He and Washington soon started the motors, the dynamos and engines.  The propeller revolved rapidly.  The adventurers were under way again.

“Jack and Mark, go into the conning tower and steer!” called Mr. Henderson from the engine room.  “Take her up about half a mile, and send her straight north by the compass.  I have to adjust some of the machinery.”

Delighted at the prospect of running the airship, the two boys hurried forward.  Mark went to the steering wheel, which was similar to the kind used on automobiles.  The Monarch was heading to the west, having no one to guide her, but Mark soon brought her around until her bow was poked directly for the north.

Under the guidance of the two boys, the airship rushed forward.  They had become somewhat used to the queer feeling of being high up in the air, and now it did not seem wonderful to be sailing among the clouds, though two weeks before they would have laughed at the idea of such a thing.  Andy and the two farmers had, likewise, become a little indifferent to the strange sensations, and, aside from being careful not to go too near the rail of the ship when it was sailing aloft, they took no more precautions than as if they were on the deck of a steamboat.

For several hours the ship was kept on her course.  The boys remained in the conning tower, gazing ahead.  Not a single thing could be observed but a monotonous expanse of whiteness.  Now and then they would run into a bank of clouds which obscured their vision as if there was a heavy fog.

“Look at the clock!” exclaimed Mark suddenly, pointing to the time-piece.

“What’s the matter with it?” asked Jack.

“Can it be right?” went on Mark.  “Surely it isn’t nine o’clock, and the sun shining as brightly as if it was noon.”

“It’s nine o’clock at night!” exclaimed the professor, entering the steering tower in time to hear Mark’s words.

“But it can’t be,” argued the boy.  “Look how the sun is shining.”

“You must realize where you are,” was the reply.  “We are so far north, my boy, that we are in the land of the midnight sun.  From now on we will have daylight all the while.  We are nearing the pole, where it is light six months of the year, and dark the other six.  We are having summer here, now.”

“I guess it don’t feel much like summer outside,” said Mark.  “The thermometer indicates fifty below zero!”

“So it does,” said Amos Henderson, glancing at the instrument which, though it was outside, could be read through the glass in the tower.  “Well, we may have struck a cold wave.  Ordinarily we will not have much more than twenty below zero when the sun shines.”

“That’s cold enough for me,” said Mark.

The professor announced that the airship’s machinery was now in good shape.  He said he expected to come to the end of the journey in about three days more, provided no accidents occurred, and there were no storms to delay the Monarch.