on and on sped the mermaid. now that the travelers felt their journey accomplished they were anxious to begin the homeward trip. they made a straight course for the village where they had so nearly met with disaster, and where the king of the giants had saved them. they went in a direct line, and did not travel here and there, as they had after they left the town. consequently they shortened the route by a great distance. yet it was long enough, and when they finally came in sight of the place the dial registered a trip of five thousand miles underground.

it was one evening when they landed almost at the spot whence they had taken flight eventually to reach the temple of the treasure. most of the giants had betaken themselves to their mound houses, but hankos was walking in the fields, and, when he caught sight of the airship hovering above him he waved his great sword in welcome.

he rushed up to shake hands with the travelers when they came out of the ship, though to greet him it was only possible for the adventurers to grasp one of his immense fingers.

as soon as the greetings were over hankos began to speak rapidly to the professor, at the same time going through many strange motions.

“it is as i feared!” suddenly exclaimed the scientist.

“what is the matter?” asked mark.

“the worst has happened!” went on mr. henderson. “the great hole by which we came into this place has been closed by an earthquake shock!”

“the hole closed?” repeated jack.

“an earthquake shock!” murmured mark.

“then how are we going to get back to earth?” asked old andy.

a terrible fear entered the hearts of the travelers. the closing of the opening by which they had come to the strange world meant, in all probability that they would have to spend the rest of their lives in this underground place.

“what good did it do us to get all those diamonds and that gold?” asked mark in a sorrowful tone.

hankos began to speak again, using his gestures which were almost as eloquent as words. the professor watched and listened intently. then there seemed to come a more hopeful look to his face. he nodded vigorously as hankos went on with what seemed to be an explanation.

“it’s worth trying, at all events!” the scientist exclaimed. “it is our only hope!”

“what is?” asked jack.

“friends,” began the professor in solemn tones. “i must admit our plight is desperate. at the same time there is a bare chance of our getting back to our own earth. as you remember, hankos went from this place to the upper regions through the upward spouting column of water.”

“if we had our submarine we might also,” interrupted jack. “but the mermaid isn’t built to sail in that fashion.”

“nor would the porpoise have served us in this emergency,” said the professor. “it would prove too heavy. but, nevertheless, i think i have a plan. now, mark, you are about to learn the secret of the storeroom. the real one, not the hiding of hankos in there, which you imagined to be the cause of my desire to keep something hidden. when we planned a trip to this underground world i had a dim idea that we might meet with trouble. so i planned and made a cylinder lifeboat.”

“a cylinder lifeboat?” repeated mark.

“yes,” replied mr. henderson. “i have it in the storeroom. i did not want any of you to see it for fear you would have faint hearts. i thought there might be no necessity of using it. but, since there is, we must do our best. i will admit it may be a fearful ordeal, but we will have to risk something in order to escape.

“i have in the storeroom a large cylinder, capable of holding us all. it will also contain food and drink for a month, but we will all have to go, packed almost like sardines in a box. my plan is to take the mermaid to the place where the column of water shoots up. there we will get into the cylinder, close it, and trust ourselves to the terrible force that may bring us back to the upper world. what do you say? shall we attempt it?”

for a few seconds no one spoke. then jack said slowly:

“i don’t see that we can do anything else. i don’t want to stay here all my life.”

“i wants a chance t’ wear some of them sparklers,” put in washington.

“then we will make the attempt,” the professor added. “now all aboard for the place where the water shoots up!”

questioning hankos, the professor learned how to reach the strange place. it was in the midst of a desolate country where none of the giants ever went, so afraid were they of the strange phenomenon.

it was a week’s journey. sometimes the mermaid flew through the air, and again it sailed on vast lakes or inland seas. on the trip they met with big waterfalls and terrible geysers that spouted a mile or more into the air. they traveled by night as well as day, though it was necessary to keep a sharp watch.

sometimes the ship passed through great flocks of birds that surrounded her and sought to pierce the aluminum hull with their sharp beaks and talons. over the mountains and valleys the ship sailed until, one evening, there sounded through the air a strange rumbling sound.

“it is thunder,” said old andy.

“it is the water column,” replied the scientist. “we are at the end of our trip. may the remainder be as successful!”

the ship was lowered to the surface, as it was deemed best to approach the column when the lights were shining. no one slept much that night, for the roaring and rumbling never ceased.

in the morning the ship was sent forward slowly. ever and ever the terrific sound increased, until it was almost deafening. they had to call to each other to be heard.

then, as the mermaid passed over a mountain, the adventurers saw, in a valley below them, the up-shooting water.

it was a vast column, nearly three hundred feet in thickness, and as solid and white as a shaft of marble. up, up, up, it went, until it was lost to sight, but there were no falling drops, and not even a spray came from the watery shafts.

“there is a terrible power to it,” the professor said. “may it prove our salvation!”

the ship was lowered about a hundred feet away from the waterspout. all around them the ground was vibrating with the force of the fluid.

“to think that connects with the world above!” exclaimed jack.

“it’s a good thing for us that it does,” mark answered.

“we must lose no time,” the professor put in. “if the earthquake destroyed the downward shaft, it may effect this one in time. we must escape while we can.”

then, for the first time, he opened the storeroom and the big cylinder was disclosed to view. it was made of aluminum, and shaped like an immense cigar. the hull was double, and it was strongly braced. inside were padded berths for the occupants, and there was just room enough for the seven adventurers. once they had entered they could not move about, but must stay in their little compartment.

compressed air in strong cylinders furnished a means of breathing, and there were tiny electric lights operated by a storage battery. there was also a chamber to be filled with the lifting gas. the cylinder was so arranged that it would float on it’s long axis if thrown into the water. a trap door hermetically sealed gave access to the interior. a small propellor, worked by compressed air, furnished motive power.

the food supply consisted of compressed capsules on which a man could subsist for several days. there was also some water, but not much, since that can not be compressed and would, therefore, take considerable room.

“the only thing for us to do,” said the professor, “is to get into the cylinder, seal it up, and trust to providence. this is what i intended to use when we were caught in the draught.”

“how can we get into the column of water after we shut ourselves into the cylinder?” asked mark.

“the cylinder fits into a sort of improvised cannon,” said mr. henderson. “it is fired by electricity and compressed air. we will aim it at the column, press the button and be projected into the midst of the water. then——” he did not finish the sentence, but the others knew what he meant.

“when are we to start?” asked mark.

“as soon as possible,” replied the professor. “i must arrange the cylinder, compress the air and lay out the food supply.”

it took the rest of the day to do this, as the inventor found it would be advisable to attach a weight to the end of the cylinder, to hold it upright in the column of water. the weight could be detached automatically when they were shot up into the midst of the ocean, where, as hankos had told them, the column spurted forth.

then some food was stored in the tiny ship that was destined to be their last hope, and some tanks of water were placed in it.

“i think we are almost ready,” mr. henderson said about noon the next day.

“what about our gold and diamonds?” asked jack suddenly. “can we take them with us in the cylinder?”

“that’s so!” exclaimed mr. henderson. “i forgot about them. i’m afraid we’ll have to leave the riches behind. we will not be able to carry them and the food we need, for it may be a week or more before we can leave the cylinder. gold and diamonds will be a poor substitute for something to eat.”

“i’m goin’ t’ take mine!” said washington with much conviction. “i might as well starve rich as starve poor!”

“we may be able to take a few diamonds,” the professor answered. “the gold will be too heavy. let each one select the largest of the diamonds he has and put them in his pockets.”

then began a sorting of the wealth. it was strange, as they recalled afterward, throwing away riches that would have made millionaires envious, but it had to be done. all the wealth in the world would not equal a beef capsule when they were starving, and they realized it. so they only saved a few pieces of gold as souvenirs, and took the best of the diamonds. but even then they had a vast fortune with them.

at last all was in readiness. the cylinder had been placed in the tube from which it was to be shot gently forth by compressed air, so that it would fall into the upward spouting column of water. the charge of compressed air was put in and the electric wires arranged.

“are we all ready?” asked mr. henderson.

“i think so,” said jack, in what sounded like a whisper, but which was loud, only the noise of the water muffled it.

“then we had better enter the cylinder,” spoke the inventor. “take a last look at the flying mermaid, boys, for you will never see again the ship that has borne us many thousand miles. she served us well, and might again, but for the freak of nature that has placed us in this position.”

for the first time the adventurers realized that they must abandon the craft in which they had reached the new world. so it was with no little feeling of sadness that they climbed up the ladder that had been arranged and slid down into the cylinder. one by one they took their places in the padded berths arranged for them. it was a snug fit, for the professor knew if there was too much room he and the others might be so tossed about as to be killed.

mr. henderson was the last to enter. standing at the manhole he took a final look at his pet creation, the mermaid. through the opened windows the colored lights came, shifting here and there. outside the terrible column of water was roaring as if anxious to devour them.

“good-bye, mermaid!” said the professor softly.

then he closed down the manhole cover and tightened the screws that held it in place. he touched a button that turned on the electric lights and the interior of the cylinder was illuminated with a soft glow.

“are you all ready?” he asked.

“jest as much as i ever will be,” replied washington, who, as the crisis approached, seemed more light-hearted than any of the others.

“then here we go!” exclaimed mr. henderson.

his fingers touched the button that connected with the electric machine, which operated the compressed air.

there sounded a muffled report. then it seemed to those in the cylinder that the end of the world had come. they shot upward and outward, through the top of the conning tower which had been removed. the cylinder, launched straight at the column of water struck it squarely and, an instant later was caught in the grasp of the giant force and hurled toward the upper world.

up and up and up the mass of metal with its human freight went. now it was spinning like a top, again it shot toward the earth’s crust like an arrow from the archer’s bow.

it was moving with the velocity of a meteor, yet because of being surrounded with water, and traveling with the same velocity as the column, there was no friction. had there been, the heat generated would have melted the case in an instant.

for the first few seconds those in the cylinder were dazed by the sudden rush. then as it became greater and greater there came a curious dull feeling, and, one after another lost consciousness. the terror of the water column, and the frightful speed, had made them senseless.

it seemed like a month later, though, of course, it could have been only a few hours or a day at most when jack opened his eyes. he saw his companions, white and senseless all around him, and at first thought they were dead. then he saw mark looking at him, and washington asked:

“is any one livin’ 'sides me?”

“i am,” replied jack decidedly.

then, one after another they regained their senses. but they were in a strange daze, for they were being carried along like a shooting star, only, as they went at the same rate as did the element carrying them, they did not realize this.

“i think i’m hungry,” said bill, who had the best appetite of any of the travelers.

“you’ll find a beef capsule in the little compartment over your head,” spoke the professor.

bill was about to reach for it, when they were all startled by a sudden side motion of the cylinder. then came a violent shock, and a sound as of splashing water. next the cylinder seemed to be falling, and, a few minutes later to be shooting upward. following this there was another splash and the cylinder began to bob about like a cork on a mill pond.

“we have reached the sea! we are afloat on the ocean!” cried the professor.

hurriedly he disengaged himself from the straps that held him to his bunk. he pushed back the lever that opened the manhole. into the opening glowed the glorious sunlight, while to the occupants came the breath of salt air.

“hurrah!” cried jack. “we are safe at last!”

“safe at last!” the professor answered, and then they all gave a cheer.

for their cylinder, which might now be termed a boat, was floating on the great atlantic. the blue sky was overhead and the air of the sea fanned their cheeks.

they had shot up from the underground earth, in the column of water, had been tossed high into the air, had fallen back when the liquid shaft broke into spray, had descended into the ocean, gone down a hundred feet or more, and then had shot up like a cork to bob about the surface.

for a week they were afloat, and then they were picked up by a passing vessel, rather weak and very much cramped, but otherwise in good shape. they said nothing of their adventures, save to explain that they were experimenting in a new kind of boat. about a month later, for the ship that had rescued them was a slow sailer, they were back on the island whence that wonderful voyage was begun.

“well, we solved the mystery of the center of the earth,” remarked jack, one evening, when they were gathered in the old shack where so many wonderful adventures had been planned.

“yes, we did,” said mr. henderson. “and no one else is ever likely to go there.”

“why?”

“because the only way of getting there was destroyed by the earthquake, and no one could ever force his way down through that upward-shooting column of water.”

“that’s so. well, we have the diamonds, anyway,” spoke mark. “they ought to make us rich.”

and the jewels did, for the stones proved to be of great value, even though the adventurers had saved only a few of the many they found in the ruined temple.

but there was money enough so that they all could live in comfort the rest of their lives. as the professor was getting quite old, and incapable of making any more wonderful inventions, he closed up his workshop and settled down to a quiet life. as for washington, andy, and bill and tom, they invested their money received from the sale of the diamonds in different business ventures, and each one did well.

“i am going in for a good education,” said jack to mark.

“just what i am going to do,” answered his chum. “and after we’ve got that——” he paused suggestively.

“we’ll go in for inventing airships, or something like that, eh?”

“yes. we’ve learned a great deal from mr. henderson, and in the course of time we ought to be able to turn out something even more wonderful than the electric monarch, the porpoise, or the flying mermaid.”

“yes, and when we’ve invented something better——”

“we’ll take another trip.”

“right you are!”

and then the two chums shook hands warmly; and here we will say good-bye.