"Oh," said Perry, weakly.

"But it does move," said Innes, encouragingly, "and the fault is easily remedied."

There being no such thing as time in Pellucidar, no, one cared how long it took to effect a change in the propeller. Everyone except Perry and a couple of his mechanics lay down in the shade, under trees or under the plane until Perry announced that the propeller had been reversed.

Innes took his place in the cockpit, a mechanic spun the prop, the engine started, the blocks were yanked away. The engine roared and pounded and leaped. The plane almost jumped from the ground in harmony with the vibration. Innes was thrown about so violently in the cockpit that he could scarcely find the controls or keep his hands and feet on them.

Suddenly the plane started forward. It gained momentum. It rushed down the long, level stretch that Perry had selected on which to build his hangar. Innes struggled with the controls, but the thing wouldn't rise. It bounced about like a ship in a heavy sea until Innes was dizzy; and then, suddenly the fabric burst into flame.

Dave Innes discovered the flames as he was nearing the end of the runway. He shut off the motor, applied the brakes, and jumped. A moment later the gas tank burst, and Abner Perry's latest invention went up in smoke.

II

EVEN THOUGH Abner Perry's first gun powder would not burn, his aeroplane would not leave the ground, and his first ship turned bottomside up when it was launched, nevertheless he had achieved a great deal since Fate and the Iron Mole had deposited him at the center of the Earth.

He had discovered ores and smelted them; he had manufactured steel; he had made cement and produced a very good grade of concrete. He had discovered oil in Sari and refined it to produce gasoline; he had manufactured small arms and cannon. He had found and mined gold, silver, platinum, lead, and other metals. He was probably the busiest man in a whole world and the most useful. The great trouble was that the men of the Stone Age, or at least most of them, were not far enough advanced to appreciate what Perry had done and could do for them.