"In that case you should master the tricks of tank driving in short order. Get familiar with it and we'll appoint you as driver. We'll be ready to go in to-night, but I am going to wait a day. Our friend was fed last night, and there is less chance he'll be about."
THE early part of the next evening was marked by howls and screams coming from the mouth of the cave. As the night wore on the noises were quite evidently coming nearer and the sentries watched the cave mouth nervously, ready to bolt and scatter according to their orders at the first alarm. About two A. M. the doctor and Carnes climbed into the tank beside Lieutenant Leffingwell, and the machine moved slowly into the cave. A search-light on the front of the tank lighted the way for them and, attached to a frame which held it some distance ahead of them, was a luckless sheep.
"Keep your eye on the mutton, Carnes," cautioned the doctor. "As soon as anything happens to it, shut off the search-light and let me try to get a picture. As soon as I have made my exposures I'll tell you, and you can snap it on again. Lieutenant, when the picture is made, turn your tank and make for the entrance to the cave. If we are lucky, we'll get out."
Forward the tank crawled, the sheep bleating and trying to break loose from the bonds which held it. It was impossible to hear much over the roar of the motor, but presently Dr. Bird leaned forward, his eyes shining.
"I smell musk," he announced. "Get ready for action."
Even as he spoke the sheep was suddenly lifted into the air. It gave a final bleat of terror, and then its head was torn from its body.
"Quick, Carnes!" shouted the doctor.
The search-light went out, and Carnes and the lieutenant could hear the slide of the ultra-violet light which Dr. Bird was manipulating open. For two or three minutes the doctor worked with his apparatus.
"All right!" he cried suddenly. "Lights on and get out of here!"
Carnes snapped on the search-light and Lieutenant Leffingwell swung the tank around and headed for the cave mouth. For a few feet their progress was unhindered and then the tank ceased its forward motion, although the motor still roared and the track slid on the cave floor. Carnes watched with horror as one side of the tank bent slowly in toward him. There was a rending sound, and a portion of the heavy steel fabric was torn away. Dr. Bird bent over something on the floor of the tank. Presently he straightened up and threw a small object into the darkness. There was a flash of light, and bits of flaming phosphorus flew in every direction. The anchor which held the tank was suddenly loosed and the machine crawled forward at full speed, while a roar as of escaping air mingled with a bellowing shriek burdened the smoke-laden air.
"Faster!" cried the doctor, as he threw another grenade.
LIEUTENANT LEFFINGWELL got the last bit of speed possible out of the tank and they reached the cave mouth without further molestation.
"I had an idea that our friend wouldn't care to pass through a phosphorus screen," said Dr. Bird with a chuckle as he climbed out of the tank. "He must have been rather severely burned the other day, and once burned is usually twice shy. Where is Major Brown?"
The commanding officer stepped forward.
"Drive a couple of cattle into the cave, Major," directed Dr. Bird. "I want to fill that brute up and keep him quiet for a while. I'm going to develope my films."
Lieutenant Leffingwell and Carnes peered over the doctor's shoulders as he manipulated his films in a developing bath. Gradually vague lines and blotches made their appearance on one
