resounded a hissing noise that told it was being forced through the
pipe into the aluminum shell above the ship proper.

"I wonder how soon it will begin to lift us," said Mark.

"It will take about half an hour," replied Mr. Henderson. "You see we
have first to fill the holder completely, since there is no gas in it.
After this we will keep some on hand, so that it will only need the
addition of a small quantity to enable the ship to rise."

He was busy watching the pointer on a dial which indicated the
pressure of the gas, and the lifting force. The boys were kept busy
making adjustments to the machinery and oiling bearings.

Suddenly, throughout the length of the craft there was felt a curious
trembling. It was as though the screw of a powerful steamer was
revolving in the water.

"What is it?" asked Jack.

"I hope it is the lifting power of the gas making itself felt," the
professor answered. "Perhaps the Flying Mermaid is getting ready to
try her wings."

The trembling became more pronounced. The gas was being generated
faster than ever. The whole ship was trembling. Tom and Bill came from
the room, where they were stationed, to inquire the meaning, but were
reassured by the professor.

"Don't be alarmed if you find yourselves up in the air pretty soon,"