A mechanic took his place at the propeller. It was so far off the ground that he had to stand on a specially constructed ladder. A man on either side stood ready to pull the blocks from beneath the wheels.

"Contact!" shouted Perry.

"Contact!" replied Innes.

The man at the propeller gave it a turn. The engine spluttered and died. "By golly!" exclaimed Innes. "It really fired. Try it again."

"Give her more throttle," said Perry.

The mechanic spun her again, and this time the engine took hold. The mechanic leaped from the ladder and dragged it away. David opened the throttle a little wider, and the engine almost leaped from its seat. It sounded as though a hundred men were building a hundred boilers simultaneously.

David shouted to the two men to pull the blocks, but no one could hear him above the din of the motor. He waved and pointed and signalled, and finally Perry grasped what he wanted, and had the blocks withdrawn. Everyone stood in wide-eyed silence as David opened the throttle wider. The engine raced. The plane moved! But it moved backward! It swung around and nearly crashed into the crowd of Sarians before Innes could cut the motor.

Perry approached, scratching his head. "What in the world did you do, David," he asked, "to make an aeroplane back up?"

Dave Innes laughed.

"What are you laughing at?" demanded Perry. "Don't you realize that we may have stumbled upon something sensational in aerodynamics? Just think of a fighter plane that could go either forward or backward! just think of how it could dodge enemy planes! Think of its maneuverability! What did you do, David?"

"The honor is wholly yours, Abner," replied Innes. "You did it."

"But how did I do it?"

"You've reversed the pitch of your propeller blades. The plane cannot go in any other direction than backward."