Somewhat crestfallen at the outcome of his investigations, but realizing that the professor could do what he wanted to aboard his own ship, Mark went back to bed.  But he could not sleep.  All the rest of the night he was wondering whether Mr. Henderson had some strange creature hidden aboard the Mermaid.  He feared lest the old scientist’s mind might be affected and, in his wildness he had made some infernal machine that would, in time, blow the whole ship apart.

But tired nature asserted itself at last, and, weary with vain imaginings, Mark fell into a slumber.  The next morning he awoke with a start from a dream that he was being devoured by an immense water snake.

He said nothing to the others about his night’s adventure, for, as it transpired, no one else had been awakened by his investigations.  The professor did not refer to his conversation with Mark.

“There’s something queer going on aboard the ship this trip,” said Mark to himself.  “But I guess it’s none of my business.  Professor Henderson seems to know what he is doing and I guess I can trust him.”

He resolved to think no more of the strange noises and movements, and, for several nights thereafter he was not disturbed by them.

The weather, which, up to this time had been fair, took a sudden turn for the worse about the fourth day after Mark’s little night expedition.  One evening the sun sank in a mass of dull lead-colored clouds and a sharp wind sprang up.

“We’re going to have a storm,” said Mr. Henderson.  “It’s liable to be a bad one, too, from the way the barometer is falling.”

He looked at the glass, and scanned the various instruments that told how high up the Mermaid was and how fast she was traveling.

“We’re pretty high up in the air,” he said, “and scooting along at about fifty miles an hour.  We are going against the wind, too, but fortunately it is not blowing hard.”

At that moment there sounded from without a peculiar howling sound, as if a siren whistle was being blown.

“‘Pears like there’s goin’ t’ be a tumultuous demonstration of sub-maxiliary contortions in th’ empherial regions contiguous t’ th’ upper atmosphere!” exclaimed Washington, entering from the engine room into the conning tower.

“What’s the trouble?” asked Mr. Henderson.

“Terrible big black cloud chasin’ us from behind!” exclaimed the colored man.

Noting the alarm in Washington’s voice the professor glanced from the rear window.  What he saw caused him to exclaim: 

“It’s a cyclone!  We must drop down to avoid it!”

He sprang to a lever controlling the gas and yanked it toward him.  There was a shrill hissing sound, and a second later the Mermaid began to sink.  The boys watching the gages on the wall of the tower, saw that the craft was falling rapidly.

But, with a rush and roar, the terrible wind was upon them.  It caught the craft in its fearful grip and heeled it over as a ship careens to the ocean blast.