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,” he remarked with a smile.  “Remember the Electric Monarch, and the flights she took.  We may not go as high as we did in her, but it will answer the same purpose.”

The gas was hissing through the big tube as it rushed into the overhead holder.  The gage indicated a heavy pressure.  The ship began to tremble more violently and to sway slightly from side to side.

“I think we shall rise presently,” said Mr. Henderson.  His voice showed the pride he felt at the seeming success with which his invention was about to meet.

Suddenly, with a little jerk, as though some one with a giant hand had plucked the Flying Mermaid from the earth, the ship gave a little bound into the air, and was floating free.

“Here we go!” cried Mr. Henderson.  “The ship is a success.  Now we’re off for the hole in the earth!”

The Flying Mermaid was indeed rising in the air.  True it did not go up so swiftly as had the Monarch, but then it was a much heavier and stronger vessel, and flying was only one of its accomplishments.

“It’s a success!  It’s a success!” shouted Mark, capering about in his excitement.

“Now we’ll see what the centre of the earth looks like,” went on Jack.  “I can hardly wait for the time to come when we are to start on the voyage.”

At that instant, when the ship was but a few feet from the ground, but slowly rising, the boys and the professor heard a shouting below them.

“What’s that?” asked the scientist.  “Is any one hurt?”

Mark ran to a small window, something like a port hole in an ocean steamer, and looked out.

“Quick!” he shouted.  “Stop the ship!  Washington will be killed!”

In fact from the agonized yells which proceeded from somewhere under the craft it seemed that the accident was in process of happening.

“Save me!  Save me!” cried the colored man.  “I’m goin’ to fall!  Catch me, some one!”

“What is it?” asked the professor, making ready to shut off the power and let the ship settle back to earth, from which it had moved about fifty feet.