"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.
"I think we will divide the night into four watches," he said. "Washington, Jack, Mark and I will take them in turn. During the day we will all be on duty, but from six in the morning to six at night we will stand watch and watch."
It was arranged that Jack should take the first period, the professor the second, Mark the third and Washington the fourth. As the first watch had passed Jack was excused and the inventor said he would take charge of the ship. Then, as every one was tired from the happenings of the day, they all went to bed, excepting Amos Henderson, who entered the tower to steer the ship.
The engines, dynamos and motors ran without much attention save such as the pilot might give them occasionally, for he could leave the ship with the steering wheel fastened, a few minutes at a time, as there was no danger of collisions. So the Monarch continued to race toward the north.
It was almost time for Mark's tour of duty to begin. The two boys, who were sleeping together, were in a deep slumber, when Washington ran in and shouted at the top of his voice:
"Wake up everybody! De perfessor is killed dead!"
Andy, Mark, Jack and the two helpers sat up in their bunks, rubbing their sleepy eyes and wondering what had happened.