With the aid of Jupe, the gas bag was inflated to a point where only a slight additional quantity of gas would cause the craft to shoot upward to the sky.  When all was ready a test of the instruments was made and they were found to be working perfectly.  The powerful alternator on the Wondership was, of course, worked by the same motor that drove the big propellers.

“Well, I guess there’s nothing to keep you back now,” said Mr. Chadwick, who looked pale and ill after his long days and nights of work on his invention.

“No, we’re as ready as we ever will be,” said Jack, making ready to climb into the machine above which the big yellow balloon bag was billowing and sending impatient quiverings through the Wondership.

“I want you to promise me one thing, dad,” said Jack, when he had climbed into the driver’s seat, in front of Tom, whose duty it was to look after the engine.

“What is that, my boy?” asked the inventor.

“That after this test, whatever the result may be, you will take a long rest.”

“Yes, I will, I must,” agreed his father.  “I’ve been working too hard, I guess, but in the excitement of perfecting the radio telephone I hardly noticed it.  But recently I’ve had dizzy spells.”

“Two weeks’ rest will make you well,” declared Jack, as he adjusted the controls.

“Good-by and good luck,” said his father.

Both boys waved their hands.

“All ready, Tom?” hailed Jack.

The other boy nodded and then turned on a valve so that with a hissing sound additional gas rushed into the bag.  Jack pulled a lever.  The big motors roared and a queer, sickly smell of burned gas filled the air.  The propellers began to revolve slowly and then increased their speed till they became a mere blur.

“Dere she go!  Gollyumption, dere she go!” cried Jupe, capering about.

As the old black spoke, the Wondership shot up like a rocket, tilting her nose slightly into the air.  But the next moment Jack had her on an even keel.  In an incredibly short space of time those watching below saw her only as a glinting, golden speck against the blue sky, circling like some strange bird far above their heads.

“Now for the tests,” said Mr. Chadwick, as he hastened to his workshop.

He set the big alternator at work at top speed.  It droned like a gaunt bee.  The inventor’s face, worn by his anxious vigils at his experiments, was as keen as a hawk’s, while he adjusted the instruments and placed his long, lean fingers on the tuning device.

Far above the earth Jack and Tom could look down upon a patchwork of villages, farms, green pastures, yellow grain fields and stretches of woodland.  They were too far up to distinguish figures, but they could see the white steam of rushing trains along the railroad tracks, and even catch the sound of the engines’ whistles.

Beyond glinted the blue of the sea flecked with sails and with here and there a steamer’s smoke smudging the horizon.  Both lads were in high spirits.  It seemed good to be navigating the air again.  Every now and then inquisitive, high-flying crows would swoop toward the machine and then dash off again with alarmed squawks.