tor that needed eliminating before it
would feed properly.

"How about the tires?" asked Ned, when he had finished the wires.

"You might pump them up. There, the motor is all right. I'm going to
try it now, while you attend to the tires."

Ned had pumped up one of the rubber circlets of the small bicycle
wheels on which the aeroplane rested, and was beginning on the second,
when a noise like a battery of machine guns going off next to his ear
startled him so that he jumped, tripped over a stone and went down, the
air pump thumping him in the back.

"What in the world happened, Tom?" he yelled, for he had to use all his
lung power to be heard above that racket. "Did it explode?"

"Explode nothing!" shouted Tom. "That's the re-built motor in action."

"In action! I should say it was in action. Is it always going to roar
like that?"

Indeed the motor was roaring away, spitting fire and burnt gases from
the exhaust pipe, and enveloping the aeroplane in a whitish haze of
choking smoke.

No, I have the muffler cut out, and that's why she barks so. But she
runs easier that way, and I want to get her smoothed out a bit.

"Whew! That smoke!" gasped his chum. "Why don't you--whew--this is more
than I can stand," and holding his hands to his smarting eyes, Ned,
gasping and choking, staggered away to where the air was better.

"It is sort of thick," admitted Tom. "But that's only because she's
getting too much oil. She'll clear in a few minutes. Stick around and
we'll go up."

Despite the choking vapor, the young inventor stuck to his task of
regulating the motor, and in a short while the smoke became less, while
the big propeller blades whirled about more evenly. Then Tom adjusted
the muffler, and most of the noise stopped.

"Come on back, and finish pumping up the tires," he shouted to Ned.
"I'm going to stop her now, and then I'll give her the pressure test,
and we'll take a trip."

Having cleared his eyes of smoke, Ned came back to his task, and this
having been finished, Tom attached a heavy spring balance, or scales,
to the rope that held the airship back from moving when her propellers
were whirling about.

"How much pressure do you want?" asked Ned.

"I ought to get above twelve hundred With the way the motor is geared,
but I'll go up with ten. Watch the needle for me."

It may be explained that when aeroplanes are tested on the earth the
propellers are set in motion. This of course would send a craft