Scotty was probing into the box. "Batteries. Two of them, twelve volts each. And I'll bet the motor in the projector is designed to operate on twelve volts. There's even a hydrometer for testing the batteries."

Rick took a look. As Scotty had said, there were two automobile batteries, their cables running up into the projector.

"Simple enough," he commented. "Let's see what's on the film."

He opened the film gate gingerly and removed the film from the sprockets. Then, without disengaging the spindles, he put the flashlight behind it and bent close. The eight-millimeter frames were pretty small, but not so small that he and Scotty couldn't make out the image.

The scene had been shot against a black background, that was clear. Only the central figure was illuminated, the figure of a Union cavalry officer.

"Meet the Blue Ghost," Rick said happily.

"Delighted," Scotty said emphatically. "I suppose I shouldn't admit it, but deep down, way back in the primitive part of my thick head, I was sometimes guilty of wondering about this creature."

Rick held out his hand. "Shake hands with another superst.i.tious chucklehead. So was I. But let it be said to our credit that neither of us was so scared we were afraid to move."

He chuckled. "Of course there were times when I just had to keep my poor icy spine from freezing solid." He replaced the film on the sprockets and closed the gate with great care.

"The projector is aimed at the wall," Scotty pointed out, "right at the end of the tunnel. How does it get to where people can see it?"

"There has to be a way," Rick said. He swept the beam of his light around and it steadied on an iron pipe. "Hey, look!"

The pipe entered through the end of the tunnel, threaded into a right-angle pipe fitting, and disappeared into the tunnel floor!

"So that's how the water comes out of the hillside!" Rick exclaimed.

"The well was originally driven straight down, as a well should be, and the horizontal pipe was added later. It misses the lower tunnel by about six feet."

"That's not the only interesting thing about this end of the tunnel,"

Scotty added. "This whole end is artificial, including part of the roof over the well. Take a look. It's mortarless stonework. No wonder the face was so seamed on the outside. Whoever did this was a terrific mason, because he selected rocks--probably from the mine itself--that duplicated the contour of the hill. But why go to all the trouble?

That's what puzzles me."

"Maybe this is the reason," Rick said. He pointed to rusty iron projecting from the wall. The iron supported a block of stone, by means of an iron pin that ran from the bottom of the stone through a hole in the iron piece projecting from the wall. At the top of the stone was a similar arrangement. It was an elementary but effective hinge, long ago rusted to disuse.

Rick studied the wall, and directly in front of the projector lens he found another of the same arrangements, but with a difference. This one was modern, and it had been painted to prevent rusting. There were traces of graphite or graphite grease where the pins went through the iron supports.

Clearly, the block of stone supported by the iron pins formed a porthole, the pins allowing the stone to be swung inward. The old, rusted one had been unused for decades, but the port in front of the projector had been repaired and lubricated. The comparison between the two gave Rick his clue.

"This is a sniper nest built by the Confederates," he guessed. "Probably to protect the mine. The upper mine tunnel opened out here, too, and then war came and the people sealed the upper one to give protection to the troops working the lower level. That means the upper level was dug out first."

"It's speculation, but it sounds good," Scotty agreed. "These are gun ports, very likely. I don't know what other purpose they could have served."

Later they learned from Dr. Miller that the ports had also served as ventilation for slaves using the mine to hide on their way North to freedom, but that was after the North had the area partly in its grip.

They also found that from these same ports the Lansdale brothers had fired the shots that killed Captain Seth Costin, for the legend was almost entirely true.

"We open this port in front of the machine and we'll be only inches above the pool," Rick said. "Look at the location of the pipe. So, to produce the ghost, the Frostola man slid open the port, dropped a piece of dry ice...."

"All properly sized to give the right amount of mist for the right time," Scotty added.

"... and turned on the machine. With only the small port for the sound to go through, it wouldn't be audible to anyone in the picnic grounds."

Scotty agreed. "And since the projector is so close to the mist we wouldn't see a beam. That lens must have a mighty wide angle, by the way. What's more, the projector must be slid closer to the opening when in use."

"True. You know, in a way we were unlucky. If we had chanced to climb a tree when the ghost was actually appearing, we would have seen the projection lens through the mist as a bright spot of light, and that would have given the show away before this. But because of the angle, only someone in a tree could see it."

Rick shook his head in admiration. "Rear-screen projection with a wide-angle lens. That's really using movie technique for all it's worth."

"Rear-screen projection?" Scotty queried.

"Sure. Movies and TV use it all the time. When the hero is supposed to be watching dinosaurs fighting it out, he's actually standing in front of a big screen of special plastic or ground gla.s.s, with the picture projected on it from behind. The mist acted as the screen, so we saw the image but not the projector beam. That's rear-screen projection."

"I know how it works," Scotty said. "You can tell in a movie when they use it, because the definition of the background isn't as sharp as real photography, but I didn't know the name of the process."

Scotty turned and studied the location of the port. "He must place the projector right on the tunnel floor, tilted upward to shine through the port. That's why the ghost was so tall. It hit the mist at an angle."

Rick bent over the port. "Not hard to smack us in the eyes with methyl chloride from here, either. There we were, on our knees, faces in good range. And I'll bet he chuckled while he was doing it. Simple weapon, too. A water pistol. Or any plastic squirt bottle."

He tugged on the port and it failed to move. Something wrong here. He studied it carefully and saw the reason. It had to be slid sideways for a quarter of an inch, a safety-lock feature. No wonder their examination of the rock face outside had shown nothing.

"Open it," Scotty said. "Let's look."

Rick did so, and instantly closed it partly shut again. "Get down here and look," he commanded. He had seen at once what had happened during their absence and his quick mind had caused him to react.

There were men outside, several of them, and they were watching a small power scoop move into position in front of the lower mine entrance.

Among them were Dr. Miller and Belsely. Away from the group, sitting on his tricycle scooter, was the Frostola man!

"We forgot about Belsely," Rick said softly. "He saw us, and may even have seen us go into the mine. Anyway, that's the first place he'd look when we turned up missing."

Scotty drew back and closed the port gently. "That power scoop can go right into the tunnel, scoop up a yard of rock and back out and dump it.

It will have the tunnel cleared in no time. We'd better get out there and let them know we're safe."

"If they were breaking their backs with hard manual labor to get us out I'd yell through the port," Rick said gleefully. "But they aren't. So we'll let the scoop operate. It will remove that stuff in an hour. And when they open up, they'll find us."

Scotty looked at him suspiciously. "The tone of voice tells me you're whomping up something that will make someone unhappy. What is it?"

"Well, if we rush out and tell the world about this, everyone will know the ghost is a fake. But that won't help us much, because we'll still need to know the answer to the biggest question of all. Why do this? So we go back, use the time covering up the break between the tunnels so no one will suspect we know, and let ourselves be rescued. The ghost continues to operate, and so do we! Then, when we have the answer, I have a great idea for unmasking the ghost."

[Ill.u.s.tration]

Scotty saw the reasoning at once. "Besides," he added, "if the Frostola man doesn't see us come out, he'll know the jig is up right now. So let's go."

They checked carefully to be sure no trace of their presence remained, then hurried back to the lower tunnel. Working carefully, they fitted rocks into the opening until a casual survey by flashlight would not reveal that the block between the tunnels had been removed. Then they spent the remaining time clearing more rocks from the original rockfall that had sealed them in.

When the power scoop finally broke through, the glare of headlights, turned on when darkness fell, revealed two dirty, disheveled, exhausted young men who were too fatigued for anything but a quick bath, a meal, and bed.

CHAPTER XIX