Andy’s manner was so earnest that Tom could not help believing him.  Then there was the evidence of the advertisement.  Clearly the squint-eyed youth had not been the mysterious visitor to the boathouse and had not unlocked the forward compartment.  But if it was not he, who could it have been and how did the keys get there?  These were questions which racked Tom’s brain.

“You can ask Ned Newton,” repeated Andy.  “He’ll prove that I couldn’t have been near your place, if you don’t believe me.”

“Oh, I believe you all right,” answered Tom, for there could be no doubting Andy’s manner, even though he and the young inventor were not on good terms.  “But how did your keys get in my boat?”

“I don’t know, unless you found them, kept them and dropped them there,” was the insolent answer.

“You know better than that,” exclaimed Tom.

“Well, I owe you a reward of two dollars for giving them back to me,” continued the bully patronizingly.  “Here it is,” and he hauled out some bills.

“I don’t want your money!” fired back Tom.

“But I’d like to know who it was that was in my boat.”

“And I’d like to know who it was took my keys,” and Andy stuffed the money back in his pocket.  Tom did not answer.  He was puzzling over a queer matter and he wanted to be alone and think.  He turned aside from the red-haired lad and walked toward his motor-boat.

“I’ll give you a surprise in a few days,” Andy called after him, but Tom did not turn his head nor did he inquire what the surprise might be.

Mr. Swift was somewhat puzzled when his son related the outcome of the key incident.  He agreed with Tom that some one might have found the ring and kept it, and that the same person might have been the one whom Tom had surprised in the boathouse.

“But it’s idle to speculate on it,” commented the inventor.  “Andy might have induced some of his chums to act for him in harming your boat, and the key advertisement might have been only a ruse.”

“I hardly think so,” answered his son, shaking his head.  “It strikes me as being very curious, and I’m going to see if I can’t get at the bottom of it.”

But a week or more passed and Tom had no clew.  In the meanwhile he was working away at his motor-boat, installing several improvements.

One of these was a better pump, which circulated the water around the cylinders, and another was a new system of lubrication under forced feed.

“This ought to give me a little more speed,” reasoned Tom, who was not yet satisfied with his craft.  “Guess I’ll take it out for a spin.”

He was alone in the arrow, taking a long course up the lake when, as he passed a wooded point that concealed from view a sort of bay, he heard the puffing of another motor-boat.

“Maybe that’s Mr. Hastings,” thought Tom.  “If I raced with him now, I think the arrow could give a better account of herself.”