"Jack! Mark!" he exclaimed. "I am very glad you came so promptly. I have something very important to communicate to you--something that I hope will make up for the loss you suffer in being taken away from college in the middle of the term. Or, to be more correct, Mr. Roumann will impart most of the information, for it is at his suggestion that I sent for you." "Are these the young assistants of whom you spoke?" asked the other man, and the boys noticed that he was a big, burly German, with a bushy, gray beard, and penetrating, blue eyes. "This is Jack Darrow," said the professor, indicating the stout youth, "and the other is Mark Sampson. They have lived with me several years now, and we have had many adventures together." "Ha! Hum! Yes!" murmured Mr. Roumann, then he said something in German. "I beg your pardon," he went on quickly. "I have a habit of talking to myself in my own language once in a while. What I said was that I did not know the lads were so young. I am somewhat apprehensive--" "Do not be alarmed on the score of their youth," cried Professor Henderson. "I assure you that they have had a peculiar training, and, in some scientific attainments, they know as much as I do. You will not find them too young for our purpose, in case we decide that the thing can be done." "I tell you it can be done, and it shall be done," insisted Mr. Roumann. "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 earth. He began to think there might be exciting times for Mark and himself. "Mr. Roumann has proposed a wonderful plan to me," went on Professor Henderson. "It is nothing more nor less than a trip to--"