"She certainly am workin'" he observed with pride. "Are we still goin'
up, Perfessor?"

"Still mounting," replied Mr. Henderson, "We are now three hundred
feet above the earth," he added as he glanced at a registering gage.

The great air pump was set going and soon from the after tube, a big
stream of the compressed vapor rushed. It acted on the ship instantly
and sent the craft ahead at a rapid rate. By elevating or depressing
the tube the craft could be sent obliquely up or down. Then, by
forcing the air from the forward tube, the Mermaid was reversed and
scudded backward.

But it was more with the ship's ability to rise and descend that
Professor Henderson was concerned, since on that depended their
safety. So various tests were made, in generating the gas and using
the negative gravity apparatus.

All worked to perfection. Obeying the slightest turn of the wheels and
levers the Mermaid rose or fell. She stood still, suspended herself in
the air, or rushed backward and forward.

Of course the machinery was new and did not operate as smoothly as it
would later, but the professor and his friends were very well
satisfied.

"Now we'll try something new," said the scientist to the two boys as
they stood beside him in the tower. "I only hope this part succeeds,
and we shall soon be off on our voyage."