"I have my doubts," went on Mr. Henderson.

Jack and Mark must have shown the wonder they felt at this talk
between the professor and his friend, for their guardian turned
to them and said:

"Boys, you must excuse me for not telling you at once the reason
why I sent for you. The truth is that Mr. Roumann has laid a
very strange proposition before me. It is so stupendous that I
hardly know whether to consider it or not. I want to talk with
you about it, and see what you think."

"They will go with us, will they not?" asked Mr. Roumann.

"That is for them to say," replied Mr. Henderson.

"Go where?" asked Jack, wondering if there was in prospect
another voyage to one of the Poles, or a trip to the interior of
the earth.

Professor Henderson looked at the other man. They were silent a
moment.

"Shall I tell them?" asked Mr. Henderson.

"Surely," assented Mr. Roumann. "It all depends on you and them
whether we go or remain on earth."

Jack started. Then there was a question of getting off the