With a sense of anger mingled with an apprehension lest some harm should have been done to his craft, the owner of the Arrow went carefully over it. He could find nothing wrong. The engine was all right and all that appeared to have been accomplished by the unbidden visitor was the opening of the locked forward compartment. That this had been done by one of the many keys on Andy Foger's ring was evident.

"Now what could have been his object?" mused Tom. "I should think if he wanted to put a hole in the boat he would have done it amidships, where the water would have a better chance to come in, or perhaps he wanted to flood it with gasoline and---"

The idea of fire was in Tom's mind, and he did not finish his half-completed thought.

"That may have been it," he resumed after a hasty examination of the gasoline tank, to make sure there were no leaks in it. "To get even with me for outbidding him on the boat, Andy may have wanted to destroy the Arrow. Well, of all the mean tricks, that's about the limit! But wait until I see him. I've got evidence against him," and Tom looked at the key ring. "I could almost have him arrested for this."

Going outside the boathouse, Tom stood on the edge of the dock and peered into the darkness. He could hear the faint sound of someone rowing across the lake, but there was no light.

"He had one of those electric flash lanterns," decided Tom. "If I hadn't found his keys, I might have thought it was Happy Harry instead of Andy."

The young inventor went back into the house after carefully locking the boat compartment and detaching from the engine an electrical device, without which the motor in the Arrow could not be started.

"That will prevent them from running away with my boat, anyhow," decided Tom. "And I'll tell Garret Jackson to keep a sharp watch tonight." Jackson was the engineer at Mr. Swift's workshop.

Tom told his father of the happening and Mr. Swift was properly indignant. He wanted to go at once to see Mr. Foger and complain of Andy's act, but Tom counseled waiting.

"I'll attend to Andy myself," said the young inventor. "He's getting desperate, I guess, or he wouldn't try to set the place on fire. But wait until I show him these keys."

Bright and early the next morning the owner of the motor-boat was down to the dock inspecting it. The engineer, who had been on watch part of the night, reported that there had been no disturbance, and Tom found everything all right. "I wonder if I'd better go over and accuse Andy now or wait until I see him and spring this evidence on him?" thought our hero. Then he decided it would be better to wait. He took the Arrow out after breakfast, his father going on a short spin with him.

"But I must go back now and work on my gyroscope invention," said Mr. Swift when about two hours had been spent on the lake. "I am making good progress with it."

"You need a vacation," decided Tom, "I'll be ready to take you and Ned in about two weeks. He will have two weeks off then and, we'll have some glorious times together."

That afternoon Tom put some new style spark plugs in the cylinders of his motor and found that he had considerably increased the revolutions of the engine, due to a better explosion being obtained. He also made some minor adjustments and the next day he went out alone for a long run.

Heading up the lake, Tom was soon in sight of a popular excursion resort that was frequently visited by church and Sunday-school organizations in the vicinity of Shopton. The lad saw a number of rowing craft and a small motor-boat circling around opposite the resort and remarked: "There must be a picnic at the grove to-day. Guess I'll run up and take a look."

The lad was soon in the midst of quite a flotilla of rowboats, most of them manned by pretty girls or in charge of boys who were giving sisters (their own or some other chap's) a trip on the water. Tom throttled his boat down to slow speed and looked with pleasure on the pretty scene. His boat attracted considerable attention, for motor craft were not numerous on Lake Carlopa.

As our hero passed a boat, containing three very pretty young ladies, Tom heard one of them exclaim:

"There he is now! That's Tom Swift."

Something in the tones of the voice attracted his attention. He turned and saw a brown-eyed girl smiling at him. She bowed and asked, blushing the while:

"Well, have you caught any more runaway horses lately?"

"Runaway horses---why---what? Oh, it's Miss Nestor!" exclaimed the lad, recognizing the young lady whose steed he had frightened one day when he was on his bicycle. As told in the first volume of this series, the horse had run away, being alarmed at the flashing of Tom's wheel, and Miss Mary Nestor, of Mansburg, was in grave danger.

"So you've given up the bicycle for the motor-boat," went on the young lady.

"Yes," replied Tom with a smile, shutting off the power, "and I haven't had a chance to save any girls since I've had it."

The two boats had drifted close together, and Miss Nestor introduced her two companions to Tom.

"Don't you want to come in and take a ride?" he asked.

"Is it safe?" asked Jennie Haddon, one of the trio.

"Of course it is, Jennie, or he wouldn't be out in it," said Miss Nestor hastily. "Come on, let's get in. I'm just dying for a motor-boat ride."

"What will we do with our boat?" asked Katie Carson.

"Oh, I can tow that," replied the youth. "Get right in and I'll take you all around the lake."

"Not too far," stipulated the girl who had figured in the runaway. "We must be back for lunch, which will be served in about an hour. Our church and Sunday-school are having a picnic."

"Maybe Mr. Swift will come and have some lunch with us," suggested Miss Carson, blushing prettily.

"Nothing would give me greater pleasure," answered Tom, and then he laughed at his formal reply, the girls joining in.

"We'd be glad to have you," added Miss Haddon. "Oh!" she suddenly screamed, "the boat's tipping over!"

"Oh, no," Tom hastened to assure her, coming, to the side to help her in. "It just tilts a bit, with the weight of so many on one side. It couldn't capsize if it tried."

In another moment the three were in the roomy cockpit and Tom had made the empty rowboat fast to the stern. He was about to start up when from another boat, containing two little girls and two slightly larger boys, came a plaintive cry:

"Oh, mister, give us a ride!"

"Sure!" agreed Tom pleasantly. "Just fasten your boat to the other rowboat and I'll tow you."

One of the boys did this, and then, with three pretty girls as his companions in the Arrow and towing the two boats, Tom started off.

The girls were very much interested in the craft and asked all sorts of questions about how the engine operated. Tom explained as clearly as he could how the gasoline exploded in the cylinders, about the electric spark and about the propeller. Then, when he had finished, Miss Haddon remarked naively:

"Oh, Mr. Swift, you've explained it beautifully, and I'm sure if our teacher in school made things as clear as you have that I could get along fine. I understand all about it, except I don't see what makes the engine go."

"Oh," said Tom faintly, and he wondering what would be the best remark to make under the circumstances, when Miss Nestor created a diversion by looking at her watch and exclaiming:

"Oh, girls, it's lunch time! We must go ashore. Will you kindly put about, Mr. Swift---I hope that is the proper term---and---land us---is that right?" and she looked archly at Tom.

"That's perfectly right," he admitted with a laugh and a glance into the girl's brown eyes. "I'll put you ashore at once," and he headed for a small dock.

"And come yourself to take lunch with us, added Miss Haddon.

"I'm afraid I might be in the way," stammered Tom. "I---I have a pretty good appetite, and---"

"I suppose you think that girls on a picnic don't take much lunch," finished Miss Nestor. "But I assure you that we have plenty, and that you will be very welcome," she added warmly.

"Yes, and I'd like to have him explain over again how the engine works," went on Miss Haddon. "I am so interested."

Tom helped the girls out, receiving their thanks as well as those of the children in the second boat. But as he walked with the young ladies through the grove the young inventor registered a mental vow that he would steer clear of explaining again how a gasoline engine worked.

"Now come right over this way to our table," invited Miss Nestor. "I want you to meet papa and mamma."

Tom followed her. As he stepped from behind a clump of trees he saw, standing not far away, a figure that seemed strangely familiar. A moment later the figure turned and Tom saw Andy Foger confronting him. At the sight of our hero the bully turned red and walked quickly away, while Tom's fingers touched the ring of keys in his pocket.