"Yes, Doctor, about thirty yards from the cabin and hidden so well that you could pass it a dozen times a day without suspecting its existence. The gas masks and other equipment which you sent to Fort Banks are in it."
"In that case we had better dispense with your company as Boon as we have 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 Carries 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," be 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 flashlight 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 it 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 my attention to it at once. I'll watch for the first hour."
AT the end of an hour Dr. Bird removed his mask with a sigh of relief and Carnes took his place at the spectroscope. For half an hour he moved the glass about and then spoke in a guarded tone.
"I don't see any of the lines you told me to look for," he said, "but in the southwest I get wide band at 310 and two lines at about 520."
Dr. Bird advanced toward the instrument but before he reached it, Carnes gave an exclamation.
"There they are, Doctor!" he cried.
Dr. Bird sniffed the air. A faint sweetish odor became apparent and he reached for his gas mask. Slowly hs hands drooped and Carnes grasped him and drew the mask over his face. Dr. Bird rallied slightly and feebly drew a bottle from his pocket and sniffed it. In another instant he was shouldering Carnes aside and staring through the spectroscope. Carnes watched him for an instant and then a low whirring noise attracted his attention and he looked up. Silently he
