“It is getting colder!” exclaimed the inventor as he glanced at an instrument near him.  “It is fifteen below zero now!”

In truth the Monarch was far to the north.  She had gone faster than the inventor calculated.  A glance downward showed that all traces of civilization had been left behind.

There was nothing to be seen but snow and ice, ice and snow, piled in fantastic heaps,—­mountains, ridges, hills and valleys.

The professor hastily made a few calculations.

“I believe we are somewhere over Greenland or Baffin Bay, but whether we are over the land or sea I cannot tell.  At any rate we are still going north,” and he glanced at the compass.

They were about to retrace their steps to the dining cabin, when there was a sudden settling of the Monarch.  It seemed to be plunging downward.

“What’s the matter?” cried Jack.

The inventor hurried to the engine room.  A glance at the registering needle of the instrument for telling the height attained, showed that the ship was sinking fifty feet a minute.

“Some conglomerous contraption has disproportionated herself,” cried Washington.  “What shall I do, Perfessor?”

“Start the gas generator at full speed!” cried the inventor.  “Heat the vapor before it goes to the bag!  The cold has contracted the gas in the holder above so that it will no longer support us!  Work quick, Washington!”

Washington sprang to set the gas machine in operation.  He seemed to be having trouble with it.

“She won’t work!” he called.  “She’s busted!”

Faster and faster the airship continued to sink.  The inventor hurried to Washington’s help, but it seemed that nothing could be done.  On board the Monarch there was deadly fear in every heart.

“I can’t keep her afloat!” the professor groaned.

Down and down went the craft.  The inventor and Washington were working furiously.  The boys, old Andy and Tom and Bill hurried to the engine room.

Then came a sudden jolt.  The airship had struck the ice!

“Shut off the engines!” cried the professor.  “Stop everything or we’ll go to smash!  We must set to work to repair the gas machine and raise the ship.”

The Monarch had settled down on a vast ice plane.  So gently had the ship sunk through the air that she had suffered no injury.  She rested on an even keel and there was still enough lifting power in the gas contained in the bag to keep that afloat, so that the vapor holder tugged gently at the confining meshes of the net.

“Ma goodness sakes alive!” cried Washington as soon as he had poked his head out of the warm engine room.  “De atmospheric conditions am such dat dey is conducive to de utmost congestion of mah circulatory systemation!”

“I suppose you mean it is too cold for your blood,” spoke the inventor, with a smile.

“Yo’ has conducted mah meanin’ to de utmost circumspection, Perfessor,” was the answer.