screech rose from the darkness behind them. Higher and higher in pitch the note rose until it ended suddenly in a gurgling grunt, as though the breath which uttered it had been suddenly cut off. The slithering, rustling noise became louder on their trail.
"Faster!" gasped the doctor, as he put his hand on Carries' shoulder and pushed him forward.
THE noise of pursuit gained slightly on them, and a sound as of intense breathing became audible. Dr. Bird paused and turned and faced the oncoming horror. His electric torch revealed nothing, but he listened for a moment, and then threw his second grenade. Keenly he watched its flight. It flew through the air for thirty yards and then struck an invisible obstruction and bounded toward the ground. Before it struck the downward motion ceased, and it rose in the air. As it rose it burst with a sharp report, and a wild scream of pain filled the cavern with a deafening roar. The doctor fled again after his companions.
By the time he overtook them the entrance of the cave loomed before them. With sobs of relief they burst out into the open. The guards sprang forward with raised rifles, but Dr. Bird waved them back.
"There's nothing after us, men," he panted. "We got chased a little way, but I tossed our pursuer a handful of phosphorus and it must have burned his fingers a little, judging from the racket he made. At any rate, it stopped the pursuit."
The major hurried up.
"Did you see it, Doctor?" he asked.
"No, I didn't. No one has ever seen it or anything like it. I heard it and, from its voice, I think it has a bad cold. At least, it sounded hoarse, so I gave is a little white phosphorus to make a poultice for its throat, but I didn't get a glimpse of it."
"For God's sake, Doctor, what is it?"
"I can't tell you yet, Major. So far as I can tell, it is something new to science and I am not sure just what it looks like. However, I hope to be able to show it to you shortly. Is there a telegraph office here?"
"No, but we have a Signal Corps detachment with us, and they have a portable radio set which will put us in touch with the army net."
"Good! Can you place a tent at my disposal?"
"Certainly, Doctor."
"All right, I'll go there, and I would appreciate it if you would send the radio operator to me. I want to send a message to the Bureau of Standards to forward me some apparatus which I need."
"I'll attend to it, Doctor. Have you any special advice to give me about the guarding?"
"Yes. Have you, or can you get, any live stock?"
"Live stock?"
"Yes. Cattle preferred, although hogs or sheep will do at a pinch. Sheep will do quite well."
"I'll see what I can do, Doctor."
"Get them by all means, if it is possible to do so. Don't worry about paying for them: secret service funds are not subject to the same audit that army funds get. If you can locate them, drive a couple of cattle or half a dozen sheep well into the cave and tether them there. If you don't get them, have your sentries posted well away from the cave mouth, and if any disturbance occurs during the night, tell them to break and run. I hope it won't come out, but I can't tell."
A HERD of cattle was soon located and two of the beasts driven into the cave. Two hours later a series of horrible screams and bellowings were heard in the cave. Following their orders the sentries abandoned their posts and scattered, but the noise came no nearer the mouth, and in a few minutes silence again reigned.
"I hope that will be all that will be needed for a couple of days," said the doctor to the commanding officer, "but
