o earth
whenever it is necessary. As for clothing, I have an abundant supply."

He opened a locker and disclosed a pile of fur garments. There were big
coats, caps and boots, everything made with a furry surface within as
well as without.

"Any one would think you were going into some cold country, professor,"
said Jack, looking at the warm garments.

"So we are! We are going to find the north pole!" exclaimed the old
inventor.

"The north pole?" cried Mark.

"That's what I said. Do you boys want to go along in the _Monarch_ to a
place where never mortal man has been?"

At that instant there came a loud knock at the door.

CHAPTER V

A PLAN TO SEEK THE NORTH POLE

"Hark! What was that?" exclaimed Professor Henderson in a hoarse
whisper.

"Sounded like some one at the door," replied Mark.

"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
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, "bu