"Who is there?" asked the professor, going close to the door.
"A friend," was the reply.
"Give the countersign."
"The North Pole, and long may it stand!" was the queer answer. It was the same the colored man had given when he sought admission after his second trip to the wreck that afternoon.
Slowly the inventor unfastened the door. As he cautiously opened it a roughly dressed man slipped in.
"What's the need of all this foolishness?" he demanded. "Why have you made it so dark? It's like a pocket. Is any one here?"
The two boys had crawled under the bed before the door was opened, in accordance with the instructions from the old man. The inventor and Washington were the only ones visible in the cabin.
"Why don't you turn up the light?" went on the visitor in fretful tones.
"Are you sure no one is here to learn our secret?"
"Do you see any one?" asked the professor, not wishing to disclose the boys' presence. "Do you think I am so foolish as to waste the labor and toil of years?"
"I didn't think so," said the man, "but as I came along I thought I saw lights in the balloon shed."