“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, “but as I came along I thought I saw lights in the balloon shed.”

“Very likely,” admitted Mr. Henderson coolly.  “Washington and I were out there doing some work.”

“All right,” was the rather ungracious answer.  “I have those chemicals you wanted.”

“Give them to me!” implored the old man in an anxious tone.  “I thought you would never bring them.”

“Oh, I don’t forget so easily.  Here you are,” and the newcomer passed over a package.  “Now when are you going to sail?”