As Penny knelt beside Salt, he stirred slightly and raised a hand to his head. She saw then that there was a tiny break in the skin which also was slightly discolored. Either the photographer had fallen or he had been slugged.

Before she could go for help, he sat up, staring at her in a bewildered manner. Penny assisted him to a chair, and dampening a handkerchief with water from the tap above the developer trays, applied it to his forehead.

“What happened?” she inquired anxiously when Salt seemed able to speak.

“Slugged,” he answered in disgust.

“By whom?”

“Don’t know. The fellow must have been in the darkroom when I came here to develop my films. Fact is, I thought I heard someone moving around. I stepped to the door to see, and bing! That’s the last I knew.”

“Has anything been taken, I wonder?” Switching on another light, Penny glanced over the room. The drawer of a filing cabinet where old films, and plates were kept, remained open.

“Someone may have been looking in there!” she commented. “Salt, whoever he is, he must be searching for a film he is afraid we’ll publish in the paper.”

“Maybe so,” Salt agreed, holding a hand to his throbbing forehead. “But I don’t know of any picture we have that would damage anyone.”

Penny stepped to the doorway of the darkroom. In the larger room, the skylight remained closed. It was impossible to tell if anyone had entered the building in that way.

Some distance down the hall was a seldom-used stairway which led to the roof. Finding a door opening into it, Penny climbed the steps to look about. The rooftop was deserted, but in the building directly across from the Star, a corridor window remained open.

“How easy it would be for a man to step out onto the roof from there,” she thought. “If the skylight or the stairway door were unlocked, he easily could enter the Star photography room without being seen.”

Across the way, in the adjoining building, a man stood at an office window, watching Penny curiously. Sun glared on the panes so his face was distorted. But from the location of the window, she felt certain it was Mr. Cordell.

After a moment, Penny turned and went back down the stairs. The exit at its base was barred by a door with a rusty key in the lock.

Passing through, Penny locked it, and slipped the key into her purse.

“That should stop our prowler for a few days,” she thought.

In the photography room again, she checked the skylight, and finding it locked, was convinced that this time the mysterious visitor had entered the building by means of the stairs. She knew the door was usually kept locked, but undoubtedly the janitor had been careless.

By this time Salt was feeling much better. While Penny waited, he explained to the editor why the photos would not be ready until morning, then declared he was ready to start for Blue Hole Lake.

“Do you really feel like going?” Penny asked dubiously.

“Sure thing,” the photographer insisted. “It takes more than a little tap on the head to put me out of running.”

Salt walked a trifle unsteadily as they went down the back stairs together, but once they were in the press car, he seemed his usual jovial self.

“Now tell me about that plan of yours for tonight,” Penny urged as they jounced along the country road.

“It’s not much of a plan,” the photographer confessed ruefully. “First, we’ve got to learn exactly what Webb does to those mines to make them explode. Then somehow we’ll have to undo the work to cause the demonstration to turn out a flop.”

“It sounds like a big order,” Penny sighed. “We’ll need plenty of luck to carry it out. Especially as we’re arriving rather late.”

Having no intention of announcing their presence, the pair drew up about a quarter of a mile from the lake, parking in a side road.

Shadows were casting long arms over the ground as they started hurriedly across the fields toward the beach. They had covered two thirds of the distance when Penny suddenly caught Salt’s arm, pointing toward the lake.

“Look!” she exclaimed. “There they are now!”

Out on the lake a barge-type boat was being steered toward the beach near the shack where Professor Bettenridge stored the mines. The watching couple recognized three persons aboard the craft, the professor, Mr. Johnson and Webb. The barge also bore a large mine, similar in type to those Penny had seen inside the shack.

“That must be the mine Mr. Johnson is supplying for the test tonight,” she whispered.

Hand in hand, Penny and Salt crept closer to the shore. The boat grated on the sand and Webb, with the professor helping him, carried the heavy mine toward the building.

“If the mine is to be exploded tonight, wouldn’t it be easier to leave it on the barge ready to drop into the lake?” Penny commented. “Webb and the professor must have a special reason for hauling it ashore.”

“I think you have something there,” Salt observed. “Obviously, they’re going to doctor it in some way. We’ll see what happens.”

Webb unlocked the door of the shack and the two men carried the mine inside. Creeping still closer to the building, Salt and Penny heard Mr. Johnson say:

“Just a minute. I see you have other mines stored here. How am I to be sure that the one exploded will be the mine I have provided?”

“You may mark it if you wish,” the professor replied. “In fact, we prefer that you do, so there can be no possible doubt in your mind. Take this pocket knife and scratch your initials on the covering of the mine. Then tonight, before it is dumped in the lake, you may check again to see there has been no substitution.”

“You understand, I don’t distrust you,” Mr. Johnson said, ill at ease. “But so much money is at stake—”

“I understand your attitude perfectly,” the professor replied. “Certainly you are entitled to take every precaution.”

A silence ensued, and Penny and Salt assumed that Mr. Johnson was scratching his initials on the mine.

“Now suppose we have dinner at the village inn,” the professor presently suggested. “Then we will have the demonstration.”

“Must we wait so long before setting off the mine?” Mr. Johnson inquired.

“Yes, village authorities gave permission for the test to be held at nine o’clock,” the professor explained. “My own preference would be to get it over immediately, but I dare not disobey their orders.”

Mr. Johnson made no reply, and a few minutes later, the three men walked away. No sooner had they disappeared up the lake than Penny and Salt came out of hiding from among the trees.

“You have to hand it to Professor Bettenridge,” commented the photographer with grudging praise. “He’s a smooth talker. I’ll bet a frosted cookie the test could be held at one time as well as another so far as the village authorities are concerned. He has a special reason for wanting it at nine o’clock.”

“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.”

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