mechanism of importance that Jack and Mark had ever completed, and they
had been quite confident, before the flying machine was shot from Mr.
Henderson's catapult, that it was as near perfect as an untried
aeroplane could be.

"Hang on, Mark!" yelled Jack, as the great machine soared and pitched
over the forest.

Her leaps were huge, and the shock each time she descended and rose
again threatened to shake the 'plane to bits. Mark swayed in his seat,
clutching first one lever and then another, while Professor Henderson
and Jack could only cling with both hands to the guys and stay-wires.

The sensation of being so high above the earth, and in imminent danger
of being dashed headlong to it, gripped Mark Sampson like a giant
hand. He felt difficulty in breathing, although it was not the height
that gave him that choking sensation. There was a mist before his eyes,
still the sun was shining brightly. The startling gyrations of the
flying machine for some time shook the lad to the core.

But Jack's cheerful cry of "Hang on!" spurred Mark to a new activity--an
activity of hand as well as brain. He knew that something had fouled
and that this accident was the cause of the machine making such
sickening bounds in the air. She was overbalanced in some way.

With Jack's encouraging shout ringing in his ears, Mark came to himself.
He _would_ hang on! His friends depended upon him to control the machine
and to save them from destruction, and he would not be found wanting.

One lever after another he gripped and tried. It was one controlling