The scientist gave a last look at everything. The boys got behind some
boards whence they could see without being in danger. Washington, who
had little fear so long as there was no danger of going under ground,
took his place at the dynamo. Andy Sudds, with Bill and Tom, stationed
themselves in safe places.

"All ready!" called the professor.

He pulled a lever toward him, turned a wheel and signalled to
Washington to start the dynamo. There was a sound of buzzing
machinery, which was followed by a hiss as the gas began to enter the
cylinder under pressure. Would it stand the strain? That question was
uppermost in every one's mind save the professor's. He only cared to
see the cylinder leave the ground, carrying the weight with it. That
would prove his long labors were crowned with success.

Faster and faster whirred the dynamo. The gas was being generated from
the air. The secret chemical made a hissing which could be heard for
some distance. The gage registered a heavy pressure. Anxiously the
professor watched the cylinder.

"There!" he exclaimed at length. "It has all the gas it can hold. Now
to see if it works!"

He disconnected the pipe leading from the generator. This left the
cylinder free. It seemed to tremble slightly. There appeared to be a
movement to the hundred pound weight which rested on the ground. It
was as if it was tugging to get loose.