The Seamew had passed through Balabac Strait and was standing out into the reef-strewn South China Sea, on the last leg of her course, when it happened.

That afternoon the diving suits and pumps had been broken out and put in order, after which the grinning Kanakas and Jerry Smith had given Mart and Bob some practical lessons in dressing up in the cumbersome water-tight outfit, and in working the pumps. In the evening they had sat up late with Captain Hollinger, talking rifles and ammunition, and they were weary enough to sleep soundly.

Mart's porthole was open that night, as usual. He woke up suddenly to find the setting moon streaming in across his face, and got up to hang a towel across the open port, in order not to exclude the fresh air. As he did so, he heard the ship's bell forward strike eight bells, and knew that it was midnight.

There came a faint pad of bare feet forward—the watches being changed. Then, as he stood for a moment gazing out at the moonlit sea, he heard the deep voice of the second mate, Liverpool Peters, who had apparently just taken charge of the deck.

"All right, Mr. Swanson. I'll keep a sharp eye on that chart. Sou'-sou'-east by a half east it is."

Mart went sleepily back to bed and thought no more of it. He knew that they were in dangerous waters, but the yacht had a splendid outfit of charts and there was no danger for her among the coral reefs. He was wakened at dawn, however, to find Bob pounding on his door.

"Hey, Mart!" came the voice of his chum excitedly. "Tumble out here."

Mart growled out an unintelligible reply, but Bob resumed his pounding, so the wireless operator reflected that there must be "something doing." Hastily flinging on his clothes, he opened the door and gained the deck.

"Well, what's up, Holly? Why, it's hardly dawn yet!"

"Shut up an' come along to the bridge!" exclaimed Bob. "Dad's up there—Joe Swanson came an' roused him up just now. That's what woke me up."

"Well, what's the matter?" demanded Mart vigorously. "We ain't struck a reef, have we?"

"I'm not quite sure myself, Mart. Swanson said something about Liverpool, so mebbe he's had another scrap. I heard dad tell him to call all hands, then he was out on deck like a house afire, and I came after you."

"Much obliged, old scout," chattered Mart, for the dawn was cold. While they talked, they had been hastening forward, and now they scrambled hastily up to the bridge deck, where they found everyone but the engine-room crew assembled. Jerry Smith was at the wheel, and he wagged his head solemnly at the boys, but they were too excited to notice him.

Pushing through the crowd, they entered the chart house. Captain Hollinger was seated at the table, but merely glanced at them with a nod. Swanson and the old rheumatic seaman Borden stood before him.

"Yes, sir," the mate was saying, and Mart noticed that his burly, rugged face looked queer. "He was all right at eight bells, sir. Borden was at the wheel when the port watch came up, an' Liverpool put Birch there in his place."

"All right, Borden," returned the captain quietly. "You may go. Tell Birch to step in here."

The boys glanced at each other, pale-faced. Each was exceedingly anxious to know what had happened, but at sight of Captain Hollinger's tight-lipped mouth and drawn face, they dared ask no questions.

The one-eyed Birch came in, ducking his head respectfully.

"When did you last see Mr. Peters, Birch?" asked the captain.

"At six bells, Cap'n. Mr. Peters said he was goin' below for a drink, but he didn't come to the bridge again, sir."

"You heard nothing suspicious?"

"Nothin', sir."

"Who else was on the bridge?"

"The quartermaster, sir."

"Send him in here. You may go."

Birch left. The two boys again met each other's eyes, hardly able to believe what they had heard. Then old Jerry shuffled in.

"Quartermaster, did Birch leave the wheel about six bells?"

"No, sir—he wasn't off the bridge at all, sir."

"Hm!" Captain Hollinger leaned forward, fixing his eyes on the old seaman. "Look here, Jerry. What do you think happened to Mr. Peters? Did he meet with foul play?"

Jerry hesitated, glancing at the open door. Swanson moved forward and closed it.

"No, sir, I don't think as he did," returned Jerry slowly. "The men didn't like him, Mr. Hollinger; I will say they fair hated him, but not so bad as that, sir. Take Birch there—he's threatened Mr. Peters' life before now, sir, but that's no more'n fo'c'sle talk, sir, as you know very well. No, sir, I think that Mr. Peters went below to get a drink, as Birch said, and in some way fell overboard. Me and Birch was on the bridge, and the rest in the port watch are Kanakas."

There ensued a brisk discussion, in the course of which the horrified boys learned that some time during the night the second mate had vanished. The ship had been searched, but he was not aboard her, nor had there been any sign of struggle. Remembering the scene which they had witnessed between Peters and Birch, Mart immediately suspected the one-eyed seaman, while Swanson openly announced his belief that the second officer had met with foul play; but in no long time all such thoughts were sent flying, when the engine-room crew came up for questioning.

Two of the Kanaka stokers, both of them simple, frank-faced fellows who were above all suspicion, stated that they had come up on deck for a breath of air shortly after six bells and had seen Peters standing by the stern rail, looking down at the swirling waters as they rose from the churn of the propeller. Having no business in that part of the ship, they had gone forward again.

"I think there's no doubt of it," exclaimed the captain at last, even Swanson nodding gloomily. "Poor Peters must have either committed suicide, or else he fell overboard. Stand by for another hour, Mr. Swanson, then put the ship on her course again."

Only then did the boys become aware that the yacht was retracing her course in the vain effort to pick up her lost second mate. Later on that morning, when all hope had been given up, Bob and Mart sat in the wireless house and talked over the matter in sober earnest. As gladly as they could have suspected Birch, however, they agreed that there was no foul play involved.

"Your dad's no fool," declared Mart positively. "He sized up everything pretty square, and Swanson didn't overlook anything either. Joe is sore at Jerry for something—prob'ly suspects him of being a pirate."

"Well, I wouldn't be surprised myself," asserted Bob. "Poor Liverpool! He was a fine chap, for all his rough ways. Still, there's no doubt that Birch was innocent. I shouldn't wonder if Liverpool got moonstruck and just pitched overboard. I've heard of that happening before, Mart. Look out—there's old Jerry coming aft now."

Sure enough, Mart looked out to see the slightly stooping figure of the old quartermaster coming aft to the wireless house. Jerry entered, ducked his head in silent greeting, and said nothing for some moments. After his pipe was filled, he looked out at the ocean, glittering in the morning sun, and then turned to glance solemnly at the two boys.

"Mystery o' the sea, lads—wave after wave! Fish down below, lads, and us up above. Fish tell no tales, fish tell no tales! Poor Liverpool Peters, he's—"

"Look here, Jerry," exclaimed Mart, breaking in abruptly on the old man's talk and forcing the bleary blue eyes to meet his. "I'd like to know just how much stock to take in your talk. How long is it since you and the rest of 'em were shipmates together aboard the Coralie, eh?"

Mart fully expected that Jerry would break out into vehement denial, and might even be surprised into making some admission. Bob, also, while no little astonished at his chum's unexpected attack, nodded his support and craned forward as he watched the quartermaster.

But to their mutual disconcertion, old Jerry's face did not change, save for a slight widening of his blue eyes as they met the hard gray ones of Mart. When he replied to the question, it was with a little chuckle as of inward amusement.

"Well, well! So you lads have heard about the old Coralie, hey? There ain't many in these seas as haven't, 'cause why, men are bound to talk. Only fish tell no tales, lads. Aye, the old Coralie was a sweet little schooner, she was! But that was all years ago—and now she's lyin' ninety fathom deep, lads, off the South Lyconia reef. Not very far from here, neither, where she went down."

Mart sent a blank gaze at his chum, as Jerry replaced his pipe in his mouth and gazed calmly out at the ocean. This cool reception of his bomb was dismaying to say the least; but Bob came promptly to the rescue, and more successfully.

"Why do they call you Shark Smith, Jerry?" he asked carelessly.

This time the boys scored visibly. The quartermaster's position did not change, but his bleared eyes suddenly flashed out quick and keen and bright, while his wrinkled old face lost its gently benignant expression as his firm mouth snapped shut on his pipe. This was not the first time the boys had seen that swift alteration of his features; and now it passed as quickly as it had passed before. Jerry turned slowly and looked at them, a slow smile crinkling up his eyes.

"Why, lads, ye main surprised me, ye did that! How come you to learn that old Jerry was called Shark Smith, now?"

"Oh, we heard about it," laughed Mart carelessly. "What's the reason, Jerry?"

The quartermaster chuckled again, tapped down his pipe, and replied frankly.

"Well, lads, I like both o' you, so I'll tell you. You mind me tellin' you about that there Pirate Shark, one day?"

They answered his questioning look with a nod.

"Well, when we was in the old Coralie, tradin' among the islands and doin' a bit o' pearl-fishin' on the side, we met up wi' that there Pirate Shark. He nipped two of our men, he did, and I been chasing him ever since, lads. I'm goin' to get him, an' I'm goin' to lay him out where he won't kill no more men, lads. My mates know this and that's why they call me Shark Smith, 'cause why I've been after that there Pirate Shark for a long time. Now I'm goin' to get him this cruise."

Mart's eyes flashed suddenly. He thought he understood everything now.

"So that's why you've got that dynamite aboard!" he cried accusingly. "You lied to Captain Hollinger about that river having gold, just to get—"

"Tut, tut, lad!" Under Jerry's reproachful glance his words died away. "No, I told no lies, lad. That river has gold in it all right. I'm goin' to get the Pirate Shark, and the cap'n gets the gold concession. Ain't that fair, lads? Ain't that fair, I asks you?"

Mart looked into the reproachful blue eyes an instant, then nodded. He suddenly felt ashamed of suspecting this gentle, half-crazy old man of any wrong. It lay plain before him now—the Pirate Shark had killed two of Jerry's shipmates, years before, and ever since that time the old quartermaster had been pursuing his enemy, until it had become a fixed mania with him. After all, he did not blame old Jerry so very much, he thought.

Bob also was quite satisfied now, as appeared after Jerry had slouched away below again and the two boys talked over the matter.

"By juniper, Mart," exclaimed Bob, "I guess dad was right. We were foolish to suspect old Jerry. He's got a bug about killing that Pirate Shark, see?"

"Sure he has," agreed Mart at once. "He's a little bit touched in the head, Holly, but that's about all. Did you notice that he never budged an eyelash when I shot out the Coralie at him?"

"Uh-huh," nodded Bob thoughtfully. "So the Coralie was just a trading schooner among the islands, eh? That straightens out things pretty well, Mart. I s'pose she was a pretty tough craft, like most of 'em were in the old days, and prob'ly she did a little pirating on the side. But just as dad says, there aren't any pirates any more. Especially on the Seamew. Believe me, we've been knocking at the wrong door."

"Looks like it to me," assented Mart. "Let's just forget the whole thing, Holly, and call it square. I guess there's no doubt that poor Liverpool fell overboard, either. But if Jerry got that dynamite put aboard to kill the Pirate Shark, I see where we're going to have some fun, Holly!"

"Say, that's right!" Bob sat up suddenly, looking at Mart. Then they both grinned.

"We'll let your dad get off after his tigers, an' when he gets back we'll have some surprising news for him, eh?"

"You bet!" agreed Bob, chortling.

But if Mart had been able to look into the future, he would hardly have greeted the prospect with such unalloyed delight. For old Jerry Smith was not quite so crazy as he was credited with being.