Part 8

"Probably to give Webb time enough to work on the mine or exchange them," Penny said, and then frowned thoughtfully. "But what if the machine actually should work? After all, the professor agreed to explode Mr. Johnson's mine, and apparently he's marked it with his initials. It won't be easy to substitute another one now."

"All the same, if I'm any good at guessing, it will be done. Now what shall we do until nine o'clock? Grab ourselves something to eat?"

Penny was about to suggest that they drive to a village cafe, when she noticed Webb returning alone from up the beach. Barely did the pair have time to duck out of sight behind a boulder before he approached.

Walking directly to the shack, he unlocked the door, and entered.

"Now this must be where the hocus-pocus begins!" Salt whispered. "We've got to find out what he does to that mine."

"Louise and I climbed up in that tree the other day and looked through the glass in the top."

"Then that's the trick for us! Come on!"

Making no sound, the pair climbed the tree close beside the shack. Noiselessly, they inched their way toward the skylight, and lying flat, peered down into the dark interior.

Webb had lighted a lantern which he hung on a wall nail. Unaware that he was being watched, he squatted in front of the mine which bore Mr. Johnson's initials, studying it thoughtfully.

Muttering to himself, he next took a powerful ratchet drill, and for a long time worked with it on the mine, boring a tiny but deep hole.

"I'm getting stiff in this position," Penny whispered. "What is he doing, Salt?"

"Don't know," the photographer admitted, puzzled. "Apparently, he's doctoring Mr. Johnson's mine so it will explode tonight, but I'm not smart enough to figure how the trick will be accomplished."

By now it was so dark that the pair in the tree no longer feared they would be seen. Keeping perfectly still, they watched the work in the room below.

"It's clear why Professor Bettenridge set nine o'clock for the demonstration," Salt whispered. "Webb needed all this time to get the mine ready."

"And that's why they brought it here instead of dumping it into the lake," Penny added. "But how can they make the mine explode at exactly the right moment?"

After Webb had worked for a while longer, he arose and stretched his cramped muscles. Going to a cupboard, he removed a white powder from a glass tube, and carefully inserted it in the hole he had just made in the mine. As a final act, he sealed the tiny hole with another material, and polished the surface so that the place did not show.

"Slick work!" Salt commented. "By the time he's through, no one ever could tell the mine has been touched! Certainly not that thick-skulled Johnson."

Apparently satisfied with his work, Webb put away his tools, made a final inspection of the mine, and then left the shack. After carefully locking the door, he disappeared into the night.

"Now what's our move?" Penny asked as she and Salt finally slid down from their uncomfortable perch. "Shall we tell Mr. Johnson what we just saw?"

"We could, but he might not believe us. Penny, I have a better idea! If we can get inside the shack--"

"But it's locked!"

"The skylight may be open." Salt climbed up on the roof to investigate, but to his disappointment, the roof window was tightly fastened from inside.

"We could smash the glass," Penny suggested dubiously.

Salt shook his head. "That would give the whole thing away. No, I think we can get inside another way, but we'll have to work fast! Now that Webb has the mine ready for the demonstration, the professor and Mr. Johnson may show up here at any minute."

CHAPTER 19 _THE LANTERN SIGNAL_

Salt explained that he intended to pick the lock of the shack door.

"When I worked the police beat, a detective taught me this trick," he explained. "You keep watch while I work."

Now that Webb had disappeared no one was to be seen near the beach. To Penny's relief, not a person appeared, and Salt, working swiftly, soon had the door open.

To make certain they would not be taken unawares, Salt relocked the door on the inside. Groping about, he found the lantern Webb had left behind, and lighted it.

Three mines lay on the floor. "Which is the right one?" Penny asked. "They all look alike!"

"Mr. Johnson's initials must be on the one Webb tampered with."

Salt turned over one of the mines, inspecting it.

"That thing might go off any minute," Penny said, edging away. "Do be careful, Salt."

Salt chuckled. "If it should go off, we'd never know what hit us," he said. "This is the one Webb tampered with all right. Penny, how are you at forging?"

"Forging?" she repeated, not understanding what he meant.

"Can you duplicate Mr. Johnson's initials on another mine?"

"Oh, I don't think so. Not so it would look the same."

"Sure, you can," Salt said, thrusting his pocket knife into her hand. "It will be dark and no one will look too carefully."

"But why do you want me to do it? You mean to substitute Mr. Johnson's mine for one of the others?"

"That's the ticket," chuckled the photographer. "Maybe my guess is wrong, but I have a sneaking suspicion if we use one of the professor's own mines, it will fail to explode."

"The mine has to be doctored with that powder we saw Webb use!"

"That's my theory, Penny."

"But maybe the other mines have already been treated."

"That's a possibility," Salt admitted thoughtfully. "No way of telling that, because the hole would be covered so skillfully. We'll have to take a chance on it."

While Salt held the lantern, Penny scratched Mr. Johnson's initials on the metal covering of the mine. Skilled in art, she was able to copy them fairly well.

"They don't look exactly the same," Salt said, comparing the two, "but they're good enough to get by unless Mr. Johnson becomes very critical."

Quickly they moved the two mines, placing Mr. Johnson's well to the back of the room, and leaving the substitute exactly where the other had been.

"Well, that job is done," Salt chuckled. "Unless I miss my guess--"

He broke off, startled to hear a murmur of voices from a short distance down the beach. Quick as a flash he blew out the lantern and hung it in its accustomed place on the wall nail.

"Salt! Those men are coming!" Penny whispered fearfully. "We're trapped here!"

It was too late to slip out the door, for already the men were very close, and unmistakably, one of the voices was that of Professor Bettenridge.

The only available hiding place was a storage closet. Barely in time, Salt and Penny squeezed into it, closing the door and flattening themselves against the wall.

The door of the shack swung open to admit the professor, Webb, and Mr. Johnson.

"Dark as pitch in here," Webb muttered. "Wait and I'll light the lantern."

In a moment the yellow glow illuminated the dingy little room.

"Which is my mine?" Mr. Johnson asked. "They all look alike."

"And for all practical purposes they are exactly alike," said the professor smoothly. "So far as my machine is concerned, it makes not a particle of difference. Webb, which is the mine that Mr. Johnson supplied?"

"Here it is," the assistant said, tapping the one Salt and Penny had substituted. "See your initials, Mr. Johnson?"

"Yes, yes," agreed the man.

Inside the closet, Penny and Salt breathed easier.

"Let's get on with the demonstration," the professor urged with sudden impatience. "Load the mine onto the boat, Webb. Go out to the center of the lake. Then when you have dropped it, give the usual signal."

"When everything is okay, I'll wave my lantern three times," Webb agreed.

The mine was trundled out and the shack became dark. However, Salt and Penny did not dare come out of hiding until they heard Webb start the motor of the boat.

"The coast is clear," the photographer then reported, peering out a crack of the outer door. "Webb has gone, and the professor and Mr. Johnson are walking up to the cabin."

From the beach, Penny and Salt watched the boat moving slowly across the water. Presently the craft stopped, and the mine was heaved overboard. The pair waited, but there was no signal from Webb. Nor did his boat move away from the locality where the mine had been dropped.

"Why doesn't he wave the lantern?" Penny fretted.

"He's waiting deliberately, and for a purpose," Salt declared. "Why not amble up the hill and watch the professor perform?"

"Not a bad idea," agreed Penny.

Walking rapidly, they arrived at the cabin quite breathless. As they tapped lightly on the door, Professor Bettenridge appeared visibly startled. He stiffened to alert, guarded attention, but relaxed slightly as his wife admitted the pair.

"Oh, it's you two again," he said none too pleasantly. "You are just in time to witness my final demonstration. We are waiting now for my assistant's signal."

"It seems to take a long while," Mr. Johnson commented, glancing at his watch.

"Webb may have had trouble getting the mine overboard," the professor soothed. "Besides, he has to move out of the danger zone."

Penny and Salt looked at each other but said nothing. They were certain that Webb had been in no haste to return to shore.

"What are you two smirking about?" the professor demanded irritably. "I suppose you think my machine won't work?"

"I'll be surprised if it does," Salt agreed, unruffled.

Mrs. Bettenridge, who stood at the window, suddenly cried: "There is the signal!"

Professor Bettenridge snapped on a switch and the ray machine began to hum. He turned on another motor and lights began to glow. Then he struck the crystal ball, producing a musical vibration.

Assuming a confident pose, he waited.

Nothing happened.

As the seconds ticked by and still there was no explosion, the professor began to wilt. He gazed desperately at his wife who looked as dismayed as he.

"My dear, something seems to be wrong. Are you sure you saw the signal? Perhaps Webb has not yet dropped the mine."

"I saw the signal. The lantern was waved three times."

The professor made several adjustments on his machine, and again struck the musical note. But there was no explosion. Enjoying his discomfiture, Salt and Penny grinned from ear to ear.

"You did something to the machine!" the professor accused them furiously. "You came here and tampered!"

"We've not been near this place tonight until a moment ago," Penny retorted. "The truth is, you weren't able to explode Mr. Johnson's mine!"

"That's not so!" The professor's face now was red with anger. "Something has gone wrong, but that doesn't prove my machine is a failure. We'll have another test."

"I'm not sure that I shall be interested," Mr. Johnson said quietly. "I've been thinking the matter over and there are so many hazards--"

"I'll make you an especially good offer," the professor declared, flipping the canvas cover over his machine. "Furthermore, we will have the test tonight. I guarantee to explode the mine before you leave here."

"But the mine I supplied is at the bottom of the lake and it failed to go off," Mr. Johnson said.

"First, we will talk to Webb and learn exactly what happened," the professor said, taking him by the arm. "I know there is a logical explanation for the failure."

Glaring at Penny and Salt, he shooed everyone out of the cabin, locking the door.

"My dear," he said to his wife, giving her a significant look, "take Mr. Johnson to the house while I find Webb. I'll be with you in just a minute."

The professor went hurriedly down the beach while Mrs. Bettenridge and Mr. Johnson walked slowly toward the rooming house. Penny and Salt remained beside the cabin until everyone was beyond hearing.

"Well, our trick worked," Salt chuckled, "but if we aren't careful, the professor will pull off a successful test yet and ruin all our plans."

"He and Webb are certain to examine the mines and discover the one with Mr. Johnson's initials still in the shack. Then they may convince Mr. Johnson there was a mix-up, and go ahead with another test which will be successful."

"We've got to do something," Salt muttered. "But what?"

"I know!" Penny exclaimed. "I'll telephone Dad and have him come here right away with Major Bryan!"

"Good!" approved Salt. "I'll stay here and hold the fort while you telephone. Tell your father to step on the gas, because we've got to move fast to queer Professor Bettenridge's game."

CHAPTER 20 _A CROOK EXPOSED_

Eager to carry out Salt Sommers' bidding, Penny ran up the hill in search of a telephone. She considered using the one at the house where Professor and Mrs. Bettenridge roomed, but decided against it, fearing that the conversation might be overheard or reported to them by the farm woman.

Hastening on, she saw a light farther down the road, and recalled having noticed a house there. Five minutes later, completely winded, she pounded on the door. A man in shirtsleeves, the evening newspaper in his hand, answered her knock.

"Please, may I use your telephone?" Penny gasped.

"Why, sure," he agreed, stepping aside for her to enter. "Anything wrong?"

Penny knew better than to mention what was happening at the lake. "I want to telephone my father in Riverview," she explained.

"The phone is in the other room," the man said, switching on a light.

Placing the call, Penny waited impatiently for it to be put through. She was uncertain whether her father would be at home. If she failed to reach him, then the only other thing was to notify the sheriff.

"Here is your party. Go ahead, please," came the long distance operator's voice. The next moment Penny heard her father's clear tones at the other end of the line.

"Dad, I'm at Blue Hole Lake with Salt," she explained hurriedly. "Can you drive here right away?"

"I suppose so," he answered, knowing from her voice that something serious was wrong. "What's up?"

"We've learned plenty about Professor Bettenridge, Dad. Unless something is done quickly, he may sell his fake machine to Mr. Johnson."

"But what can _I_ do about it?" the publisher asked.

"Can you get hold of the Major and bring him with you?" Penny pleaded. "Professor Bettenridge may be the man he's after!"

"Maybe I can reach him!" Mr. Parker agreed. "If I have luck I'll be out there within twenty or thirty minutes. I'll come as fast as I can."

Before hanging up the receiver, Penny gave her father detailed instructions for reaching the lake and told him where to park. Leaving a dollar bill to pay for the call, she then hastened back to find Salt.

The photographer was nowhere near the cabin and she was afraid to call his name lest she be overheard by the Bettenridges.

As she stood in the shadow of the building, she heard voices from the beach. Someone with a lighted lantern was coming up the trail, and soon she distinguished two figures--Professor Bettenridge and Webb.

"That's the story you'll have to tell Johnson," she heard the professor say. "Tell him that somehow you got the two mines mixed up as you were loading them onto the boat and dumped one that was never meant to explode."

"But he saw us load the mine."

"It was dark and he may not be sure. Anyway, the mine with Johnson's initials is still in the shack. We'll show it to him."

"What bothers me is how did the mistake happen?" Webb muttered. "I know the mine I loaded on the boat had Johnson's initials. It should have gone off."

"Someone is onto our game, and tampered with the mines. It may have been a trick of that newspaper pair."

"In that case, we're in a dangerous spot. We ought to clear out while the clearing is good. If the authorities get onto what we're doing--"

"They won't--at least not tonight," the professor said confidently. "The sheriff is as dumb as they come, and is convinced I am a genius second only to Thomas Edison. We'll have to pull off a successful test tonight with Johnson's mine, collect what we can, and clear out."

"Okay," Webb agreed, "but this is my last job. The game is too dangerous. I served one stretch in the pen and I don't look forward to another."

"If we can explode Johnson's mine tonight, we'll collect the money and be away from here as soon as we cash the check. Can you pull off the job without any blunder?"

"Sure I can unless someone tampers with the mine! This time I'll make sure they don't!"

"Okay," the professor agreed. "Now I want you to talk to Johnson. Put up a good story, and get him to look at the mine that has his initials on it. If he refuses, we're licked, but it's worth a final try."

"I'd like to find the guy who broke into the shack!" Webb muttered.

"We may have time for that later. Just now our most important job is to convince Johnson we have something to sell."

The two men now were very close. Penny flattened herself against the building wall, fearful of being seen. The light from their lantern illuminated her for an instant, but the men were so absorbed in their discussion, they failed to see her. Going on up the hillside path, they vanished into the farmhouse.

What had become of Salt, Penny did not know. Thinking he might have gone down to the lake, she walked rapidly in that direction. As she approached the shack where the mines were stored, she heard a low whistle.

"Is that you, Salt?" she called softly.

He came from behind a clump of bushes to join her. Quickly they compared notes. Salt had overheard no conversation, but he had watched Professor Bettenridge and Webb as they reexamined the mines in the shack.

"They're onto our game, and it won't work twice," he said. "We've got to delay the test, but how?"

"Maybe we could cut the boat loose!"

"A capital idea!" Salt approved, chuckling. "Penny, you really have a brain!"

As they scurried over the stones to the water's edge, Penny suddenly stopped short.

"Listen!" she commanded.

"I didn't hear anything," Salt said.

"An automobile stopped by the roadside. I'm sure of it. Maybe it's my father!"

"He couldn't have reached here so soon."

"You don't know Dad," Penny chuckled. "He drives like the wind. It certainly sounded like the engine of our car."

"Let's have a look before we cut the boat loose," Salt said, slipping a knife back into his pocket.

"I'll go," Penny offered. "You wait here."

Before Salt could stop her, she darted away into the darkness. Crawling under a barbed wire fence, she took a short cut to the road. Even before she saw the car, she heard a voice which she recognized as her father's.

"Dad!" she called softly.

He was with another man whom Penny hoped was Major Bryan. As the two came toward the fence, she saw that it was indeed the Army officer.

"Dad, how did you get here so quickly?" she greeted him. "Salt and I didn't expect you for at least another twenty minutes."

"I was lucky enough to get hold of Major Bryan right away," Mr. Parker answered, climbing over the fence. "Now I hope you haven't brought us on a wild chase, Penny. What's up?"

"Come with me and I'll show you," she offered. "That's easier than explaining everything."

Major Bryan, a well-built man of early middle age, asked Penny several questions about Professor Bettenridge as the three walked hurriedly toward the lake.

"From your description, he seems to be the man I'm after," he declared grimly. "If he's the same person, his real name is Claude Arkwright and he's wanted for impersonating an officer and on various other charges. He pulled a big job in New York three months ago, then vanished."

Salt was waiting at the lake. "What's our move?" he asked, after relating everything that had occurred that night. "Shall we cut the boat loose?"

"First, let me examine those mines," the major requested. "Can we get into the shack?"

"I can pick the lock, but it takes time," Salt offered.

"We'll break it," the major decided. "Those men may return here at any minute, so there's no time to lose."

The door was forced open and Penny was placed on guard to watch the hillside for Webb or anyone in the professor's party.

There was no light in the shack, but both Mr. Parker and the major had brought flashlights. Salt pointed out the mine which had been doctored by Webb. Carefully, the Army officer examined it.

"I can't tell much by looking at it for the work has been cleverly concealed," he admitted. "But from what you've told me, I am quite certain how the mine is made to explode."

"How is it done?" Salt demanded.

"After the hole is made, a chemical--probably sodium--is inserted. Then another substance which melts slowly in water is used to seal up the opening."

"Then that explains why Webb delayed so long in giving the signal after the mine had been dropped into the water!" Penny exclaimed from the doorway. "He was waiting for the substance to melt!"

"Exactly," agreed the major. "If my theory is correct, only the action of water is required to explode this mine. The professor's machine, of course, has nothing whatsoever to do with it."

"Why don't we explode the mine now?" Penny suddenly proposed. "That would put an end to the professor's little scheme."

"It might also prevent us from arresting him," the major said. He debated a moment. Then he exclaimed: "It's worth trying! We'll load the mine on the boat and dump 'er in the lake!"

The men would not permit Penny to help with the dangerous work. Carefully they transported the mine to the boat. Salt was about to start the motor, but the major stopped him.

"No, we don't want the sound of the engine to give us away," he said. "We'll row out into the lake."

Penny was eager to accompany the men, but they would not hear of it. To her disappointment, she was compelled to remain on the beach.

Sitting down on the sand, she nervously watched until the boat faded into the black of the night. Presently, she heard a splash which told her that the mine had been lowered overboard. Anxiously, she waited for the boat to return.

"Why don't they come?" she thought, straining to hear the sound of oars. "If the explosion should go off while they're still out there--"

Then she heard the boat coming and breathed in relief. Soon the craft grated on sand, and the three men leaped out.

"Perhaps my theory is wrong," the major commented, as they all huddled together, waiting. "The mine should have gone off by this time."

Several minutes elapsed and still nothing happened. And then, as the group became convinced their plan had failed, there came a terrific explosion which sent flame and water high above the lake's surface.

CHAPTER 21 _IN SEARCH OF WEBB_

"Beautiful! Beautiful!" chuckled the major as the flames began to die away. "That proves our theory. No machine is required to set off the mines--only the action of water."

"Professor Bettenridge must have heard the explosion!" Penny exclaimed, fairly beside herself with excitement. "What will happen now?"

"If human nature runs true to form, he will soon come here to investigate," the major predicted.

The four stepped back into the dense growth of trees to wait. Within five minutes they observed two shadowy figures scurrying down the path toward the shack where the mines were stored. As they came closer, Penny recognized the professor and his wife.

"And someone is following them," she discerned. "It looks like Mr. Johnson."

Professor Bettenridge and his wife now were near the trees. Their voices, though low, carried to those in hiding.

"That stupid lout, Webb!" the professor muttered. "He has ruined everything now by setting off the mine too soon."

"But how could it have been Webb?" his wife protested. "He was at the farmhouse only five minutes ago. He wouldn't have had time."

"Then it was someone else--" Professor Bettenridge paused, and cast a quick alert glance about the lake shore. He noted that the boat was tied, but that the door of the shack was wide open.