked the Lieutenant, as he reached the bridge.

Trent withdrew his head from the hood-screen on the port side, and pointed to a couple of vertical red lights some distance ahead.

"Steamer broken down," he said; "thought it might be your bogey ship."

Lawless picked up the night glasses and gazed through them for some moments.

"It's the Black Diamond, right enough," he said. "Queer she should have a mishap right in our course, though!"

Trent laughed scornfully.

"I don't suppose they're hanging about there for fun," he observed. "If they were really up to any games, they wouldn't show the warning lights."

"That's just it," replied Lawless. "They know it would at once excite suspicion if they didn't show them." And he again studied the vessel through his glasses.

"Trent," he exclaimed suddenly, "did you see anything?"

The junior officer, although he had been gazing in the same direction as Lawless, had seen nothing of a startling character.

"No," he replied. "What was it?"

"I'll swear I saw something slipped overboard," said Lawless; "but it's so dark I couldn't see what it was even with these glasses. Look here," he added, with unusual gravity, "there's some mischief going on. I'm certain of it."

"Then we'd better have the searchlight on her," replied Trent; and was about to give an order when the Lieutenant stopped him.

"No," he said decisively, "that would only serve to warn them if they are up to any artful dodges."

"They wouldn't have lowered a mine, if that's what you're thinking about, because they know we should alter our course as soon as we saw a disabled steamer ahead."

"No, it was nothing of that sort. It looked more like a sort of cage as far as I could make out."

"Perhaps they're fishing for crabs," suggested Trent with a laugh. For the life of him he could not understand what Lawless was worrying about, since even the best-regulated steamers are apt to break down.

"Look here," said Lawless suddenly, "I'm not going to run the risk of being made a fool of by signalling to the Mars. I'm going to investigate this matter for myself." And, turning to a seaman, he told him to get one of the Berthon boats lowered alongside.

"But if you intend to pull out to her, we shall leave you behind," said Trent. "Besides, what can you do on your own?"

"I shall see," replied Lawless. "As to the rest, if you can't pick me up, I can pull back to the Kite, which by that time will be nearly abreast of me. I know her skipper, and he won't give me away."

Trent merely shrugged his shoulders. If his commander had a bee in his bonnet it was not his place, as a junior officer, to argue; so he held his peace and thought the more. The Berthon having been launched, Lawless dropped in and started to pull towards the apparently disabled steamer with powerful, noiseless strokes, having taken the precaution of muffling the oars. The distance between the Knat and the steamer having been considerably lessened by this time, it was not long before the Lieutenant reached her. He had rowed in a direct line with her stern, that he might not be observed by any possible look-out aboard, and at length he rested on his oars and turned round to make observations.

"Phew!" he whistled softly.

Floating on the water alongside the steamer, and screened by her hull from the observation of those on the war vessels, was a large waterplane. As far as he could make out, it had accommodation for two men, but what attracted his attention most was a machine, in appearance not unlike a very small inverted howitzer, and evidently designed for aiming bombs. There was no one in the waterplane, and for a moment Lawless thought of rowing with all speed to the Mars and warning her captain; but a moment's reflection told him that it was more than likely that she would have passed him before he could get near, and, in any case, the warning might by that time be too late.

"I'll have to trust to luck, that's all," he muttered.

How he was going to prevent the airmen from carrying out their plans, the nature of which he had very little difficulty in guessing, Lawless had no very clear idea; but, after a moment's consideration, he shipped his paddles, and slipped noiselessly into the water. A few strokes brought him alongside the steamer, and then, keeping well under the shadow of her hull, he continued swimming until he was able to reach one of the wire stays on the floating machine. So far he had escaped notice.

He had no sooner secured a firm hold on the waterplane than a man, suspended by a rope from one of the steamer's derricks, swung outboard and gently lowered himself into the seat of the machine. Then another man followed, and, after an interchange of remarks in a language which Lawless had no difficulty in recognising, the two airmen took their seats, and the engines were set going. A second or two afterwards the waterplane began to glide off, and the Lieutenant found himself being dragged through the water.

The weight of his body, in addition to the resistance of the water, caused the thin steel wire to cut into his hands; and, in order to get a firmer and less painful grip, he threw his arms over the narrow end of the car. As he did so, the waterplane rose in the air, with the result that it dipped and swerved so violently owing to the sudden shifting of his weight that Lawless thought it was going to capsize. The pilot, however, righted it with wonderful skill, but at the same time his companion caught sight of the Lieutenant hanging on.

For a moment he stared in blank astonishment at the dark figure dangling beneath him, and then, evidently under the impression that it was one of his compatriots who had somehow contrived to get into this awkward position, he leant over and extended his hand. Lawless gripped it, and, with a tremendous effort, managed to lift himself up and throw his legs over the canoe-shaped body, so that he sat astride facing his unknown helper.

"Look here——" he began thoughtlessly, and then stopped. It occurred to him, too late, that his speech had betrayed his nationality, and therefore his motives.

The other seemed to grasp the situation in a flash, and, before he had time to consider what to do next, Lawless found himself looking straight into the barrel of a revolver. But the sight of the weapon had the same effect upon him as a douche of cold water has upon a swooning man. It restored his presence of mind, and his brain worked with lightning-like rapidity.

In the face of death no man could be calmer than the reckless Lieutenant. For a moment they stared at each other, the airman raised his pistol, and then, to his amazement, the Englishman laughed.

In all probability, it was this cynical laugh which saved his life, but defeated his immediate object. Lawless had realised that if the man fired at him it must inevitably attract the attention of those on board the Mars—for revolver shots in mid-air would be hardly likely to pass unnoticed—and they would at once open fire with the anti-aircraft guns. But the same thought had evidently occurred to the other, for, instead of firing, he slipped the revolver back into his pocket.

Lawless braced himself for the coming struggle, and twisted his legs round some wire guys to secure a firmer grip of the machine and leave his arms free. Giving a quick glance below, he saw the lights of the Mars and her attendant destroyers standing out like tiny specks in a gulf of unfathomable blackness. The two red lights of the Black Diamond were no longer visible, and he concluded that, having launched the waterplane, she had proceeded on her way. To the left he could see a searchlight, which he assumed to be operated from the Knat, rapidly sweeping the horizon in great semi-circles.

The whole panorama passed before his eyes like a flash, and a moment later he had come to grips with his antagonist.

With arms enlocked and faces almost touching, the combatants swayed gently to and fro, as though testing each other's strength. Then, loosening their hold, and drawing a little apart for a second, they closed again, and this time they wound their arms about each other like bands of steel, till the terrible pressure made their veins stand up like whipcord, and their breath came in short, choking gasps. But neither could get sufficient advantage over his adversary to swing him off, and more than once it seemed as if both of them must tumble into the sea beneath.

Had the pilot been able to come to the assistance of his companion, Lawless would not have stood a dog's chance; but, fortunately for him, the man had all he could do to maintain control of the machine and prevent it from capsizing under the constantly shifting weight of the opponents as they swayed dangerously from side to side.

Suddenly, from a point seemingly immediately beneath them, came a faint bellowing sound, which Lawless at once recognised as the Mars' syren.

Apparently his antagonist recognised it also, for, throwing caution to the winds, he flung his body forward, carrying the Lieutenant with him so that they both nearly went over the side with the sudden impetus.

Lawless was now underneath, his back resting on the narrow deck of the car, while the airman struggled to get a grip of his throat. Uppermost in the mind of each was the desire to kill. Hate gleamed from their bloodshot eyes, and they ground their teeth savagely. Having the advantage of position, the airman at last managed to get his fingers round the Lieutenant's throat, and the latter, in a desperate endeavour to free himself from the choking grip, relaxed his hold, and was swung over the side of the car. For a moment he remained head downwards, suspended only by the steel wires into which he had twisted his legs. Then he began to slip, and a second later disappeared from sight into the yawning abyss beneath.

Quick as thought the airman leaned back, and, reaching the lever which operated the bomb-throwing machine, pulled it; but, as he did so, the waterplane gave a terrific lurch, and the bomb, instead of dropping on the Mars, which at that moment was directly underneath, fell into the water and exploded some distance astern of her. The cause of this failure was due to Lawless gripping in his fall one of the stays which held the rudder, and his weight, operating on the extreme end of the machine, had destroyed its balance, and therefore altered the course of the bomb.

Instantly the night was illuminated with the beams of searchlights, which, like huge phosphorescent feelers, swept the sky, until at last they were focussed on the waterplane. Then came a cracking sound from beneath, and shot after shot whizzed upwards from the anti-aircraft guns of the Mars, striking the car, smashing and splintering the slender wooden spars which supported the planes, and occasionally snapping a steel guy.

By a miracle Lawless had managed to retain his hold of the wire, though it cut through his flesh, and he knew that he must drop before many seconds elapsed, even if he was not struck by one of the shots from below. He was on the point of letting go through sheer pain and exhaustion, when the machine suddenly tilted, turned right over, and then swooped downwards with a velocity that made his brain reel.

The terrible sensation of falling from a tremendous height terminated abruptly, and next moment the Lieutenant felt the water close above his head, and he was sinking down, down, fathoms deep, it seemed to him, into the sea. Then came a feeling of agonising suffocation, a terrible pressure on his chest, a sensation as though his lungs were about to burst, and he rose to the surface, to find himself near some floating remnants of the wrecked waterplane.

Catching hold of one of the floats, Lawless endeavoured to draw himself up, but fell back in the water as a numbing pain shot through his left arm. He rose to the surface again in a semi-dazed condition, and made another feeble attempt to get on the floats; but his strength was exhausted, and he was suffering agonies from the injured arm. At this moment there came the sound of men's voices and the click of oars as they swung in the rowlocks; strong arms lifted him out of the water, and then he lost consciousness.

When Lawless came to himself, he was lying in his bunk on board the Knat, and the absence of any movement told him that she must be at anchor in harbour. In a dreamy fashion he fell to wondering why he was lying there, with the sunlight streaming through the port-holes; he even made an effort to rise, but fell back as a pain shot from his left elbow to his shoulder.

That set him thinking, and gradually the events of the previous night—he supposed it was the previous night—came back to him. Recollections of the mysterious steamer, the fight in mid-air, his fall into the sea—all passed through his brain in a fragmentary, disjointed manner, like the confused memories of a dream. Then, as he was trying to sort them out, the cabin door opened, and Sub-Lieutenant Trent entered.

"Hallo! So you've come to at last!" he exclaimed. "How do you feel?"

"As if I'd had a beastly nightmare," answered Lawless. "Where are we?"

"In Portsmouth Harbour," replied Trent, as he seated himself on a locker beside the bunk and started to fill his pipe.

"I'm beginning to remember things a bit more clearly now," said the Lieutenant. "Tell me what happened after I left the Knat."

"There's not much to tell, so far as I'm concerned," answered Trent as he lit his pipe. "We kept our course, and just as we came abreast of your bogey ship she hauled down her signal lights and steamed away. Then——"

"Ah, that must have been immediately after the waterplane rose," interrupted Lawless. "I thought there wasn't much wrong with her machinery."

"Well, as I was saying, as soon as she steamed off I turned the searchlight on her, but could discover nothing of a suspicious nature," continued Trent. "Neither, for that matter, could I see anything of you, though we caught sight of the empty boat drifting. Nothing fresh happened till I heard a terrific explosion astern, and turned round just in time to see a ball of flame sink into the water near the Mars. I couldn't imagine what it was for a moment; but the Mars people got their searchlight going in less than a jiffy, and then we spotted an aeroplane directly overhead.

"That settled the mystery of the explosion, and the Mars began to pot at her with the vertical guns, though at first it was impossible to tell whether any hits were made. But after a few shots, all doubts were set at rest, for she came tumbling down like a winged partridge. As soon as she struck the water, boats were lowered, and they rowed to the floating wreckage, where they found you and also the body of a man who had evidently been killed by one of the shots. As the heavier parts of the machine had sunk, we could not tell what sort of weapons she carried, or the number of the crew.

"After that, I went aboard the Mars and reported to the skipper what you had told me about your suspicions concerning the Black Diamond. He was precious ratty at not having been told before, and I got orders to chase the steamer and search her. We overhauled her in less than half an hour. She's now safely anchored near the Hard, and her crew are prisoners."

"It's pretty clear that the waterplane was carried aboard the steamer in sections," said Lawless after a pause. "No doubt they were put together after she left the quay. The whole thing was wonderfully engineered, I must say."

"But what about you?" asked Trent. "I imagine you must have had a rather exciting time."

"Exciting!" ejaculated the Lieutenant grimly, and then proceeded to relate all that had happened to him after he left the Knat.

"Well," said Trent at the conclusion, "you scored that time, anyway."

"I s'pose so," answered the other, with a sigh. "But if it had only been a Zep and I'd brought it down, there'd have been a cool thou.' waiting for me."