"Essence of spook," Scotty replied. "Spook essence does that to water.

Seriously, we poked in the bottom of the pool and found nothing."

"That doesn't mean there was nothing while the ghost was performing,"

Rick pointed out. "Only that no trace was left."

"You thinking about chemicals?" Scotty lengthened his stride toward the inviting lights of the Miller farmhouse. "And speaking of same, I need some for these mosquito bites."

"Chemicals can produce a mist," Rick pointed out, "without leaving a visible trace. We didn't taste the water in the pool. I'm going to take a sample tomorrow and see what I can find out."

The girls and the Millers were on the screened porch, waiting anxiously.

"No show," Rick called, antic.i.p.ating the questions from the four on the porch. "Not a sign of a spook. Only mosquitoes."

"I have something for those bites," Mrs. Miller replied quickly. "The mosquitoes are fierce this year. Come into the kitchen and we'll treat both of you."

Between applications of the aromatic ointment the boys reported on their experience, or lack of it. Rick concluded, "So the ghost performs only before an audience, and then only when notified in advance."

Dr. Miller smiled. "A pretty sweeping conclusion from a pretty small sample, Rick. One experiment doesn't do more than give a single point on the curve. You need more evidence than tonight's failure."

"We'll try again," Rick answered. He outlined the plan to let it be known that they would be watching.

"That will be added evidence, but not conclusive," the scientist warned.

"But you're on the right track, I'd say. Now, let's leave ghosts and go on to something more tangible. I have an interesting device made up of alternate black and red squares, on which various carved pieces, resembling royalty ..."

Rick held up a hand. "Say no more. I will be delighted to take you on for a game of chess."

Barby and Jan returned to their own project, creating monograms to be embroidered on their summer clothes, while Scotty and Mrs. Miller settled down with books.

Rick knew from the start that he was no match for Dr. Miller, but he resolved to give him as good a game as possible. An hour pa.s.sed before it was clear that Rick would be checkmated in two moves. He sighed.

"You've got me, sir. I guess ..."

The sentence was never completed. The quiet was abruptly shattered by the strident blasting of the plane's alarm system!

Rick and Scotty were on their feet and running on the instant. Rick reached the door first and threw it open, almost upsetting Belsely, the tenant farmer.

The man's eyes were wide, and his face was pale under the tan.

"It's the ghost!" he shouted. "It's him! In the field, by the plane!"

CHAPTER VI

The Dark Pit

The plane's klaxon horn wailed through the night with a noise audible for miles. The boys pushed past the tenant farmer and ran through the screen door on the porch. The plane was not yet in sight and it was very dark out. The moon was hidden by a bank of low-lying clouds, a precursor of rain.

Rick ran as fast as his long legs would carry him, which was fast enough to hold a track record or two at Whiteside High. Scotty, in spite of his greater weight, was not far behind.

At least one question was answered, Rick thought as he sped through the trees, ducking now and then as he caught a glimpse of a low branch. The ghost could set off an alarm system! He fumbled in his pocket to be sure that he had the keys to the plane, and wondered if he would be in time to keep the apparition from causing damage.

In the next instant he burst through the fringe of the orchard and broke stride as he saw a pale-blue light dancing in the air around the dark shadow of the Sky Wagon!

Scotty was right behind him. He, too, paused for an instant as he saw the light, then both boys were moving at their best speed again.

Rick tried to control his breathing. The spurt was taking its toll, but if he kept going he would get his second wind. He had to get to the plane! He wondered briefly if a supernatural being could do physical damage, then discarded the thought. He wasn't ready to accept that anything supernatural could trigger purely physical alarm systems!

The light seemed almost to have features as Rick drew closer, like a pale-blue jack-o'-lantern, but it was soon clear that this was no hollowed pumpkin head. It was like a human head illuminated from within by some ghastly luminescence.

"It's moving," Scotty called, his voice shaky. Rick saw at the same time that the apparition was retreating, slowly, away from the plane.

It kept the distance constant, always retreating as the boys neared.

Their own pace had slowed; the initial sprint couldn't be kept up.

Rick ran directly for the plane, jumped the low wire fence, and inserted his key in the door. He turned the key and the deafening blast of the horn cut off, leaving a deep silence. He turned the key back again, resetting the alarm system, then he jumped the fence once more. "Where is it?"

"There." Scotty pointed to the bank of the creek. The ghostly blue light was swaying, as though in invitation, but it was no longer retreating.

"What is it?" Rick asked. "It looks like a human head lighted from within. But it's too far in the air to be at head level, unless this Union bluecoat was seven feet tall."

Scotty replied with conviction. "It has to be someone carrying a light."

"Can you see anyone under it?"

"No, but that means nothing. The trees make a dark background. I thought I caught a glimpse of a body under it while we were running, but I can't be certain."

"There's one way to find out," Rick said, and was astonished to find that he didn't get cold chills at the thought. "Let's catch him!"

Scotty's reply was to take off in a racing start toward the blue light.

Rick had to stretch his legs to catch up, and saw the ghost begin its retreat again, always maintaining the distance between itself and the boys. It danced in the air like a will-o'-the-wisp, as though inviting the boys to hurry.

The pace was slower now, because the relatively smooth surface of the field had been left behind and the course led through bunch gra.s.s with an occasional clump of brambles. The ghost danced along the creek bank.

Whatever might be under the light was constantly invisible against the fringe of trees. Then it vanished among the trees for a moment, only to reappear.

Rick thought grimly that it was going to be a long chase. Once he stopped in his tracks and whispered to Scotty to do the same. Both listened, but there was no sound other than the normal night noises.

Rick knew their own pa.s.sage had been noisy, marked by the crunching of dry bunch gra.s.s, the crack of an occasional small twig of brush, and other sounds of hurrying feet, but the ghost moved with the silence of a--well, a ghost!

In spite of himself Rick felt a moment's chill, then he pressed his lips tightly together and hurried on. It was no ghost, he told himself. _It was no ghost!_ Someone was carrying a light, that was all. Ghosts do not carry lights.

The chase led into the trees, and onto rising ground. There were rocky outcroppings now, and Rick knew they had reached the foothills. The creek cut its way through the foothills for a short distance, then turned to follow an easier path on its way to the sea.

The underbrush was thicker now. This was typical Virginia second-growth forest, full of low brush and creepers. Rick knew it only by feel, however, because it was so dark he could only sense the presence of trees before crashing into them. The blue light vanished periodically behind trees, only to reappear again as though urging them on.

Then, as they broke into a denser thicket, the light vanished completely. Scotty muttered under his breath. Rick peered through the blackness eagerly, taking deep breaths. He had thought they were actually gaining for a moment.

He stood still, his chest heaving. Scotty stopped beside him. There was no sound. Even the night noises of the forest had ceased. There was a weird feeling of hollowness in the air, as though they stood in some great cavern. Rick whispered, "Where did it go?"