Monarch," the cars were wrecked near where Professor Henderson was
building his strange craft.

The boys were cared for by the scientist, and, after their recovery
from hurts received in the collision, they accepted his invitation to
make the trip through the upper regions in the airship, to search for
the north pole. With them went Andy Sudds, an old hunter, and Tom
Smith and Bill Jones, two farmers, but who were hired as helpers on
the voyage. The party had many adventures on the trip, having battles
with savage animals and more savage Esquimaux, and were tossed about
in terrible storms. After making some scientific observations, which
the professor was much interested in, they started back home.

Having found he could successfully sail in the air, Mr. Henderson
resolved to try what it might be like under water.

He moved his machine shop to a lonely spot on the Maine coast, and
there, with the help of the boys, Washington, Andy and two machinists
constructed a submarine boat, called the Porpoise.

In this the professor resolved to seek the south pole, he having a
theory that it was surrounded by an open sea. After much hard work the
Porpoise was made ready for the voyage.

What occurred on this great trip is described in the second book of
this series, called "Under the Ocean to the South Pole, or the Strange
Cruise of the Submarine Wonder." In that is told how once more Tom and
Bill, with Andy, the boys and Washington, accompanying Professor
Henderson, had many thrilling experiences.