and clouds caused by the water which poured down from the ocean with
the force of a Niagara.

Gazing down into the big hole Mark suggested it might lead to the
centre of the earth, which some scientists claim is hollow. The
professor admitted that the cavity looked as though it led to China.

They had no means of investigating further the mystery of the opening
and returned to their submarine, completing the voyage to the south
pole.

It was now about two years since they had come back from that eventful
trip. One of the first things the professor did, after docking the
Porpoise, was to shut himself up in his study and begin to draw plans.
To the questions of the boys he returned no answer for several days.
Then he announced he was working on a craft which could both sail on
top of the water and navigate the air.

In time the plans were done, and, in order to keep the work secret,
the shop was moved to an island which the professor owned.

Parts of the Monarch and the Porpoise were used in constructing the
new craft, so there was no need to get other help than that which the
boys, Washington and Bill and Tom could give, since the two latter
accepted an offer of the professor to remain and work for him. The
boys, of course, would not leave their friend.

The professor realized that he had a more difficult task in his new
venture than he had set himself on other occasions. For a ship to be
light enough to rise in the air, and, at another time, and with no