the airship rose rapidly. washington had continued to operate the gas machine until there was a sufficient quantity of vapor to overcome the contracting influence of the cold atmosphere.

"forward once more!" cried the professor, hurrying to the engine room.

"this time we may reach the north pole!"

he and washington soon started the motors, the dynamos and engines. the propeller revolved rapidly. the adventurers were under way again.

"jack and mark, go into the conning tower and steer!" called mr. henderson from the engine room. "take her up about half a mile, and send her straight north by the compass. i have to adjust some of the machinery."

delighted at the prospect of running the airship, the two boys hurried forward. mark went to the steering wheel, which was similar to the kind used on automobiles. the monarch was heading to the west, having no one to guide her, but mark soon brought her around until her bow was poked directly for the north.

under the guidance of the two boys, the airship rushed forward. they had become somewhat used to the queer feeling of being high up in the air, and now it did not seem wonderful to be sailing among the clouds, though two weeks before they would have laughed at the idea of such a thing. andy and the two farmers had, likewise, become a little indifferent to the strange sensations, and, aside from being careful not to go too near the rail of the ship when it was sailing aloft, they took no more precautions than as if they were on the deck of a steamboat.

for several hours the ship was kept on her course. the boys remained in the conning tower, gazing ahead. not a single thing could be observed but a monotonous expanse of whiteness. now and then they would run into a bank of clouds which obscured their vision as if there was a heavy fog.

"look at the clock!" exclaimed mark suddenly, pointing to the time-piece.

"what's the matter with it?" asked jack.

"can it be right?" went on mark. "surely it isn't nine o'clock, and the sun shining as brightly as if it was noon."

"it's nine o'clock at night!" exclaimed the professor, entering the steering tower in time to hear mark's words.

"but it can't be," argued the boy. "look how the sun is shining."

"you must realize where you are," was the reply. "we are so far north, my boy, that we are in the land of the midnight sun. from now on we will have daylight all the while. we are nearing the pole, where it is light six months of the year, and dark the other six. we are having summer here, now."

"i guess it don't feel much like summer outside," said mark. "the thermometer indicates fifty below zero!"

"so it does," said amos henderson, glancing at the instrument which, though it was outside, could be read through the glass in the tower. "well, we may have struck a cold wave. ordinarily we will not have much more than twenty below zero when the sun shines."

"that's cold enough for me," said mark.

the professor announced that the airship's machinery was now in good shape. he said he expected to come to the end of the journey in about three days more, provided no accidents occurred, and there were no storms to delay the monarch.

"i think we will divide the night into four watches," he said. "washington, jack, mark and i will take them in turn. during the day we will all be on duty, but from six in the morning to six at night we will stand watch and watch."

it was arranged that jack should take the first period, the professor the second, mark the third and washington the fourth. as the first watch had passed jack was excused and the inventor said he would take charge of the ship. then, as every one was tired from the happenings of the day, they all went to bed, excepting amos henderson, who entered the tower to steer the ship.

the engines, dynamos and motors ran without much attention save such as the pilot might give them occasionally, for he could leave the ship with the steering wheel fastened, a few minutes at a time, as there was no danger of collisions. so the monarch continued to race toward the north.

it was almost time for mark's tour of duty to begin. the two boys, who were sleeping together, were in a deep slumber, when washington ran in and shouted at the top of his voice:

"wake up everybody! de perfessor is killed dead!"

andy, mark, jack and the two helpers sat up in their bunks, rubbing their sleepy eyes and wondering what had happened.

"what's the matter?" asked jack.

"mr. henderson is dead! he's in the engine room!"

"what killed him?" inquired andy.

"he must hab got a shock from de dynamo!"

andy jumped from his bunk and hurried to the engine room. there, as the negro had described, professor henderson was lying white and senseless on the floor.

the old hunter stooped over the inventor and felt of his heart.

"he is alive!" he exclaimed. "his heart beats! we must carry him to a bunk!"

aided by washington, andy carried the professor to the sleeping room, where he was made comfortable in one of the beds. the captain was so near death that he could not be seen to breathe, and only the faint flutter of his heart told that life existed.

"we're lost!" cried washington. "if he dies we'll never get back home again. he is de only one dat knows how to mix de chemicals for de gas!"

this was alarming news. without the mysterious vapor the ship would not keep afloat long, nor could it be worked back from the desolate land of ice and snow. how much of the gas was left no one knew.

"did he ever get a shock like this before?" asked the hunter of

washington.

"once, but not so strong."

"what did you do for him? what medicine did he take?"

"glory! glory!" shouted the negro, jumping up and down in his excitement. "i remembers it now. wait a second!"

he hurried to the engine room, and came back in a little while with a small bottle.

"perfessor done say," he began, "dat if eber i seed him senselike, when he done gone and got a shock from de 'lectrisititeness, i was to gib him two spoons full ob dis."

andy took the bottle, which contained a red liquid. bill got a spoon from the locker where the dishes were kept. with hands that trembled the old hunter poured out some of the fluid. then, with jack's help he forced open the inventor's mouth and put the medicine in.

"i hope it works!" murmured andy.

he poured out a second spoonful. this was administered to the unconscious man. in a few seconds his face that had been pale showed a little color. his chest expanded as he drew a long breath. then the old inventor opened his eyes and asked faintly:

"what happened? where am i?"

"you are all right now," spoke andy in a gentle voice. "you are out of danger i hope, and safe on the monarch."

"is the airship all right?" asked the captain eagerly.

"yes, and sailing along like a bird," was the answer. in fact every one had forgotten that the craft was forging ahead, and that all the machinery was working.

a look of relief crossed professor henderson's face, and he sat up.

"i remember now," he said. "i was adjusting the dynamo, and i touched a live wire. the current was very strong. it is a wonder i was not killed. but how did you bring me around again?"

"washington happened to remember some medicine you had told him to use in emergencies."

"oh, yes: i'm glad i had some on board. it is a remedy for those shocked with electricity. but i must see to the machinery."

"no, you must not," said andy firmly. "you are too weak to get up yet, and you have a bad burn on your hand."

"all right," agreed amos henderson, for he felt weak and sick from the shock. "some one had better see to the steering now," he added, and then he leaned back in the bunk and closed his eyes.

jack ran to the conning tower. he found that the ship, under the influence of a strong wind, was going due west, instead of to the north. he shifted the steering wheel and brought the monarch on her course again, pointing to the north pole. then he called for mark, and the two boys arranged that between them they would run the ship until professor henderson recovered.

andy and washington, who were watching beside the professor's bunk, where he reclined, seemingly in a deep slumber, were startled as he suddenly sat upright.

"hark!" the old man exclaimed. "listen! do you hear it?"

"hear what?" asked the hunter, in a soothing tone, wishing to humor the sick man.

"do you not hear a terrible rushing, roaring wind? the ship! the ship is in danger!"

the thrilling words sent a chill to the hearts of the watchers. there was no sign of a storm. in fact it was strangely quiet outside, the only noise heard being that of the engines of the ship.

"his mind wanders," said andy.

he had no sooner spoken than a cry from jack, who with mark was in the steering tower, startled them. his voice ran out through the cabins as he cried:

"a whirlwind! a whirlwind! we are running straight into a whirlwind!"