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.