it's our only hope," muttered Mr. Sharp. "We must go up, but the
trouble is the gas doesn't generate so fast when there's too much heat.
We're bound to have to stay over this fiery pit for some time yet."

"We're going up a little!" spoke Tom hopefully, as he glanced at a
gauge near him. "We're fifteen hundred feet now, and we were only
twelve a while ago."

"Good! Keep the elevation rudder as it is, and I'll see what I can do
with the gas," advised the balloonist. "It's our only hope," and he
hurried into the engine room, which, like the other parts of the cabin,
was now murky with choking vapor and soot.

Suddenly the elevation gauge showed that they were falling. The airship
was going down.

"What's the matter?" called Mr. Damon, from the cabin floor.

"I don't know," answered Tom, "unless the rudder has broken."

He peered through the haze. No, the big elevation rudder was still in
place, but it seemed to have no effect on the shim.

"It's a down draught!" cried Mr. Sharp. "We're being sucked down. It
won't last but a few seconds. I've been in 'em before."

He seemed to have guessed rightly, for, the next instant the airship
was shooting upward again, and relief came to the aeronauts, though it
was not much, for the heat was almost unbearable, and they had taken
off nearly all their clothing.

"Lighten ship!" sung out Mr. Sharp. "Toss over all the things you think
we can spare, Tom. Some of the cases of provisions--we can get more--if
we need 'em. We must rise, and the gas isn't generating fast enough!"

There was no need for the young inventor at the steering wheel now, for
the craft simply could not be guided. It was swirled about, now this
way, now that, by the currents of heated air. At times it would rise a
considerable distance, only to be pulled down again, and, just before
Tom began to toss overboard some boxes of food, it seemed that the end
had come, for the craft went down so low that the upward leaping
tongues of flame almost reached the lower frame.

"I'll help you," gasped Mr. Damon, and while he and Tom tossed from the
cabin windows some of their stores, Mr. Sharp was frantically
endeavoring to make the gas generate faster.

It was slow work, but with the lightening of the ship their situation
improved. Slowly, so slowly that it seemed an age, the elevation
pointer went higher and higher on the dial.

"Sixteen hundred feet!" sung out Tom, pausing for a look at the gauge.
"That's the be