"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?"

"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."

"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 usual."

in a few minutes the colored man had made rude but comfortable beds on two bunks, like shelves that folded against the wall. then, with an armful of bed clothes, he retired to the big shed.

"better use a little more liniment," advised the old man. "i don't want you sore and stiff if you go with me."

accordingly jack and mark rubbed their arms and legs well. something in the stuff must have been very soothing for they soon fell asleep.

it was broad day when the boys awoke. at first they could not realize where they were. they saw a colored man moving about and cooking something on the gasolene stove.

"did yo' gen'men obtain a sufficient percentage of restful slumberation?" he asked with a broad grin.

"we slept fine," said mark.

"washington, is breakfast ready?" asked mr. henderson, coming in from the balloon shed.

"it am prepared," was the reply.

"hello, boys! how did you sleep?" asked the inventor, observing that

mark and jack were awake.

"fine!" they said in a chorus and with a smile.

"well, wash up and we'll have something to eat. you'll find soap, water and towels out in the shed," and he pointed to where he had just come from.

the boys found two big tubs full of cool water. in an instant they had stripped and were splashing around like ducks. it was a treat to get a good bath. they came back into the cabin glowing. not even a reminder of the soreness and stiffness of the railroad accident remained. they did full justice to the meal of coffee and ham and eggs washington had prepared.

"now, washington, you had better take the prisoner something, and get your own breakfast," the professor said. "i want to have a talk with the boys."

whistling a merry tune, the colored man took out a tray of food to

taggert, who was still bound so he could not escape.

"now i'd like to hear your names, and all about you," the old man said.

the lads told their simple stories from the time each of them had started to shift for himself until they had accidentally met, and been hurled from the train.

"and have you thought over what i asked you last night?" asked the professor, when they had finished.

"i have," said jack, "and i'd like to go along."

"good! you shall go!" exclaimed the inventor. "how about you, mark?"

"i'll go, too."

"all right. now we have plenty to do," the old professor went on. "the actions of this man taggert will hasten my plans. there are a few finishing touches to put on the ship. come out into the shed."

delighted at the chance of helping about the mysterious monarch, the boys followed the professor. they found the shed lighted by windows in the roof, from which the curtains had been rolled back. the windows on the side were not opened.

by daylight the airship looked larger than before. it was a wonderful machine. the professor and his colored helper busied themselves in the engine room. now and then the two boys were allowed to aid.

as he hurried about from one part of the ship to the other the professor told them how he had come to build the monarch. he said he was an old bachelor and alone in the world, and had long desired to sail to the north pole. the failure of many land expeditions had convinced him that an airship was the only feasible method. accordingly he had come to this rather deserted part of the country, built his cabin and shed, and then had begun the putting together of his airship.

the engine parts, the various pieces of apparatus, and the machinery, he bought from many different sources, so as not to excite suspicion. at last after much labor the great undertaking was done.

"the monarch has never been tested," said the professor, "but i know it will sail. i have made many small models and they worked perfectly."

several busy hours were spent. much more machinery was put in the ship, the food lockers were stored with supplies, the gasolene tanks filled, and the supply of fur clothing increased.

"there!" exclaimed the professor at length. "we are about ready to sail. i could start in an hour if necessary. all i have to do is to fill the silk bag with my wonderful gas, which is all ready to generate."

"den you'd better start to generationess it right off quicker than sooner!" shouted washington, running from the rear of the shed. "hurry up, perfessor!"

"what's the matter?" asked mr. henderson anxiously.

"dat prisoner man has escaped!" cried washington. "he's clean gone!

flown away! jumped his bail!"

"that's bad!" exclaimed the professor. "he'll work some mischief now! i guess we'll have to start on our trip at once!"