"It's a graft, if you ask me," Penny declared. "Poor Mr. Johnson seems hypnotized."

"I wish we could see that test tomorrow night!"

"So do I. In fact, I'd be willing to bet the machine won't work."

"What makes you think so?"

"Just my doubting nature, I suppose. No, there's more to it than that, Lou. Didn't you notice how startled the Bettenridges were when their star customer appeared?"

"They did look a bit upset."

"And the professor refused to give a demonstration tonight, although obviously it would have been to his advantage."

"He explained he had an engagement with Navy men."

"Which I suspect was all a made-up story. No, Lou, there must be another reason why the professor was unwilling to give the demonstration. He probably knows his machine won't work."

"You're convinced he's a fraud."

"Yes, I am," Penny said. "Furthermore, I believe he knows what became of Salt's camera."

"We can't prove anything."

"No, but if he would steal a camera he might also take to swindle in a big way."

"There's nothing we can do unless we want to report him to the police."

"I have a little idea," Penny confessed. "As soon as we reach Riverview I propose to check the Gables Hotel and learn if any Navy men have registered there. By talking to them, we may get at the truth."

CHAPTER 12 _THE PROFESSOR'S HELPER_

Climbing the hill, Penny and Louise sought their parked car. The day had been an interesting one, replete with surprises, and yet another surprise was in store.

As they were ready to drive away, a man came slowly down the road, cut across the Leonard yard and vanished down a path which led toward Blue Hole Lake.

"See that fellow!" Penny exclaimed.

"Why, yes," agreed Louise, surprised by her chum's tense manner. "What about him?"

"I'm sure he's Webb!"

"Webb?"

"The man who was pushed off the _Snark_ and who stole Ben's watch!"

"What would he be doing here?"

"That's exactly what I want to learn! I'd like to get Ben's watch back for him!" Quickly Penny pushed open the car door and jumped out.

"What are you going to do, Penny?" Louise asked anxiously.

"Follow that man and learn for certain who he is!"

"But it's late," Louise protested. "Besides, he looks like an unpleasant sort of individual."

Penny paid no heed, but started off in pursuit of the stranger. He had already disappeared among the trees and was well on his way toward the lake. Not wishing to be deserted, Louise quickly followed her chum.

"He's going to the professor's shack!" Penny observed a moment later.

"Perhaps he is another prospective buyer of the secret ray machine,"

Louise speculated. "Business seems to be rushing today."

Penny was not convinced. "I'm sure it is Webb," she declared. "If we can force him to admit his ident.i.ty, we may get Ben's watch back."

Not wishing to attract attention, the girls paused behind a large rock on the hillside. From there they could watch the man without being seen.

He walked directly to the shack and tapped on the door. In a moment it was opened by the professor, who looked anything but pleased to see the new arrival. Closing the door behind him as if fearful that Mr. Johnson would hear, he stepped out of doors.

The girls were too far away to overhear the conversation, but they saw the two men talk earnestly together for a moment. Then the man they believed to be Webb, walked on down the hill toward the lake's edge.

Professor Bettenridge reentered the shack.

"Now what?" inquired Louise, straightening up from a cramped position behind the rock.

"Let's follow Webb. I have a hunch he may be the a.s.sistant Professor Bettenridge told Mr. Johnson about."

"But the man wasn't expected here until tomorrow."

"Which may or may not have been true, Lou. There's more to this deal than meets the eye. Let's see what we can learn."

Already the man had disappeared from view, so the girls walked swiftly after him. Reaching the lake's edge, they saw him striding along the sandy beach. Apparently he had no suspicion that he was being trailed, for he did not glance backward.

Presently the girls noticed another shack which had been erected in a clump of trees a few yards back from the beach. It was much smaller than the other little house, a mere box-like structure with a flat, low roof.

Walking directly toward it, the man unfastened a padlock and went inside.

He closed the door behind him.

"Now what is he doing in there?" Penny fretted, as minutes elapsed and the man remained inside the building. "Listen!"

Both girls could hear a peculiar grinding sound as if from machinery turning inside the shack. The building was windowless, so it was impossible to see what was going on.

"This is maddening!" Penny muttered with increasing impatience. "I wonder--?" She broke off, and gazed thoughtfully at the flat roof of the shack.

"Lou, how is that building lighted?" she demanded.

"From here it looks as if there might be double panels of gla.s.s in the roof--a make-shift skylight."