Everyone who witnessed the spectacular demonstration was awed by the sight of the flames rising above the lake. As they died away, Professor Bettenridge, strutting a bit, walked back to his machine and covered it with the canvas hood.

“Now are you satisfied?” he inquired triumphantly. “Is there anyone here who doubts the remarkable possibilities of my invention?”

“It was a fine demonstration! Magnificent!” approved Mr. Johnson, fairly beside himself with excitement. “I am convinced of the machine’s worth and if we can agree upon terms I will write you a check tonight.”

Professor Bettenridge’s expression did not change, but the brief glance he flashed his wife was not lost upon Penny or Salt.

“You understand, of course,” he said smoothly, “that the Navy probably will insist upon ultimate purchase of the machine even if I relinquish ownership?”

“Certainly,” agreed Mr. Johnson. “I should expect to make such a sale. The machine would have no practical use except in warfare.”

Penny was tempted to ask the man if he considered it patriotic to try to obtain control of a machine in the hope of selling it to the government at a high profit to himself. But she wisely remained silent.

Salt, however, had a few pointed remarks to offer.

“How come,” he observed, “that if this invention is so remarkable, the Navy hasn’t already snapped it up?”

Professor Bettenridge froze him with a glance. “Young man,” he said cuttingly, “you evidently do not understand how government business is conducted. Negotiations take months to complete. My wife and I need cash, so for that reason, we are willing to sell the machine quickly.”

“Yesterday I understood you to say that Navy men were ready to complete the deal,” Penny interposed innocently. “Did they change their minds?”

“Certainly not!” Professor Bettenridge’s dark eyes flashed, and only by great effort did he maintain control of his temper. “You understand that while their recommendation would eventually be acted upon, a sale still would take many months to complete.”

“Will your machine explode mines on land as well as in the water?” Salt inquired.

“Of course!”

“Then why not give us a land demonstration?”

“Us!” the professor mocked, his patience at an end. “Young man, you were not invited here, and I might add that your presence irritates me! Are you in any way associated with Mr. Johnson?”

“I am not.”

“Then kindly do not inject yourself into our negotiations.”

“The young man raises an interesting point,” Mr. Johnson interposed, frowning thoughtfully. “Perhaps we should have a land demonstration before I pay over the money.”

“So you doubt my honesty?” the professor demanded.

“Not at all. It’s only that I must be very careful before I purchase such an expensive machine. I must satisfy myself that it will do everything you claim for it.”

“You have just witnessed a successful demonstration. What more do you ask?”

“A successful water test,” Salt remarked softly, “does not necessarily mean a successful land test.”

“I think we should have a land test,” Mr. Johnson decided. “If you convince me that the machine will work equally well under such circumstances, I will write the check instantly.”

“A land test is impossible,” the professor said stiffly.

“But why?” inquired Mr. Johnson.

“The dangers are too great. Windows would be smashed for many miles around. Authorities would not permit such a test. Only with the greater difficulty did I obtain permission to discharge the mines under water.”

“I had not thought of that,” Mr. Johnson acknowledged. He hesitated, and it was evident that in another moment he would decide to purchase the machine without further tests.

“Mr. Johnson, I suppose you have witnessed tests made with your own mines,” Penny interposed. “Or have they all been made with those supplied by the professor?”

The remark infuriated Mr. and Mrs. Bettenridge, as she had expected it would.

“You and this young man are trying to discredit my machine!” he exclaimed wrathfully. “Please leave.”

“Why, certainly,” agreed Penny, but made no move to depart.

Seeds of suspicion already had been implanted in Mr. Johnson’s mind.

“The young lady is right,” he said. “I should request a test on a mine which I provide myself.”

“Ridiculous!” snapped the professor. “The machine will work equally well on any mine.”

“Then surely you should not object to one further test?”

“The delay is unnecessary.”

“I am certain I can provide a mine within two days,” insisted Mr. Johnson. “Suppose we set the next test for Thursday night at this same hour?”

Thus trapped, Professor Bettenridge could not refuse without losing the sale. Scowling, he gave in.

“Very well. But this will be the final demonstration. If you are not satisfied Thursday night, the deal is off.”

“Agreed,” said Mr. Johnson.

Bowing to the Bettenridges, he departed. Others who had witnessed the demonstration began to melt away. Only Salt and Penny remained.

Professor Bettenridge closed the door so that the conversation would not be overheard. Then he turned angrily to the pair.

“Now what’s your little game?” he demanded. “You deliberately tried to queer my sale? Why?”

“Mr. Johnson seems like such an innocent little rabbit, maybe we thought he ought to be protected,” Salt drawled.

“Protected! Why, he’s being given the opportunity of a lifetime! How much is it worth to you to keep out of my affairs?”

“Not a cent,” Salt retorted. “We don’t want any part of your deal. But there’s something I did come here for—my camera.”

“I’ve already told the young lady I know nothing about it. If you were stupid enough to throw it into a passing automobile, then you deserve to lose it. Now get out!”

Salt was tempted to argue the matter, but Penny took his arm, pulling him toward the door. The professor slammed it hard behind them as they went out into the night.

“That fellow is a crook!” Salt exploded. “I’ll bet a cookie he has my camera too!”

“Well, we can’t prove it,” Penny sighed. “After all, we did act in a high-handed way. We may have queered his sale to Mr. Johnson.”

“A good thing if we have.”

“But we have no proof the machine is a fake. With our own eyes we saw the mine explode. Of course, we think Webb tampered with it in some fashion, but we’re not certain of that either.”

“The proof of the pudding will come Thursday night when and if the old boy explodes Mr. Johnson’s mine.”

“He’s just clever enough to do it, too,” Penny said gloomily.

The couple had walked only a short distance up the lane when they were startled to hear a shrill whistle in the darkness. It came from the beach.

Halting, they waited. In a moment the sound was repeated. Then to their surprise, came an answering whistle from inside Professor Bettenridge’s cabin.

“That must be Webb,” Salt whispered, observing a shadowy form approaching. “The whistle evidently is a signal to make certain the coast is clear.”

As they watched, the hunched figure emerged from the darkness, was silhouetted momentarily in the light which came from the cabin, then disappeared inside.

“I wish we knew what they were talking about in there,” Penny said. “It might clear up some of the mystery.”

“Why not see what we can learn?” proposed Salt. “It’s safe enough.”

Taking care to walk softly, the pair stole back to the cabin. Crouching by the window, they could hear a low murmur of voices inside. At first it was difficult to catch the trend of the conversation, but gradually Professor Bettenridge’s voice grew louder.

“I don’t like it any better than you do,” Salt and Penny heard him say, “but that’s the way it is. If we’re to finish the deal, we’ve got to explode one of Johnson’s mines Thursday night. The question is, can we do it?”

“Depends on the type of mine,” Webb replied gruffly. “How soon can we have it ahead of the test?”

“I’ll insist that he deliver it here at least by afternoon. Will that give you time enough?”

“Sure, it won’t take more than a half hour to fix ’er for the test, providing it can be done. But I ain’t makin’ no promises until I see the mine.”

“It’s a chance we have to take,” Professor Bettenridge said. “The deal would have gone through tonight if it hadn’t been for a couple of young newspaper fools who came nosing around here. They may make us trouble Thursday night too.”

“I ain’t aimin’ to get mixed with the police,” Webb said uneasily. “If this deal don’t go through Thursday night, I’m quitting. We’re in a mighty risky business.”

“But we stand to make at least $200,000,” the professor reminded him. “You’ll get a third cut. If Johnson holds off Thursday night, I’ll drop to $100,000. The thing we’ve got to do is to pull off that test okay and clear out.”

Penny and Salt had heard enough to be certain that the men with whom they were dealing were crooks of the first rank. Slipping noiselessly away, they trudged to the car.

“Now what do we do?” Penny questioned. “Notify the police?”

“We could,” Salt debated, “but so far, it’s only our word against Professor Bettenridge’s. He’d probably convince the police he was only a crack-pot inventor who thought he had a wonderful machine. They might let him go.”

“Any other ideas?”

“A slick trick would be to fix that mine so it won’t explode. That automatically would cause complications and probably delay the deal with Mr. Johnson.”

“Just how do you propose to fix Mr. Johnson’s mine?” Penny inquired. “It would take some doing.”

“The mines are all kept in that shack on the beach?”

“Yes, Louise and I saw Webb working on one of them there. Evidently it was the one the professor exploded tonight.”

“He must have doctored it in some special way. Probably an untampered mine won’t explode.”

“He’ll fix Mr. Johnson’s mine the same way, and then the test will appear successful.”

Salt nodded gloomily. He was lost in thought for several minutes, and then he grinned.

“Maybe I have an idea!”

“What is it, Salt?”

The photographer switched on the car ignition. “Wait until Thursday night,” he replied. “Can you get away from the office early?”

“Well, I really shouldn’t—”

“I’ll take care of that part,” Salt said briskly. “Just sit tight, Penny. You and I will have some fun out of this affair yet, and maybe we’ll save Mr. Johnson a tidy sum of money.”

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