n," replied the negro. "He
mustn't expostulate sounds too freely 'cause it might keep us awake."

In a few minutes Washington returned.

"I made him as comfortableness as de existin' circumstanceableness would
permit ob," he announced.

"That's right. I did not want to do this, but I was forced to," the
inventor said. "I will release him as soon as we are ready to sail. But
I am forgetting the boys. Come out," he called, and Jack and Mark, much
mystified and somewhat frightened by what had taken place, crawled from
under the bed.

"I am sorry you witnessed what you did," the professor said to them.
"But I could not have this man spoil my plans. Some time ago he
discovered my secret, and to keep him from publishing it broadcast I was
forced to take him into my confidence. He has given me some aid in
getting rare chemicals, but he wants a heavy price. He demands a half
interest in the _Monarch_, and to be taken to the north pole."

"Then you are really going to search for the pole?" asked Jack.

"I am, my boy, and, what is more, I am going to find it. Why, it is
simple with the wonderful gas I have discovered. That is the whole
secret of what will be my success. It is easy enough to make an airship
that will move, but the trouble is no one has yet been able to make a
gas strong enough to lift the heavy weight of the ship high into the
air. That is where I have the advantage."

"I wish I could see your ship sail," said Jack.

"You may if you like," exclaimed the old man. "Do you remember what I
asked you when the knock interrupted us? I asked you if you wanted to go
to the north pole. Now I have taken a great liking to both you boys. I
haven't even asked your names yet, but I like you. I need some help in
running the ship, also in making my explorations in the frozen north.
Would you like to go along?"

For a few seconds the boys did not know what to say. It was a strange
and sudden proposition. They had been through so many adventures in the
last few hours that their brains were fairly bewildered. But to both of
them there came a great desire to make this wonderful trip through the
air. Before they could make a reply Professor Henderson spoke again:

"Perhaps you had better think it over a bit," he said. "I realize that
it comes rather suddenly. Supposing you go to bed, and we'll talk more
in the morning. Come, Washington, make up a couple of bunks for the boys
in this room. You can sleep in the balloon shed as