“In about a week,” answered the inventor.

“Then I guess I’ll stay until you go,” spoke the stranger.  “I don’t want to be left behind.”

At this the old professor seemed strangely excited.  His hands trembled as he placed the chemicals on a shelf.

“You don’t like it, I see,” observed the stranger with a sort of snarl.  “But I know you too well, Professor Henderson.  You would be only too glad to go and leave me behind after all I have done for you.”

“My only desire, and you know it, James Taggert,” broke in the old man, “is to preserve my secret from the world until I see whether I can succeed or not.  I do not want to be laughed at if I fail.  I admit you have been of service to me, but, rather than risk failure, rather than run the chance of having my plans made known before I am ready to have them, I would do anything.  I know you too well to imagine that you have aided me from pure love.”

“Well, go on,” snarled the man, as the professor paused.

“You have some object back of it all,” continued the professor.  “I do not know what your motive is, but I say, rather than have my plans spoiled, I will make you a prisoner and keep you here until after I have sailed.  I am all ready to start,—­tonight, if need be!”

“So that’s your game, is it?” cried Taggert.  He turned toward the old man with an ugly look.

“Washington!” cried the professor.  “Bind him!  Put him in the little room and see that he does not escape!”

The next instant the big negro had folded his arms around Taggert.  The white man struggled, but he was like a baby in the grasp of a giant, for Washington was very powerful.  He procured a strong cord, and, before Taggert could resist had him firmly bound.  Then, picking the man up in his arms, Washington carried him back into the balloon shed.

“Help!  Help!” cried Taggert, and then his cries were smothered.

“Don’t hurt him!” cautioned the professor, calling into the darkness to Washington.

“I only guv him a soft piece ob wood to bite on,” 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.”