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 quantity 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, circumstantial evidence is permitted in court. Speculation
can give us the circumstances 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 assumption that the ghost is man-made. On this
assumption, 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-buttons for a
marker, then he and Scotty walked in ever-widening circles, scanning the
ground for any trace of the ghosts.

Scotty's k