>From then on, for several days, the young inventor and his new friend lived in an atmosphere of airships. They talked them from morning until night, and even Mr. Swift, much as he was exercised over his loss, took part in the discussions.

In the meanwhile efforts had not ceased to locate the robbers and recover the stolen goods, but so far without success.

One afternoon, about two weeks after the thrilling rescue of John Sharp, Tom said to the balloonist:

"Wouldn't you like to come for a ride in the motor-boat? Maybe it will help us to solve the puzzle of the airship. We'll take a trip across and up the opposite shore."

"Good idea," commented Mr. Sharp. "Fine day for a sail. Come on. Blow the cobwebs from our brains."

Mr. Swift declined an invitation to accompany them, as he said he would stay home and try to straighten out his affairs, which were somewhat muddled by the robbery.

Out over the blue waters of Lake Carlopa shot the Arrow. It was making only moderate speed, as Tom was in no hurry, and he knew his engine would last longer if not forced too frequently. They glided along, crossed the lake and were proceeding up the opposite shore when, as they turned out from a little bay and rounded a point of land, Mr. Sharp exclaimed:

"Look out, Tom, there's rowboat just ahead!"

"Oh, I'll pass well to one side of that," answered the young inventor, looking at the craft. As he did so, noting that there were four men in it, one of the occupants caught a glimpse of the Arrow. No sooner had he done so than he spoke to his companions, and they all turned to stare at Tom. At first the lad could scarcely believe his eyes, but as he looked more intently he uttered a cry.

"There they are!"

"Who?" inquired Mr. Sharp.

"Those men---the thieves! We must catch them!"

Tom had spoken loudly, but even though the men in the rowboat did hear what he said, they would have realized without that that they were about to be pursued, for there was no mistaking the attitude of our hero.

Two of the thieves were at the oars, and, with one accord, they at once increased their speed. The boat swung about sharply and was headed for the shore, which they seemed to have come from only a short time previous, as the craft was not far out in the lake.

"No, you don't!" cried Tom. "I see your game! You want to get to the woods, where you'll have a better chance to escape! If this isn't great luck, coming upon them this way!"

It was the work of but a moment to speed up the engine and head the Arrow for the rowboat. The men were pulling frantically, but they had no chance.

"Get between them and the shore!" cried Mr. Sharp. "You can head them off then." This was good advice and Tom followed it. The men, among whom the lad could recognize Happy Harry and Anson Morse, were all excited. Two of them stood up, as though to jump overboard, but their companions called to them to stop.

"If we only had a gun now, not to shoot at them but to intimidate them," murmured the balloonist, "maybe they'd stop."

"Here's one," answered Tom, pointing to the seat locker, where he kept the shotgun Mr. Duncan had given him. In a moment Mr. Sharp had it out.

"Surrender!" he cried, pointing the weapon at the men in the small boat.

"Don't shoot! Don't fire on us! We'll give up!" cried Happy Harry, and the two with the oars ceased pulling.

"Don't take any chances," urged Mr. Sharp in a low voice. "Keep between them and the shore. I'll cover them." Tom was steering from an auxiliary side wheel near the motor, and soon the Arrow had cut off the retreat of the men. They could not land and to row across the lake meant speedy capture.

"Well, what do you want of us?" growled Morse. "What right have you got to interfere with us in this fashion?"

"The best of right," answered Tom. "You'll find out when you're landed in jail."

"You can't arrest us," sneered Happy Harry. "You're not an officer and you haven't any warrant."

Tom hadn't thought of that, and his chagrin showed in his face. Happy Harry was quick to see it.

"You'd better let us go," he threatened "We can have you arrested for bothering us. You haven't any right to stop us, Tom Swift."

"Maybe he hasn't, but I have!" exclaimed John Sharp suddenly.

"You! Who are you?" demanded Featherton, alias Simpson, the man who had run the automobile that carried Tom away.

"Me. I'm a special deputy sheriff for this county," answered the balloonist simply. "Here's my badge," and, throwing back his coat, he displayed it. "You see I got the appointment in order to have some authority in the crowds that gather to watch me go up," he explained to Tom, who plainly showed his astonishment. "I found it very useful to be able to threaten arrest, but in this case I'll do more than threaten. You are my prisoners," he went on to the men in the boat, and he handled the shotgun as if he knew how to use it. "I'll take you into custody on complaint of Mr. Swift for robbery. Now will you go quietly or are you going to make a fuss?" and Mr. Sharp shut his jaw grimly.

"Well, seeing as how you have the drop on us, I guess we'll have to do as you say," admitted Happy Harry, alias Jim Burke. "But you can't prove anything against us. We haven't any of Mr. Swift's property."

"Well, you know where it is then," retorted Tom quickly.

Under the restraining influence of the gun the men made no resistance. While Mr. Sharp covered them, Tom towed their boat toward shore. Then, while the young inventor held the gun, the balloonist tied the hands and feet of the thieves in a most scientific manner, for what he did not know about ropes and knots was not worth putting into a book.

"Now, I guess they'll stay quiet for a while," remarked Mr. Sharp as he surveyed the crestfallen criminals. "I'll remain on guard here, Tom, while you go notify the nearest constable and we'll take them to jail. We bagged the whole lot as neatly as could be desired."

"No, you didn't get all of us!" exclaimed Happy Harry, and there was a savage anger in his tones.

"Keep quiet!" urged Morse.

"No, I'll not keep quiet! It's a shame that we have to take our medicine while that trimmer, Tod Boreck, goes free. He ought to have been with us, and he would be, only he's trying to get away with that sparkler!"

"Keep quiet," again urged Morse.

Tom was all attention. He had caught the word "sparkler," and he at once associated it with the occasion he had heard the men use it before. He felt that he was on the track of solving the mystery connected with his boat.

He looked at the men. They were the same four who had been involved in the former theft---Appleson, Featherton, Morse and Burke. Were there five of them? He recalled the man who had been caught tampering with his boat---the man who had tried to bid on the Arrow at the auction. Where was he?

"Boreck didn't get what he was after," resumed Happy Harry, "and I'm going to spoil his game for him. Say, kid," he went on to Tom, "look in the front part of your boat---where the gasoline tank is."

Tom felt his heart beating fast. At last he felt that he would solve the puzzle. He opened the forward compartment. To his disappointment it seemed as usual. Morse and the others were making a vain effort to silence Happy Harry.

"I don't see anything here," said Tom.

"No, because it's hidden in one of those blocks of wood you use for a brace," continued the man. "Which one it is, Boreck didn't know, so he pulled out two or three, only to be fooled each time. You must have shifted them, kid, from the way they were when we had the boat."

"I did," answered the young inventor, recollecting how he had taken out some of the braces and inserted new ones, then painted the interior of the compartment. "What is in the braces, anyhow?"

"The sparkler---a big diamond---in a hollow place in the wood, kid!" exclaimed Happy Harry, blurting out the words. "I'm not going to let Tod Boreck get away with it while we stay in jail."

"Take out all the braces that haven't been moved and have a look," suggested Mr. Sharp. Tom only had to remove two, those farthest back, for all the others had, at one time or another, been changed or taken away by the thief.

One of the blocks did not seem to have anything unusual about it, but at the sight of the other Tom could not repress a cry. It was the one that seemed to have had a hole bored in it and then plugged up again. He remembered his father noticing it on the occasion of overhauling the boat.

"The sparkler's in there," said the tramp as he saw the brace. "Boreck was after it several times, but he never pulled out the right one."

With his knife Tom dug out the putty that covered the round hole in the block. No sooner had he done so than there rolled out into his hand a white object. It was something done up in tissue paper, and as he removed the wrapper, there was a flash in the sunlight and a large, beautiful diamond was revealed. The mystery had been solved.