"Where did this diamond come from?" demanded Mr. Sharp of the quartette of criminals.

"That's for us to know and you to find out," sneered Happy Harry. "I don't care as long as that trimmer Boreck didn't get it. He tried to do us out of our share."

"Well, I guess the police will make you tell," went on the balloonist. "Go for the constable, Tom."

Leaving his friend to guard the ugly men, who for a time at least were beyond the possibility of doing harm, Tom hurried off through the woods to the nearest village. There he found an officer and the gang was soon lodged in jail. The diamond was turned over to the authorities, who said they would soon locate the owner.

Nor were they long in doing it, for it appeared the gem was part of a large jewel robbery that had taken place some time before in a distant city. The Happy Harry gang, as the men came to be called, were implicated in it, though they got only a small share of the plunder. Search was made for Tod Boreck and he was captured about a week after his companions. Seeing that their game was up, the men made a partial confession, telling where Mr. Swift's goods had been secreted, and the inventor's valuable tools, papers and machinery were recovered, no damage having been done to them.

It developed that after the diamond theft, and when the gang still had possession of Mr. Hastings' boat, Boreck, sometimes called Murdock by his cronies, unknown to them, had secreted the jewel in one of the braces under the gasoline tank. He expected to get it out secretly, but the capture of the gang and the sale of the boat prevented this. Then he tried to buy the craft to take out the diamond, but Tom overbid him. It was Boreck who found Andy's bunch of keys and used one to open the compartment lock when Tom surprised him. The man did manage to remove some of the blocks, thinking he had the one with the diamond in it, but the fact of Tom changing them, and painting the compartment deceived him. The gang hoped to get some valuables from Mr. Swift's shops, and, to a certain extent, succeeded after hanging around for several nights and following him to Sandport, but Tom eventually proved too much for them. Even stealing the Arrow, which was taken to aid the gang in robbing Mr. Swift, did not succeed, and Boreck's plan then to get possession of the diamond fell through.

It was thought that the gang would get long terms in prison, but one night, during a violent storm, they escaped from the local jail and that was the last seen of them for some time.

A few days after the capture as Tom was in the boathouse making some minor repairs to the motor he heard a voice calling:

"Mistah Swift, am yo' about?"

"Hello, Rad, is that you?" he inquired, recognizing the voice of the colored owner of the mule Boomerang.

"Yais, sa, dat's me. I got a lettah fo' yo'. I were passin' de post-office an' de clerk asted me to brung it to yo' 'case as how it's marked 'hurry,' an' he said he hadn't seen yo' to-day."

"That's right. I've been so busy I haven't had time to go for the mail," and Tom took the letter, giving Eradicate ten cents for his trouble.

"Ha, that's good!" exclaimed Tom as he read it.

"Hab some one done gone an' left yo' a fortune, Mistah Swift?" asked the negro.

"No, but it's almost as good. It's an invitation to take part in the motor-boat races next week. I'd forgotten all about them. I must get ready."

"Good land! Dat's all de risin' generation t'inks about now," observed Eradicate, "racin' an' goin' fast. Mah ole mule Boomerang am good enough fo' me," and, shaking his head in a woeful manner, Eradicate went on his way.

Tom told Mr. Sharp and his father of the proposed races of the Lanton Motor-boat Club, and, as it was required that two persons be in a craft the size of the Arrow, the young inventor arranged for the balloonist to accompany him. Our hero spent the next few days in tuning up his motor and in getting the Arrow ready for the contest.

The races took place on that side of Lake Carlopa near where Mr. Hastings lived, and he was one of the officials of the club. There were several classes, graded according to the horse-power of the motors, and Tom found himself in a class with Andy Foger.

"Here's where I beat you," boasted the red-haired youth exultantly, though his manner toward Tom was more temperate than usual. Andy had learned a lesson.

"Well, if you can beat me I'll give you credit for it," answered Tom.

The first race was for high-powered craft, and in this Mr. Hastings' new Carlopa won. Then came the trial of the small boats, and Tom was pleased to note that Miss Nestor was on hand in the tiny Dot.

"Good luck!" he called to her as he was adjusting his timer, for his turn would come soon. "Remember what I told you about the spark," for he had given her a few lessons.

"If I win it will be due to you," she called brightly.

She did win, coming in ahead of several confident lads who had better boats. But Miss Nestor handled the Dot to perfection and crossed the line a boat's length ahead of her nearest competitor.

"Fine!" cried Tom, and then came the warning gun that told him to get ready for his trial.

This was a five-mile race and had several entrants. The affair was a handicap one and Tom had no reason to complain of the rating allowed him.

"Crack!" went the starting pistol and away went Tom and one or two others who had the same allowance as did he. A little later the others started and finally the last class, including Andy Foger. The Red Streak shot ahead and was soon in the lead, for Andy and Sam had learned better how to handle their craft. Tom and Mr. Sharp were worried, but they stuck grimly to the race and when the turning stake was reached Tom's motor had so warmed up and was running so well that he crept up on Andy. A mile from the final mark Andy and Tom were on even terms, and though the red-haired lad tried to shake off his rival he could not. Andy's ignition system failed him several times and he changed from batteries to magneto and back again in the hope of getting a little more speed out of the motor.

But it was not to be. A half-mile away from the finish Tom, who had fallen behind a little, crept up on even terms. Then he slowly forged ahead, and, a hundred rods from the stake, the young inventor knew that the race was his. He clinched it a few minutes later, crossing the line amid a burst of cheers. The Arrow had beaten several boats out of her own class and Tom was very proud and happy.

"My, but we certainly did scoot along some!" cried Mr. Sharp. "But that's nothing to how we'll go when we build our airship, eh, Tom?" and he looked at the flushed face of the lad.

"No, indeed," agreed the young inventor. "But I don't know that we'll take part in any races in it. We'll build it, however, as soon as we can solve that one difficulty."

They did solve it, as will be told in the next book of this series, to be called "Tom Swift and His Airship; or, The Stirring Cruise of the Red Cloud." They had some remarkable adventures in the wonderful craft, and solved the mystery of a great bank robbery.

This ended the contests of the motor-boats and the little fleet crowded up to the floats and docks, where the prizes were to be awarded. Tom received a handsome silver cup and Miss Nestor a gold bracelet.

"Now I want all the contestants, winners and losers, to come up to my house and have lunch," invited Mr. Hastings.

As Tom and the balloonist strolled up the walk to the handsome house Andy Foger passed them.

"You wouldn't have beaten me if my spark coil hadn't gone back on me," he said, somewhat sneeringly.

"Maybe," admitted Tom, and just then he caught sight of Mary Nestor. "May I take you in to lunch?" he asked.

"Yes," she said, "because you helped me to win," and she blushed prettily. And then they all sat down to the tables set out on the lawn, while Tom looked so often at Mary Nestor that Mr. Sharp said afterward it was a wonder he found time to eat. But Tom didn't care. He was happy.