me size as the
searchlight aperture in the path of the beam and concentrate all the
light, and heat, at one spot, the focal point of the lens, the
temperature at that point is the same as the temperature of the source
of the light, less what has been lost by radiation. You understand that,
do you not?"

"Certainly."

"Suppose that we place at the center of the aperture of the searchlight
a small opaque disc which is permeable neither to heat nor light, in
such a manner as to interrupt the central portion of the beam. As a
result, the beam will go out in the form of a hollow rod, or pipe, of
heat and light with a dark, cold core. This core will have the
temperature of the surrounding air plus the small amount which has
radiated into it from the surrounding pipe. If we now pass this beam of
light through a lens in order to concentrate the beam, both the pipe of
heat and the cold core will focus. If we place a temperature measuring
device near the focus of the dark core, we will find that the
temperature is lower than the surrounding air. This means that we have
focused or concentrated cold."

"That sounds impossible. But I can offer no other criticism."

* * * * *

"Nevertheless, it is experimentally true. It is one of the facts which
lead me to consider cold as negative heat. However, this is true of
cold, as it is of the other negative forces; they exist and manifest
themselves only in the presence of the positive forces. No one has yet
concentrated cold except in the presence of heat, as I have outlined.
How this cold belt which the T. A. C. plane encountered came to be there
is another question. The thing which we have to determine is whether it
was caused by natural or artificial forces."

"Both of the packets which the plane carried are gone, Doctor," observed
Carnes.

"Yes, and that seems to add weight to the possibility that the cause was
artificial, but it is far from conclusive. The packets might not have
been on the men when the plane fell, or someone may have passed later
and taken them for safekeeping."

The doctor's remarks were interrupted by the guides.

"Someone has been here since the wreck, Doctor," said Bill. "Walter and
I found tracks where two men came up here and prowled around for some
time and then left by the way they came. They went off toward the
northwest, and we followed their trail for about forty rods and then
lost it. We weren't able to pick it up