"Quick, Washington! Put out the lights! You boys creep back and hide
under the bed. My secret must not be discovered now when everything is
ready for the trial!"

The boys started back toward the living room, Washington began putting
out the lights and then, with the professor, joined the boys. The shed
containing the airship was in total darkness, and the negro, turning
down the lamp in the cabin, shrouded that in gloom also.

Once more the knock was repeated. It was a peculiar one; first two raps,
then a silence, then three blows, followed at intervals by six single
raps.

"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