again."

"Thanks, Bill," replied the doctor. "Well, Carnes, that seems to add
more weight to the theory that the spot of cold was made and didn't just
happen. If a prospecting party had just happened along they would either
have left the wreck alone or would have made some attempt to inter the
bodies. That cold belt must have been produced artificially by men who
planned to rob this plane after bringing it down and who were near at
hand to get their plunder. Is there any chance of following that
trail?"

"I doubt it, Doc. Walter and I scouted around quite a little, but we
couldn't pick it up again."

"Is there any power line passing within twenty miles of here?"

"None that Walter and I know of, Doc."

"Funny! Such a device as must have been used would need power and lots
of it for operation. Well, I'll try my luck. Carnes, help me unpack and
set up the rest of my apparatus."

* * * * *

With the aid of the operative, Dr. Bird unpacked two of the burros and
extracted from cases where they were carefully packed and padded some
elaborate electrical and optical apparatus. The first was a short
telescope of large diameter which he mounted on a base in such a manner
that it could be elevated or depressed and rotated in any direction. At
the focal point of the telescope was fastened a small knot of wire from
which one lead ran to the main piece of apparatus, which he sat on a
flat rock. The other lead from the wire knot ran into a sealed container
surrounded by a water bath under which a spirit lamp burned. From the
container another lead led to the main apparatus. This main piece
consisted of a series of wire coils mounted on a frame and attached to
the two leads. The doctor took from a padded case a tiny magnet
suspended on a piece of wire of exceedingly small diameter which he
fastened in place inside the coils. Cemented to the magnet was a tiny
mirror.

"What is that apparatus?" asked Carnes as the doctor finished his set-up
and surveyed it with satisfaction.

"Merely a thermocouple attached to a D'Arsonval galvanometer," replied
the doctor. "This large, squat telescope catches and concentrates on the
thermocouple and the galvanometer registers the temperature."

"You're out of my depth. What is a thermocouple?"

"A juncture of two wires made of dissimilar metals, in this case of
platinum and of platinum-iridium alloy. There is another similar
junction in this case, which is k