“Water rat,” he murmured, “or maybe muskrats.  I must set some traps.”

As Tom entered the boathouse he started back in alarm, for a bright light flashed up, almost in his eyes.

“Who’s here?” he cried, and at that moment someone sprang out of his motor-boat, scrambled into a rowing craft which the youth could dimly make out in front of the dock and began to pull away quickly.

“Hold on there!” cried the young inventor.  “Who are you?  What do you want?  Come back here!”

The person in the ’coat returned no answer.  With his heart doing beats over-time Tom lighted a lantern and made a hasty examination of the arrow.  It did not appear to have been harmed, but a glance showed that the door of the gasoline compartment had been unlocked and was open.  Tom jumped down into his craft.

“Some one has been at that compartment again!” he murmured.  “I wonder if it was the same man who acted so suspiciously at the auction?  What can his object be, anyhow?”

The next moment he uttered an exclamation of startled surprise and picked up something from the bottom of the boat.  It was a bunch of keys, with a tag attached, bearing the owner’s name.

“Andy Foger!” murmured Tom.  “So this is, how he was trying to get even!  Maybe he started to put a hole in the tank or in my boat.”

CHAPTER VI

TOWING SOME GIRLS

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.”