"Not all ghosts are transparent," Barby insisted.

"She's got you." Dr. Miller chuckled.

Scotty spoke up. "Ghosts do not drive cars."

"And you've no proof the ghosts you saw in the field came from the car,"

Barby defended hotly. "Did you see them get in the car and drive away?"

Scotty held up his hands in surrender. "No. I pa.s.sed them on my way back from the car."

"Evidence not sufficient," Dr. Miller said with a grin. "The ghosts may or may not be human. Your first fact needs more proof, Rick. Carry on."

Rick sighed. "All right. I'll start over again. First, we have uncovered cement bags that contained radioactive ore, pulverized into a fine dust.

I'll amend that. The bags contain a small quant.i.ty of radioactive ore, which gives some reason for believing they were once full of such ore."

The group laughed. "Now you're on the beam," Dr. Miller approved. "State only what you know as fact and identify what you infer from the facts as inference or speculation."

"Glad you all approve. Second, we believe the Frostola man was interested in the cement bag Scotty carried. It is a fact that when we returned from town the cement bag that we put in the trash can, and the cement bags we left where we found them, had been removed. Because of the Frostola man's apparent interest, we are of the opinion he took the bags."

Jan Miller giggled. "You sound like a lawyer."

"I feel like one," Rick returned with a grin.

"Third, the Blue Ghost led Scotty and me on a wild chase that ended up with me dropping into the quarry. The facts are that the ghost somehow triggered the plane alarm. We will not argue whether or not a real ghost could have set off a purely physical, nonspiritual alarm."

Barby nodded soberly, but there was a twinkle in her blue eyes.

"Continuing with the facts of that incident, the ghost stayed ahead of us without difficulty. A real ghost could have done that, I suppose, but so could any person in reasonable physical shape who knew the terrain.

Now, the ghost's light went off as he reached the edge of the quarry, or somewhere in the vicinity of the edge."

Rick looked at his sister. "I will stipulate that a real ghost need not have any reason for his actions. But a person imitating a ghost would have had to turn off his light in order to go around the quarry, otherwise we would have seen that he made a detour. A ghost would presumably float right over the quarry."

"Ghosts do float," Barby agreed solemnly.

"Uh-uh. Since this one did not, and since it reappeared--or the light did--on the opposite side of the quarry, we believe there was a deliberate attempt to lead us into said quarry."

He paused and took a deep breath. "How am I doing, coach?"

Dr. Miller nodded approval. "Fine. See how easy it is to separate fact and conjecture?"

"So what do we conclude from this one event? We conclude it is reasonable to believe that a person, and not a spook, triggered the plane alarm and led us to the quarry. We speculate that the person did not know about the alarm and set it off by accident, probably while inspecting the plane, since we see nothing to be gained by sabotage. We speculate that the chase was to frighten us, not primarily to harm us, the reason being that we rushed the ghost during the ghost act and are therefore potentially dangerous. We reach this conclusion because the ghost picked a side of the quarry where we would land in the water, which is plenty deep by the way, and not on the rocks."

"Okay. Scotty, take over. I'm worn out from trying to be precise."

The scientist grinned. "Lack of practice, I'm afraid. If we all sought precision in our speech many of the world's misunderstandings could be avoided."

"I don't know what we can say," Scotty objected. "We have few facts. We have only some observations. We can try to interpret our observations, but we can't prove them. For instance, there is the fact that we were given a bath of something by the Blue Ghost that seemed to freeze our faces. There is the fact that the Frostola man bought a quant.i.ty of methyl chloride. There is the fact that methyl chloride could have produced the effect we felt. But how can we say that it's a fact that the Frostola man somehow doused us with chemical?"

"You can't," Jan Miller agreed.

"So if we stick to demonstrable facts, we don't get far," the scientist concluded. "But can we settle for mere speculation?"

"No, sir," Rick stated. "But let's admit that speculation has its uses.

After all, circ.u.mstantial evidence is permitted in court. Speculation can give us the circ.u.mstances that need to be proved, and that tells us where to put our efforts. I think that's fair enough."

"So do I," Dr. Miller agreed.

Rick arose. "Then we'll continue working the way we've been doing it.

It's not the best way, but at least we're uncovering little items that can be tied together if we find just two missing facts."

"Like what?" Barby demanded.

"We go back to our a.s.sumption that the ghost is man-made. On this a.s.sumption, the things we need to know are _how_ and _why_ is the ghost produced?"

CHAPTER XVI

Trapped!

It was, as Rick said, time for action and not for words. He and Scotty set out to track down every possible shred of evidence. They armed themselves with flashlights, and Rick made sure he had his pocket lens, and they started out.

The first stop was in the field, to locate the places where last night's ghostly party had paused.

As the boys walked across the field toward the plane, Rick wondered aloud. "What did the ghost want with the plane?"

"Sabotage?" Scotty asked.

"Maybe. But if so, why?"

"Because he was afraid of what we might see from the air, maybe."

Rick considered. "It could be, I suppose, but we've examined the whole area from the plane. I didn't see anything suspicious or particularly interesting."

"Not a thing," Scotty confirmed. "But it might be a good idea to take another look."

"Okay. We can do it later this afternoon. Now, according to what I remember, the first stop the ghosts made was right about here. Let's work like hunting dogs and see what we can turn up."

Rick dropped his handkerchief on a clump of bachelor's-b.u.t.tons for a marker, then he and Scotty walked in ever-widening circles, scanning the ground for any trace of the ghosts.

Scotty's keen eyes saw the first sign, a heelprint in a bare place in the gra.s.s. The boys examined it. "Doesn't match anyone's shoes," Scotty said. "Not of our gang. Leather heels, a little worn, run down on the outside edge. You can see the nail marks. No rubber heels would make those marks."

There were other prints, now that they were searching closely. Clearly, three men had walked the field last night. But nowhere did they find a clue to what the men had searched for. There was no raw dirt, no impressions left where something had been removed.

"Fact," Rick stated. "Three men were here."

Scotty laughed. "This does not mean there were not also three ghosts who left no tracks."