"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.