"Let's get away! I can smell them burning," she shuddered and covered her face with her hands.
Brian grabbed her and shook her.
"Snap out of it," he barked. "That's no worse than shooting helpless men in parachutes. We can't go, yet; we're not finished here."
"Oh, let them shoot us! I can't go through that again!"
"You don't have to. Wait here."
He climbed back into one of the ships and cut the richness of the fuel mixture down till the exhaust was a lambent, shuddering stutter, verging on extinction. He dashed to the other ship and repeated the maneuver, fussing with the throttle till he had the fuel mixture adjusted to critical fineness. The beat of the stuttering exhaust seemed to catch up to the other and built to an aching pulsation. In a moment the whole mass of air in the cavern hit the frequency with a subtle, intangible thunder of vibration.
Crystal screamed. "Brian! There's more police cutting in around the entrance."
Brian clambered out of the ship and glanced at the glowing points in the rock where the police were cutting their way through outside the line of the exhaust flames. The pulsating thunder in the cavern crescendoed to an intolerable pitch. A huge mass of stalactites crashed to the floor.
"It's time to check out," Brian shouted.
Crystal led the way as they fled down the escape tunnel. The roaring crash of falling rock was a continuous, increasing avalanche of sound in the cavern behind them.