ve eaten a bite, and retire to it. On second thought, we will eat in
it. Carnes, we will go to our downy couches at once and leave our
substitutes in possession of the cabin. I trust, gentlemen, that
things come out all right and that you are in no danger."

Major Trowbridge shrugged his heavy shoulders.

"It is as the gods will," he said sententiously. "It is merely a
matter of duty to me, you know, and thank God, I have no family to
mourn if anything does go wrong. Neither has Corporal Askins."

"Well, good luck at any rate. Will you guide Carnes to the tent and
then return here and I'll join him?"

* * * * *

Huddled in the tiny concealed tent, Dr. Bird handed Carnes a haversack
on a web strap.

"This is a gas mask," he said. "Put it on your neck and keep it ready
for instant use. I have one on and one of us must wear a mask
continually while we are here. We'll change off every hour. If the gas
used is lethane, as I suspect, we should be able to detect it before
its gets too concentrated, but some other gas might be used and we
must take no chances. Now look here."

With the aid of a flash-light he showed Carnes a piece of apparatus
which had been set up in the tent. It consisted of two telescopic
barrels, one fitted with an eye-piece and the other, which was at a
wide angle to the first, with an objective glass. Between the two was
a covered round disc from which projected a short tube fitted with a
protecting lens. This tube was parallel to the telescopic barrel
containing the objective lens.

"This is a new thing which I have developed and it is getting its
first practical test to-night," he said. "It is a gas detector. It
works on the principle of the spectroscope with modifications. From
this projector goes out a beam of invisible light and the reflections
are gathered and thrown through a prism of the eye-piece. While a
spectroscope requires that the substance which it examines be
incandescent and throw out visible light rays in order to show the
typical spectral lines, this device catches the invisible ultra-violet
on a fluorescent screen and analyzes it spectroscopically. Whoever has
the mask on must continually search the sky with it and look for the
three bright lines which characterize lethane, one at 230, one at 240
and the third at 670 on the illuminated scale. If you see any bright
lines in those regions or any other lines that are not continually
present, call m