And the next day the try-out came.  And such an ordeal as it was!  Gobs had surely never been put to a test like that in any navy-yard training station!  For five long hours they dived and rose and dived again.  They rose suddenly, rose slowly; they tipped, glided, shot through the water.  They passed for miles beneath the ice-floe, to emerge at last and bump a cake, or lift themselves toward a dark spot not larger than the sub itself—­a patch of open water in the midst of the floe.

With mind all in a whirl, Dave gave the final command to make for port.  It had been a great day.

That night, after “chow,” the Doctor called Dave into his room at the hospital.

“Young man,” he said, motioning the boy to a seat, “you and your crew have surprised me beyond belief.  I feel that we shall be risking little in attempting what, to many, might seem the most difficult task ever undertaken by a submarine.  I do not yet feel free to tell you what that trip will be; you’ll have to take that on faith.  I can only tell you that we will proceed from here directly to Nome, Alaska.  There we will get more oil and provisions.  We will then sail through Behring Strait due North.”

For a time the two sat in silence.  The Doctor’s face grew mellow, then sad at recollections of years that had gone.

“I don’t mind telling you,” he said after awhile, “that I am an explorer, you almost might say ‘by profession;’ that some years ago another explorer and I sought the same goal.  We went from different points; both claimed to have reached it.  But he got the honors.”

“And you really reached—­”

“Doesn’t matter now what I did in the past,” interrupted the Doctor quickly.  “What I am to do in the future is all that counts, and the immediate future is big with possibilities.”

“The crew will be with you to a man,” Dave assured him, as he rose to go.

As he stepped into the cool night air, Dave found that his face was hot with excitement.  There was left in his mind not one doubt as to their final destination:  it was to be a try for the Pole.  Only one thought saddened him; that his good friend, Blake, would not continue as one of the party.

Two days later they crossed over to the island of the illicit wireless station.  They found the apparatus in perfect condition, and the Doctor at once began sending messages.

“I’m letting the world know of our purpose,” he explained.  “At least, trying to.  Sending messages by code to a friend of mine in Chicago.  Hope Seattle will pick it up, and if not, perhaps that radical operator who is supposed to be relaying messages to Canada and the States from the north-central portion of the Continent will catch it, and, thinking it one of his own messages in a new code, pass it on.”

Had the doctor known what kind of radicals were in control of the station on Great Bear Lake at that moment, perhaps he would have been more careful what messages he sent.