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Five Thousand Miles Underground
by Roy Rockwood
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Five Thousand Miles Underground

Or

The Mystery of the Centre of the Earth

by Roy Rockwood, 1908

CHAPTER I

WASHINGTON BACKS OUT

"WASHINGTON! I say Washington!"

Throughout a big shed, filled for the most part with huge pieces of machinery, echoed the voice of Professor Amos Henderson. He did not look up from a small engine over which he was bending.

"Washington! Where are you? Why don't you answer me?"

From somewhere underneath an immense pile of iron, steel and aluminum came the voice of a colored man.

"Yas sir, Perfesser, I'se goin' t' saggasiate my bodily presence in yo' contiguous proximity an' attend t' yo' immediate conglomerated prescriptions at th' predistined period. Yas, sir!"

"Well, Washington, if you had started when you began that long speech you would have been at least half way here by this time. Hurry up! Never mind tightning those bolts now. Find the boys. I need them to help me with this engine. They must be around somewhere."

"I seen 'em goin' fishin' down by th' brook a little while ago," answered the negro, crawling out from under what seemed to be a combined airship and watercraft. "Jack says as how yo' gived him permission t' occupy his indisputatious period of levity in endeavorin' t' extract from th' liquid element some specimens of swimmin' creatures."

"If you mean I said he and Mark could go fishing in the brook, you're right, Washington," replied the professor with a smile. "But you waste a lot of time and breath trying to say it. Why, don't you give up using big words?"

"I reckon I was brought up t' it," replied the colored man grinning from ear to ear. He did not always use big words but when he did they were generally the wrong ones. Sometimes, he spoke quite correctly.

"Well, I suppose you can't help it," resumed Mr. Henderson. "However, never mind that. Find the boys and send them to me."

"With th' least appreciatableness amount of postponement," answered the messenger, and he went out.

Washington White, who in color was just the opposite to his name, a general helper and companion to Professor Henderson, found Mark Sampson and Jack Darrow about a quarter of a mile from the big shed, which was in the center of a wooded island off the coast of Maine. The lads were seated on the bank of a small brook, fishing.

"Perfesser wants yo' immediate," said Washington.

"But we haven't caught a single fish," objected Mark.

"Them's the orders from headquarters," replied the colored man. "Yo' both got t' project yo'selves in th' vicinity of th' machine shop. I reckon th' new fangled contraption that th' perfesser is goin' t' navigate th' air an' sail th' angry seas in, am about done. He want's t' try th' engine."

"Come on then," said Jack. "We probably would not catch any fish, anyhow, Mark."

Accompanied by Washington, the youths, each of whom was about eighteen years old, started toward the big shed.

While they are on their way opportunity may be taken to tell a little about them, as well as about Washington and the professor, and the curious craft on which the scientist was working.

A few years before this story opens Mr. Henderson had invented a wonderful electric airship. He had it about completed when, one day, he and the two boys became unexpectedly acquainted, and, as it developed, friends.

Mark and Jack were orphans. After having rather a hard time knocking about the world trying to make a living, they chanced to meet, and resolved to cast their lots together. They boarded a freight train, and, as told in the first volume of this series, entitled, "Through the Air to the North Pole; or the Wonderful Cruise of the Electric Monarch," the cars were wrecked near where Professor Henderson was building his strange craft.

The boys were cared for by the scientist, and, after their recovery from hurts received in the collision, they accepted his invitation to make the trip through the upper regions in the airship, to search for the north pole. With them went Andy Sudds, an old hunter, and Tom Smith and Bill Jones, two farmers, but who were hired as helpers on the voyage. The party had many adventures on the trip, having battles with savage animals and more savage Esquimaux, and were tossed about in terrible storms. After making some scientific observations, which the professor was much interested in, they started back home.

Having found he could successfully sail in the air, Mr. Henderson resolved to try what it might be like under water.

He moved his machine shop to a lonely spot on the Maine coast, and there, with the help of the boys, Washington, Andy and two machinists constructed a submarine boat, called the Porpoise.

In this the professor resolved to seek the south pole, he having a theory that it was surrounded by an open sea. After much hard work the Porpoise was made ready for the voyage.

What occurred on this great trip is described in the second book of this series, called "Under the Ocean to the South Pole, or the Strange Cruise of the Submarine Wonder." In that is told how once more Tom and Bill, with Andy, the boys and Washington, accompanying Professor Henderson, had many thrilling experiences.

They were caught in the grip of the grass of the terrible Sargasso Sea. Monstrous suckers grasped the boat in their powerful arms, and had to be fought off. They were caught in a sea of boiling water and imprisoned between big fields of ice.

By means of strong diving suits they were able to leave the ship and walk about on the bottom of the sea. They visited a graveyard of sunken ships, saw many strange monsters as well as many beautiful fish in the great depths to which they sunk. Many times they were in dire peril but the resources of the professor, the bravery and daring of the boys, no less than the help Washington and Andy Sudds, the hunter, rendered at times, brought them through.

Those of you who read of their adventures will recall the strange island which they came upon in the Atlantic Ocean, far from the coast of South America.

When they first drew near this island they were almost sucked into the depths of a great whirlpool, caused by water pouring down a big hole that seemed to lead far into the earth. They reversed their ship just in time.

But, on going to another side of the island they were able to approach safely, as at this point the great hole was farther from the shore. Then they landed and investigated.

They found the island was almost circular, and the hole was also round, but not in the center of the land. It was an immense cavity, so wide they could not see across, and as for the depth they could only guess at it. Looking down they could only see rolling masses of vapor and clouds caused by the water which poured down from the ocean with the force of a Niagara.

Gazing down into the big hole Mark suggested it might lead to the centre of the earth, which some scientists claim is hollow. The professor admitted that the cavity looked as though it led to China.

They had no means of investigating further the mystery of the opening and returned to their submarine, completing the voyage to the south pole.

It was now about two years since they had come back from that eventful trip. One of the first things the professor did, after docking the Porpoise, was to shut himself up in his study and begin to draw plans. To the questions of the boys he returned no answer for several days. Then he announced he was working on a craft which could both sail on top of the water and navigate the air.

In time the plans were done, and, in order to keep the work secret, the shop was moved to an island which the professor owned.

Parts of the Monarch and the Porpoise were used in constructing the new craft, so there was no need to get other help than that which the boys, Washington and Bill and Tom could give, since the two latter accepted an offer of the professor to remain and work for him. The boys, of course, would not leave their friend.

The professor realized that he had a more difficult task in his new venture than he had set himself on other occasions. For a ship to be light enough to rise in the air, and, at another time, and with no change, to be strong enough to navigate the ocean, was indeed something to tax Mr. Henderson's ingenuity.

However, in the course of a little over a year the larger part of the work was done. Inside the big shed was the huge affair which, it was hoped, would enable its owner to be master of both air and water.

"Did the professor say anything special?" asked Mark of Washington.

"Nope. I reckon he were too busy problamatin' the exact altitude projected in an inverse direction by th' square root of th' new engine when operated at a million times inside of a few seconds, but he didn't say nothin' t' me. I were busy underneath th' ship, fixin' bolts when he tole me t' find yo'. I wouldn't be s'prised if he had th' thing goin' soon."

"Do you think he'll be generating the new gas to-day?" asked Jack eagerly. "That's the most troublesome part; to get that gas right."

"He didn't say nothin' t' me 'bout it," Washington stated, as he walked along beside the two boys. "He jest seemed anxious like."

"We'd better hurry," advised Mark. "He may be at an important part in his experiments and probably needs us. I hope it will work. He has spent many days on it, and we all have worked hard. It ought to be a success."

"Perfesser allers makes things work," declared Washington stoutly.

"That's a good way to feel about it, anyway," observed Mark. "Well, we'll soon know."

The three hurried to the shed which they could see as they rounded a turn of the path through the wood. They noticed an elderly man approaching with a gun on his shoulder. On one arm he carried a game bag.

"Guess Andy got something for dinner," remarked Jack.

"I hopes so, honey," put in Washington. "I'se got a sort of gone feelin' in my stomach!"

"Any luck, Andy?" called Mark, when he came within hailing distance.

"Fine," replied Andy Sudds. "Rabbits and quail. We'll have a good dinner to-morrow."

While Andy entered the living part of the big shed to put away his gun and game, the boys and Washington kept on to the engine room. They found the professor, with Bill and Tom, busy fitting pipes to the small engine which was set up at one side of the structure.

"Come, boys, I need your aid," remarked Mr. Henderson as they entered. "Take off your coats and pitch in. Tighten up these bolts, Jack. Mark, you mix up those chemicals the way I taught you, and see that the dynamo is in working order for Washington to attend to."

In a little while the shop was a veritable hive of industry, and it resounded to the sound of hammers, wrenches and machinery. In the background was the big ship, which seemed like two immense cigars, one above the other, the lower one the larger.

"Where was you calalatin' t' take this here ship when it gits done, Perfesser?" asked Washington, during a lull in the operations.

"Do you remember that big hole in the island we visited on our trip to the south pole?"

"I suah does," answered the colored man.

"We are going to explore that," went on the scientist. "We are going to make a voyage to the interior of the earth in our Flying Mermaid."

"Go down into th' earth!" exclaimed Washington, his eyes big with fright.

"Certainly; why not?"

"Not for mine!" cried the colored man, dropping the wrench he was holding. "No sire I'm not goin' t' project myself int' a grave while I'se alive. Time enough when I kicks th' bucket. No sir! If yo' an' the boys wants t' risk yo' se'ves goin' down int' th' interior of th' earth, where th' Bible says there's fiery furnaces, yo' kin go, but Washington White stays on terra cotta! That's where he stays; He ain't ready t' be buried, not jest yet!" and the frightened colored man started to leave the shed.

CHAPTER II

THE FLYING MERMAID

"HERE! Stop him!" cried Professor Henderson. "Don't let him get away. We still need his help to get the ship in shape. He needn't be frightened. We're not going to start at once."

Mark and Jack ran after Washington, whose progress was somewhat impeded because he kept looking back as if he feared the new ship was chasing him.

"Come on back!" said Mark. "There's no danger, and if there was we're not going to start to-day."

"Ain't yo' foolin' me?" asked Washington, pausing and looking doubtfully at the boys.

"Of course not," answered Mark. "You know Professor Henderson would not make you do anything you didn't want to do, Wash. He wishes you to stay and help him get ready, that's all."

"Well, Washington," observed the aged scientist. "I didn't think you'd go back on me."

"I'd do mos' anything fer yo', Perfesser," said the colored man, "but I got t' beg off this time," and he looked at the Flying Mermaid as if he thought the metal sides would open and devour him.

Then help me get things in shape to generate the gas," the scientist said. "I want to give the new vapor the first real test in lifting power to-day. On the success of it depends the future of the ship."

Seeing there was no immediate danger of being carried to the centre of the earth, Washington resumed his labors. The professor, the boys, Bill and Tom were also hurrying matters to enable a test to be made before night.

As will readily be seen, even by those not familiar with the construction of airships and submarines, the chief problem was to find some agent strong enough to lift from the earth a weight heavier than had ever before been put into an apparatus that was destined to traverse the clouds. For the Flying Mermaid was not only an airship but an ocean voyager as well. It had to be made light enough to be lifted far above the earth, yet the very nature of it, necessitating it being made heavy enough to stand the buffeting of the waves and the pressure of water, was against its flying abilities.

Professor Henderson realized this and knew that the chief concern would be to discover a gas or vapor with five times the lifting power of hydrogen, one of the lightest gases known, and one sometimes used to inflate balloons.

After long study he had been partially successful, but he knew from experiments made that the gas he had so far been able to manufacture would not answer. What he wanted was some element that could be mixed with the gas, to neutralize the attraction of gravitation, or downward pull of the earth.

While he was seeking this, and experimenting on many lines, the construction of the air-water ship went on. In general the outward construction was two cigar shaped hulls, one above the other. Aluminum, being the lightest and strongest metal that could be used for the purpose, formed the main part of both bodies.

The upper hull was one hundred feet long and twenty feet in diameter at the widest part. It tapered to points at either end. It was attached to the lower hull by strong braces, at either end, while from the center there extended a pipe which connected with the lower section. This pipe was intended to convey the lifting gas to the part which corresponded to the bag of the balloon, save that it was of metal instead of silk, or rubber as is usual.

There were two reasons for this. One was that it would not be liable to puncture, particularly in the proposed underground trip, and the other was that it did not have to be so large as a cloth bag would have had to be. It was also a permanent part of the ship, and on a voyage where part of the time the travelers would be in the air and part on the water, and when the change from one to the other would have to be made quickly, this was necessary. It would have taken too long to raise the ship in the air had a cloth bag been used to contain the gas.

The lower hull or main part of the craft was one hundred and fifty feet long, and forty feet through at the largest part, in the centre.

It was divided into four sections. The forward one contained the sleeping quarters of Professor Henderson and his crew. There was a small stateroom for each one. Above was a conning or observation tower, reached by a small flight of steps. From this tower the ship could be steered, stopped and started, as could also be done from the engine room, which was in the after part of the hull.

As in the Porpoise and Monarch, electricity formed the motive power and was also used for many other purposes on board. Engines operated by gas produced the current which heated, lighted and moved the ship, as well as played a part in producing the wonderful gas.

The ship moved forward or backward by means of a novel arrangement. This was by the power of compressed air. From either end of the lower hull there projected a short pipe working in a ball and socket joint, so it could be turned in any direction. By means of strong pumps a current of compressed air could be sent out from either pipe. Thus when floating above the earth the ship was forced forward by the blast of air rushing from the pipe at the stern. It was the same principle as that on which a sky rocket is shot heavenward, save that gases produced by the burning of powder in the pasteboard rocket form its moving impulse.

In the case of the Flying Mermaid, it could be made to move backward by sending the air out of the forward tube. Thus, when in the water, the compressed air rushing from the pipe struck the fluid and forced the ship forward or backward as was desired. It floated on the surface, the deck being about three feet out of water, while the aluminum gas bag was overhead.

The engine room was a marvel of machine construction. It contained pumps for air and water, motors, dynamos, gas engines, and a maze of wheels and levers. Yet everything was very compact and no room was wasted.

The use of the air method of propulsion did away with the necessity of a large propellor such as most airships have to use, a propellor which must of necessity be very light and which is easily broken.

Next to the engine room was the kitchen. It contained an electric range and all necessary appliances and utensils for preparing meals. There were lockers and a large reserve storeroom which when the time came would be well stocked with food. Forward of the kitchen was the living and dining room. It contained comfortable seats, folding tables and a small library. Here, also were many instruments designed to show how the various machines were working. There were gages, pointers and dials, which told the direction the ship was traveling, the speed and the distance above the earth or below the surface. Similar indicators were in the conning tower, which had a powerful search light.

The ship was lighted throughout by incandescent lamps, and there was even a small automatic piano worked by the electric current, on which popular airs could be played.

If the gas and the gravity neutralizer worked as Professor Henderson hoped they would, as soon as the ship was completed, all that would be necessary to start on the voyage would be to fill the aluminum bag and set the air compressor in motion.

The gas was made from common air, chemically treated and with a secret material added which by means of a complicated machine in a measure did away with the downward pull of the earth. Thus all that was necessary to carry on a long voyage was a quantity of gasolene to operate the engine which worked the electric machines, and some of this secret compound.

The professor and his helpers had been working to good advantage. At last all was in readiness for the gas test.

It was proposed to try it on an experimental scale. Some of the fluid was to be generated and forced into an aluminum cylinder under the same pressure it would be used in the air ship. To this cylinder were attached weights in proportion to the weight of the Flying Mermaid with its load of human freight, engines and equipment.

"This cylinder is just one one-hundredth the size of the cylinder of the ship," said the professor. "I am going to fasten to it a hundred pound weight. If it lifts that our latest contrivance will be a success."

"You mean if the little cylinder pulls a hundred pounds up the big ship will take us and the machinery up?" asked Mark.

"Certainly," answered the professor. "If this cylinder lifts a hundred pounds, one a hundred times as big (as that of the Mermaid is), will lift a hundred times as much, or ten thousand pounds. That is five tons, or more than a ton over what I figure to be the weight of our ship and contents. The latest war balloon can lift one ton with ease, and if my machine can not do five times as well I shall be disappointed."

The last adjustments were made, pipes were run from the gas generator to the cylinder, and the hundred pound weight was attached.

"Everybody look out now," said Mr. Henderson. "I am going to start the machine and let the gas enter the cylinder. It is a very powerful gas and may break the cylinder. If it does you must all duck."

The scientist gave a last look at everything. The boys got behind some boards whence they could see without being in danger. Washington, who had little fear so long as there was no danger of going under ground, took his place at the dynamo. Andy Sudds, with Bill and Tom, stationed themselves in safe places.

"All ready!" called the professor.

He pulled a lever toward him, turned a wheel and signalled to Washington to start the dynamo. There was a sound of buzzing machinery, which was followed by a hiss as the gas began to enter the cylinder under pressure. Would it stand the strain? That question was uppermost in every one's mind save the professor's. He only cared to see the cylinder leave the ground, carrying the weight with it. That would prove his long labors were crowned with success.

Faster and faster whirred the dynamo. The gas was being generated from the air. The secret chemical made a hissing which could be heard for some distance. The gage registered a heavy pressure. Anxiously the professor watched the cylinder.

"There!" he exclaimed at length. "It has all the gas it can hold. Now to see if it works!"

He disconnected the pipe leading from the generator. This left the cylinder free. It seemed to tremble slightly. There appeared to be a movement to the hundred pound weight which rested on the ground. It was as if it was tugging to get loose.

"There it goes! There it goes!" cried Mark, joyfully.

"Hurrah!" shouted Jack. "There she rises!"

"It suttinly am projectin' itself skyward!" yelled Washington, coming from the dynamo.

Sure enough the cylinder was slowly rising in the air, bearing the weight with it. It had lifted it clear from the ground and was approaching the roof of the big shed.

"It will work! It will work!" exclaimed the professor, strangely excited.

The next instant the cylinder, carrying the weight, sailed right out of an open skylight, and began drifting outside the shop, and across the fields.

"Quick! We must get it back!" cried Mr. Henderson. "If it gets away my secret may be discovered and I will lose all! We must secure it!"

But the cylinder was now two hundred feet in the air and being blown to the east, the weight dangling below it, making it look like a miniature airship.

"We can never catch that!" cried Mark.

CHAPTER III

WASHINGTON DECIDES

"WE must catch that cylinder!" the professor exclaimed. "Some one may find it when it comes down and analyze the gas. Then he would discover how to make it. The cylinder must come down!"

"Don't see how we can proximate ourselves inter th' vicinity of it lessen we delegate th' imperial functions of orinthological specimens t' some member of this here party," observed Washington.

"If you mean we can't catch that there contraption unless we turn into birds I'll show you that you're mistaken!" cried Andy Sudds. "I guess I have a trick or two up my sleeve," and the old hunter quickly threw open the breech of his gun and inserted a couple of cartridges.

He raised the piece to his shoulder and took quick aim. There was a sliver of flame, a puff of smoke and a sharp report. The professor and the boys who were watching the cylinder saw it vibrate up in the air. Then there came a whistling sound. An instant later the metal body began to descend, and it and the weight fell to the earth.

"I'm sorry I had to put a bullet through it, Professor," said old Andy with a queer smile, "but it was the only way I saw of bringing it down. Hope it isn't damaged much."

"It doesn't matter if it is," the scientist answered. "I can make more cylinders, but I don't want that secret of the gas to become known. Your bullet served a good turn, Andy, for it let the compressed vapor out just in time."

"Then we may consider the experiment a success," said Mark, as Washington went to where the cylinder had fallen, to detach it from the weight and bring both to the shed.

"It seems so," Mr. Henderson answered. "True, it was only an experiment. We have yet to test the ship itself."

"When can we do that?" asked Jack.

"I hope by Monday," the scientist answered.

"Will you try it in the water or air first?" asked Mark.

"I'm almost certain it will float in the water," the aged inventor said. "It does not require much work to make a ship which will do that. But the air proposition is another matter. However, since the cylinder rose, I am pretty sure the Flying Mermaid will.

"But we have done enough work to-day. Let's rest and have something to eat. Then, with Sunday to sit around and talk matters over, we will be ready for Monday's test."

Some of the game Andy had killed was soon on the table, for Washington, in addition to his other accomplishments, was an expert cook. During the evening the boys and their friends sat in the living room of the big shed and talked over the events of the day.

Sunday was spent in discussing what adventures might lie before them should they be able to descend into the big hole. Washington did not say, much, but it was easy to see he had no notion of going. He even began to pack his few belongings in readiness to leave the service of Mr. Henderson, for whom he had worked a good many years.

No one remained long abed Monday morning. Even Washington was up early in spite of the interest he had lost in the professor's voyage.

"I jest wants t' see yo' start fer that place where they buries live folks," he said.

In order to properly test the Flying Mermaid it was necessary to move the craft from the shed from which place it had never been taken since it's construction was started. It had been built on big rollers in anticipation of this need, so that all which was now necessary was to open the doors at the end, and roll the craft out.

This was accomplished with no small amount of labor, and it was nearly noon before the big ship was moved into the open. It was shoved along to a little clearing in front of the shed, where no trees would interfere with its possible upward movement.

Everyone was bustling about. The professor was busiest of all. He went from one machine to another; from this apparatus to that, testing here, turning wheels there, adjusting valves and seeing that all was in readiness for the generating of the powerful gas.

As the airship was half round on the bottom and as it rested in a sort of semi-circular cradle; it brought the entrance some distance above the ground. To make it easier to get in and out while preparations for the trial were going on, Bill and Tom had made an improvised pair of steps, which were tied to the side of the ship with ropes.

Up and down these the professor, the boys and Andy went, taking in tools and materials, and removing considerable refuse which had accumulated during the building of the craft.

Finally all was in readiness for starting the making of the gas. The ship was not wholly complete and no supplies or provisions for the long voyage had been taken aboard. The Flying Mermaid was about a ton lighter than it would be when fully fitted out, but to make up for this the professor had left in the ship a lot of tools and surplus machinery so that the craft held as much weight as it would under normal conditions. If the gas lifted it now it would at any other time.

"Start the generator," said Mr. Henderson, to Mark. "We'll soon see whether we are going to succeed or fail."

The boy turned a number of levers and wheels. The machine which made the powerful vapor was soon in operation. The professor had already added enough of the secret compound to the tank containing the other ingredients, and the big pump was sucking in air to be transformed into the lifting gas.

The boys and the professor were in the engine room. Andy Sudds, with Bill and Tom, had taken their places in the living room, to more evenly balance the ship, since the things in it were not yet all in their proper places. As for Washington he was busy running from the shed to the ship with various tools and bits of machinery the professor desired.

The gas was being generated rapidly. Throughout the ship there resounded a hissing noise that told it was being forced through the pipe into the aluminum shell above the ship proper.

"I wonder how soon it will begin to lift us," said Mark.

"It will take about half an hour," replied Mr. Henderson. "You see we have first to fill the holder completely, since there is no gas in it. After this we will keep some on hand, so that it will only need the addition of a small quantity to enable the ship to rise."

He was busy watching the pointer on a dial which indicated the pressure of the gas, and the lifting force. The boys were kept busy making adjustments to the machinery and oiling bearings.

Suddenly, throughout the length of the craft there was felt a curious trembling. It was as though the screw of a powerful steamer was revolving in the water.

"What is it?" asked Jack.

"I hope it is the lifting power of the gas making itself felt," the professor answered. "Perhaps the Flying Mermaid is getting ready to try her wings."

The trembling became more pronounced. The gas was being generated faster than ever. The whole ship was trembling. Tom and Bill came from the room, where they were stationed, to inquire the meaning, but were reassured by the professor.

"Don't be alarmed if you find yourselves up in the air pretty soon," he remarked with a smile. "Remember the Electric Monarch, and the flights she took. We may not go as high as we did in her, but it will answer the same purpose."

The gas was hissing through the big tube as it rushed into the overhead holder. The gage indicated a heavy pressure. The ship began to tremble more violently and to sway slightly from side to side.

"I think we shall rise presently," said Mr. Henderson. His voice showed the pride he felt at the seeming success with which his invention was about to meet.

Suddenly, with a little jerk, as though some one with a giant hand had plucked the Flying Mermaid from the earth, the ship gave a little bound into the air, and was floating free.

"Here we go!" cried Mr. Henderson. "The ship is a success. Now we're off for the hole in the earth!"

The Flying Mermaid was indeed rising in the air. True it did not go up so swiftly as had the Monarch, but then it was a much heavier and stronger vessel, and flying was only one of its accomplishments.

"It's a success! It's a success!" shouted Mark, capering about in his excitement.

"Now we'll see what the centre of the earth looks like," went on Jack. "I can hardly wait for the time to come when we are to start on the voyage."

At that instant, when the ship was but a few feet from the ground, but slowly rising, the boys and the professor heard a shouting below them.

"What's that?" asked the scientist. "Is any one hurt?"

Mark ran to a small window, something like a port hole in an ocean steamer, and looked out.

"Quick!" he shouted. "Stop the ship! Washington will be killed!"

In fact from the agonized yells which proceeded from somewhere under the craft it seemed that the accident was in process of happening.

"Save me! Save me!" cried the colored man. "I'm goin' to fall! Catch me, some one!"

"What is it?" asked the professor, making ready to shut off the power and let the ship settle back to earth, from which it had moved about fifty feet.

"It's Washington," explained Mark. "He evidently tried to walk up the steps just as the boat mounted skyward. He rolled down and managed to grab the end of the rope which was left over after the steps were tied. Now he's swinging down there."

"Are you going to lower the ship?" asked Jack.

"Of course!" exclaimed the professor. "I only hope he hangs on until his feet touch the earth."

"Keep a tight hold!" shouted Mark, from out of the small window.

"That's th' truest thing yo' ever said!" exclaimed Washington. "You bet I'm goin' to hold on, and I'm comin' up too," which he proceeded to do, hand over hand, like a sailor.

The boys and the professor watched the colored man's upward progress. The ship had hardly begun to settle as, in the excitement, not enough gas had been let out. Closer and closer came Washington, until he was able to grasp the edge of the opening, to which the steps were fastened.

"I thought you weren't coming with us," observed the professor, when he saw that his helper was safe.

"I changed my mind," said the colored man. "It's jest luck. Seems like th' ship done wanted me t' go 'long, an' I'm goin'. I'll take my chances on bein' buried alive. I ain't never seen th' centre of th' earth, an' I want's to 'fore I die. I'm goin' 'long, Perfessor!"

CHAPTER IV

WHAT DID MARK SEE?

"WELL, I'm glad you've decided at last," the professor remarked. "Now come inside and we'll see how the ship works."

Once over his fright, Washington made himself at home on the craft he had helped build. He went from one room to another and observed the engine.

"She certainly am workin'" he observed with pride. "Are we still goin' up, Perfessor?"

"Still mounting," replied Mr. Henderson, "We are now three hundred feet above the earth," he added as he glanced at a registering gage.

The great air pump was set going and soon from the after tube, a big stream of the compressed vapor rushed. It acted on the ship instantly and sent the craft ahead at a rapid rate. By elevating or depressing the tube the craft could be sent obliquely up or down. Then, by forcing the air from the forward tube, the Mermaid was reversed and scudded backward.

But it was more with the ship's ability to rise and descend that Professor Henderson was concerned, since on that depended their safety. So various tests were made, in generating the gas and using the negative gravity apparatus.

All worked to perfection. Obeying the slightest turn of the wheels and levers the Mermaid rose or fell. She stood still, suspended herself in the air, or rushed backward and forward.

Of course the machinery was new and did not operate as smoothly as it would later, but the professor and his friends were very well satisfied.

"Now we'll try something new," said the scientist to the two boys as they stood beside him in the tower. "I only hope this part succeeds, and we shall soon be off on our voyage."

He turned several levers. There was a hissing sound as the gas rushed from the container, and the ship began to settle down.

"What's th' matter? Are we goin' t' hit th' earth?" yelled Washington, rushing from the engine room.

"Keep quiet," ordered the professor. "We are only going down, that's all."

"But good land! Perfesser!" exclaimed the colored man. "The ocean's right under us! You forgot you sailed sway from the island! We'll be drowned suah!"

"Leave it to me," said Mr. Henderson. "The Flying Mermaid is going to take a bath!"

"As long as it swims it will be all right," observed Mark in a low tone to Jack. "I'm glad I can take care of myself in the water."

Before Jack could reply the Mermaid seemed to take a sudden dive through the air. The next instant she struck the water with a splash that sent the waves rolling all about. The craft rocked violently to and fro on the surface of the sea. For a while there were anxious hearts aboard, for there was no certainty but that the ship might not sink to the bottom.

But the old professor had not calculated and builded in vain. After rocking about like a vessel newly launched, the strange craft rode safely and upright on the water. It set down far enough to bring the propelling tubes well under, but not so far but that the conning tower was well out and there was a small deck available.

"Now to see if we can conquer the water as we did the air!" cried the professor. "Mark, start the air pump. Jack, you steer, for I want to watch the machinery under the additional strain."

From the rear tube rushed such a volume of air that the ocean near it bubbled and foamed. The ship trembled from stem to stern, and then, after hanging for an instant as if undecided what to do, it began to move forward as easily as though it had never sailed any other element than the sea.

"She fits her name!" the professor cried. "She is indeed the Flying Mermaid, for she sails the ocean as easily as she navigates in the clouds!"

For a mile or two the craft was sent ahead over the waves. Then it was reversed and run backwards. Satisfied that his long months of work had not gone for naught, the professor after trying several experiments, decided to try and raise the ship while in motion.

With Jack and Mark to look after the air pumps, while Washington, Tom and Bill busied themselves in the engine room, Mr. Henderson began to generate the gas and start the negative gravity apparatus. All the while the craft was forging ahead.

There was again the hissing sound that told of the aluminum holder being filled. For a few minutes there seemed to be no change, the Mermaid plowing forward.

Then like a bird rising from the waves, or like a flying fish leaping from the sea to escape some pursuing monster of the deep, the new ship shot up diagonally from the surface and winged its way into the upper regions of the air.

"Success! Success!" cried the professor. "This proves all I wanted to know. Now. we are ready for our great trip!"

Great were the rejoicings in the camp that night. It was like living over again the days when they were aboard the diving Porpoise or the flying Monarch. To the recollections were added the anticipations of what was before them in the trip to the interior of the earth.

Busy days followed, for there was still much to be done to the Flying Mermaid. The machinery, which was only partly completed, had to be finished. Besides this the professor was working on some apparatus, the use of which he did not disclose to any one. It was stored aboard the ship at the last minute.

Plenty of provisions had to be taken aboard, and many supplies needed to work the Mermaid and insure that it would go to the end of the voyage. The materials for generating the gas and negative gravity, spare parts, records for the automatic piano and other things were stored away.

Some guns and ammunition were taken along as were a few revolvers, since old Andy had said it was best to prepare for any thing in the shape of enemies or wild beasts that might be met with in the interior regions.

It was decided to make the start by sailing along the surface of the sea for several days, as in the event of any weakness in the machinery being discovered there would be less danger. If, at the end of four days, no trouble developed, the professor said he would send the Mermaid into the air and make the rest of the voyage through the sky.

The night before the start was to be made the professor, with the boys, Washington and the other helpers, went about through the various shops and buildings, locking them up securely. For they could not tell how long they would be away, and they had to leave behind much valuable material.

As there were several things that needed attention they divided the work up. Mark had finished his share and was walking back toward the living cabin where they were all quartered, when, down at the shore, near where the boat was moored, he fancied he saw, in the gathering darkness, a moving figure.

"I wonder who that can be," he thought. "All the others are near the machine shop, for I just left them there. Perhaps it's some one trying to spy out how the Mermaid is built."

Knowing the professor wanted his secret well guarded, Mark walked softly toward the little dock that served as a place whence the Mermaid could be easily boarded. As he approached he saw the figure moving. Something struck the boy as peculiar.

Though the object had some of the characteristics of a man it did not walk like a human being, but shuffled along more like a huge ape or monkey. It seemed bent over, as if it stooped toward the ground.

"Who are you?" called Mark suddenly.

For an instant the figure halted and then hurried on faster than before, with a curious, shuffling walk. It was approaching the ship.

Somehow it struck Mark as if it was an uncanny being; an inhabitant of some other world. Then he laughed at his half-fear, and started on a run toward the dock.

"If it's some tramp trying to find a place to sleep he'd better not go aboard the ship, he might do some damage," the boy thought.

He could hardly see the figure now as it had passed into the shadow cast by the boat. He was about to summon the professor to make an investigation, when Washington started going the search light which was placed just over the door of the living cabin. It was kept there as a sort of beacon light, as, near the island was a dangerous ledge of rocks.

Then, in the blinding white glare from the big lantern as Washington accidentally swung it toward the Mermaid, Mark beheld a strange sight.

The figure he had been watching stood out in bold relief. Though it was shaped like a human being it was not like any person the boy had ever seen. It seemed covered with a skin twice too large for it; a skin, which, in spite of the clothes that concealed it, hung in folds about the arms and legs, dropping pendent like from the neck like a big garment, and flapping in the wind.

For an instant Mark was so startled he cried out, and the professor and the others ran to see what was the matter.

"There— by the ship! A horrible creature!" exclaimed Mark.

Shouting to Washington to keep the light steady in the direction of the dock, Mr. Henderson ran toward the moored Mermaid. Jack, Andy, Bill and Tom, with Mark in the rear followed him.

"Nothing here," said the scientist, after a careful search about. "Are you sure you saw something, Mark?"

"Positively," replied the lad with a shudder. He described the vision of the darkness.

"I guess it was a big otter, or maybe an enormous turtle," the professor said.

CHAPTER V

ATTACKED BY A WHALE

BUT Mark was certain it was nothing like that, though a careful search failed to reveal anything or any person near the ship. It was too dark to examine for footprints, and even Mark, after taking a look all about, felt he might have been deceived by shadows. Still he was a little nervous, and could hardly sleep for imagining what the thing he saw could have been.

The next day every one was so busy that no one, not even Mark, recalled the little excitement of the night before. Shortly after noon, final preparations having been made, they all got aboard the Mermaid and started off.

It was a bright sunshiny day, and the craft, speeding away from the island where it had been constructed, over the dancing blue waves, must have presented a strange sight had there been any spectators. For surely no such ship had ever before sailed those waters.

However, there was no other vessel in sight, and the island, as far as the professor and his friends knew, had never been inhabited.

"We will not try for any great speed," Mr. Henderson remarked as he, with Mark and Jack, stood in the conning tower managing the Mermaid. "We don't want to strain any joints at the start or heat any engine bearings. There will be time enough for speed later."

"Yes, and we may need it more when we get into the centre of the earth than we do now," observed Mark.

"Why so?" asked Jack.

"No telling what we may run up against underneath the ground," went on Mark. "We may have to fight strange animals and stranger beings. Besides, the atmosphere and water there can't be the same as up here; do you think so, Professor?"

For a few minutes the scientist was silent. He seemed to be thinking deeply.

"I will tell you what I believe," he said at length. "I have never spoken of it before, but now that we are fairly started and may eventually have a chance to prove my theory, I will say that I think the centre of this earth on which we live is hollow. Inside of it, forming a core, so to speak, I believe there is another earth, similar to ours in some respects which revolves inside this larger sphere."

They were well out to sea now, as they could observe when they emerged on the little deck. Above their heads was the aluminum gas holder, which served as a sort of protection from the sun that was quite warm. The Mermaid rode with an easy motion, being submerged just enough to make her steady, yet not deep enough to encounter much resistance from the water. In fact it could not have been arranged better for speed or comfort.

"I think we will sail well to the eastward before making our course south," Mr. Henderson said. "I do not care to meet too many ships, as those aboard will be very curious and I do not want too much news of this venture to get out. We will take an unfrequented route and avoid delays by being hailed by every passing vessel whose captain will wonder what queer craft he had met with."

The boys enjoyed the sail, for the weather could not have been better. Even old Andy, who seldom said much, seemed delighted with the prospect of having strange adventures. He had his rifle with him, and, indeed, he seldom went anywhere that he did not carry it.

"For there's no telling when you may see something you want to shoot or that ought to be shot," he used to say, "and it's always the man without a gun who needs it most. So I'm taking no chances."

They sailed all that afternoon without meeting with a craft of any kind. Straight to the east they went, and when night began to settle down Washington got supper. It was decided to run slowly after dark until all hands were more familiar with the ship.

Morning found the Mermaid about a hundred miles from the island where she had been launched. The night had been uneventful, except that Mark told Jack he heard some strange noise near his bunk several times. He was nearest the storeroom where spare parts, and the curious cylinder the professor had brought aboard, were kept.

"I guess it was rats," said Jack. "They are always in ships."

"Old wooden ships, yes," admitted Mark. "But I'll bet there's not a rat aboard the Mermaid."

"Then you were dreaming," said Jack, as if that settled it.

Mark did not speak further of the noise, but he did considerable thinking. However, the next night there was no further disturbance.

The fourth day out, when everything had passed off well, the engines doing their best, the professor decided to speed them up a bit, since he was satisfied they had "found" themselves as mechanics term it.

"We'll see how fast we can go through the water," said Mr. Henderson, "and then I think we can safely turn our course south. We are well beyond the ordinary lines of travel now."

Having oiled the bearings well, and seen that everything was in place and properly adjusted, the professor and the boys took their places in the conning tower, while Washington, Tom and Bill remained in the engine room. Andy stayed on deck with his gun.

"I might see a big fish, and we could vary our bill of fare," he said with a laugh.

"Here we go!" exclaimed the professor as he shifted the levers and turned some wheels and valves. "Now we'll see how fast we can travel."

As he spoke the Mermaid responded to the added impulse of the compressed air and shot through the water at a terrific speed. The sudden increase in momentum almost threw the boys from their feet, and they would have fallen had they not grasped some projecting levers.

"I guess that will do," Said the scientist. "I think we have speed enough for almost any emergency. I'll let her run at this rate for a while, and then we'll slack up."

Looking ahead, the boys could see the green waters parting in front of the bow of the Mermaid, as if to make room for her. Two huge waves were thrown upon either side.

Suddenly, dead ahead, there loomed up a big black object.

"Look out you'll hit the rock!" cried Mark to the professor, who was steering.

With a turn of his wrist Mr. Henderson moved the wheel which controlled the tube. It was deflected and sent the boat to larboard.

At that instant from the rock two small fountains of water rose in the air, falling back in a shower of spray through which the sun gleamed.

"That's not a rock! It's a big whale!" cried Jack. "And we're going to hit him!"

The professor had miscalculated the speed of the craft, or else had not thrown her far enough to larboard, for, a second later, the Mermaid was almost upon the big leviathan.

With a desperate twirling of the steering wheel the professor veered the craft as far as possible. But all he could do did not suffice, for the craft hit the whale a glancing blow on the side, and the ship careened as if she would turn turtle.

At the same time there rang out from upon deck the sound of a rifle shot. Old Andy had taken a chance at the enormous creature of the deep.

"Hurrah!" the bays heard him shout. "I give him one plumb in the eye! A fine shot! And we hit him besides with the boat. I guess he's a goner!"

"I'm afraid not," muttered the professor. "That was a bad blow we struck him, but I think it will only ruffle his temper. We'll have to look sharp now, boys."

By this time the ship had rushed past the whale, but the boys, looking through a window in the rear of the tower could see the huge body. Now the fountains of water which the whale spouted were tinged with red.

"He's bleeding!" exclaimed the professor. "I guess Andy hit him in a vital spot."

"But not vital enough!" cried Mark. "See! He's coming after us!"

And so it proved. The whale, angered, and, probably half crazed by the pain of the bullet and the blow, was coursing after the ship, coming on with the speed of an express train. Straight at the Mermaid he lunged his huge bulk.

"We must escape him!" cried Mr. Henderson. "If he hits us he'll send us to the bottom!"

He had made ready to slow up the Mermaid to see if it had sustained any damage from the impact with the whale, but when he saw the monster coming after the boat he knew the only safety lay in flight.

"Let us go up into the air and so escape him!" cried Jack, with sudden inspiration.

For an instant neither Mark nor the professor grasped what Jack meant. Then, with an exclamation, the professor pulled forward the lever that generated the gas and set working the gravity neutralizer, which would enable the ship to rise.

Faster through the water went the Mermaid, and faster after her came the whale. Above the hum of the engines was heard the hiss of the powerful gas. The ship trembled more violently.

"We are rising!" exclaimed the professor, as he looked at a gage.

The boys could feel the craft lifting from the waves which clung to her as if they hated to lose her. The boys knew the gas was beginning to operate.

"If it is not too late!" whispered Mark, half to himself.

For the monster of the seas was coming on, lashing the water to foam with his terrible flukes, and sending aloft a bloody spray. His speed was awful.

Now he was but ten feet away from the fleeing craft— now but eight— now five! Ten seconds more and the big head, like the blunt stern of a battle ship, forced forward by the tons of blubber, flesh, bone and fat behind it would strike the Mermaid and crush it like an egg shell.

Now if ever was the need for the Flying Mermaid to prove herself worthy of the name. Now, if ever, was the time for her to leave the watery element and take to the lighter one.

And she did. With a last tremble, as if to free herself from the hold of the waves, the gallant craft soared up into the air, leaving the water, which dripped from her keel like a fountain's spray, and shooting aloft like a bird, escaped her terrible enemy which passed under her, so close that the lower part of the Flying Mermaid scraped the whale's back.

"Saved!" exclaimed the professor.

CHAPTER VI

THE CYCLONE

IT was only in the nick of time, for a second later and the big mammal of the ocean would have struck the ship and split it from stem to stern.

Higher and higher into the air mounted the Flying Mermaid, while in the water below, the whale, incensed by missing his prey, was lashing the waves to foam.

"Well, that was a narrow squeak; as close as I ever care to come to it!" exclaimed Andy as he let go of the steel rail to which he was clinging and entered the conning tower. "I had no idea of hitting the big fish."

"I guess he would have taken after us whether you had fired at him or not," said Mr. Henderson. "He was probably looking for trouble, and took the first thing that came in his way, which happened to be us. Some whales are like that, so I have read; big bull creatures, exiled from the school to which they once belonged, they get like mad creatures and know neither friend nor foe. Something like rogue elephants, I imagine."

Now, having thus unexpectedly risen into the air, the professor decided to continue travel in that style for a while at least. It would require less force to propel the ship, and the going would be more comfortable, since in the upper regions the Mermaid rode on an even keel, while in the water there was more or less rolling, due to the action of the waves.

Once recovered from their fright caused by the whale, and having lost sight of the enormous creature, for they were now far above the ocean, the adventurers began to think of something to eat.

Washington lost little time in preparing a meal, and it was eaten with a relish. The electric cooking stove worked to perfection, for the colored man had learned how to use that aboard the Porpoise and Monarch, and could be depended on to turn out appetizing dishes.

"What do you say to traveling through the air at night?" asked Mr. Henderson, as he arose from the table.

"Suits me," replied Mark. "There's less danger than in the water, I think,"

Bill, Tom and Washington arranged to stand the night watch, and, when the professor had examined the engines and given orders about keeping the ship on her course through the air, he retired to his bunk. Jack and Mark soon followed.

It must have been about midnight when Mark was awakened by a movement that seemed to come from the storeroom next to where his sleeping place was located. At first he thought he had been dreaming, but, as he found he was wide awake, he knew it was no imagination that had affected him.

"I certainly heard something," he said to himself. "It sounded just as it did the other night. I wonder if I ought to investigate."

He thought over the matter carefully as he sat upright in his bunk in the darkness. True the noise might be a natural one, due to the vibration of the engine, or to some echo from the machinery. As Mark listened he heard it again.

This time he realized it was the slow movement of some heavy body. He felt a cold shiver run over him and his hair evinced an uncomfortable tendency to stand upright. But he conquered his feelings and resolved to keep cool and see if he could discover what had awakened him.

He got up and moved softly about the little room that contained his bunk. He could hear better now, and knew it was no echo or vibration that had come to his ears.

Once again he heard the strange sound. It was exactly the same as before; as if some big creature was pulling itself over the floor.

"Maybe it's a snake; a water snake!" thought Mark. "It may have crawled aboard when we did not notice it."

Then he remembered that the ship had not been open in any way that would enable a serpent to come on it, since it had been started on its ocean trip. Before that, he was sure no snake had entered the Flying Mermaid. Still it sounded more like a snake than anything else.

"I'm going to make a search," decided the boy.

He took a small portable electric light, run by a storage battery, and, slipping on a pair of shoes and a bath robe, he left his stateroom.

He had decided that the noise came from the storage compartment and so made for that. The door he knew was not locked, since he had seen Mr. Henderson go in late that afternoon, and the professor had used no key.

Moving softly, Mark left his room and soon found himself in a corridor, on either side of which were located the sleeping quarters of the others. He did not want to awaken them, and, perhaps, be laughed at for his curiosity.

To get to the storeroom Mark had to go first from the corridor into the dining room. He soon reached the door that guarded what he thought might be a strange secret. Trying the knob softly he found it giving under his fingers.

"I wonder if I had better go in," he thought. "Perhaps, after all, it was only rats, as Jack said."

But, even as he listened he again heard the odd sound coming from the room. This determined him. He would solve the mystery if possible.

Cautiously he turned the knob. The door was slowly swinging open when Mark was startled by a noise from behind him. He turned suddenly to see Professor Henderson confronting him.

"What is it, Mark? Is the ship on fire? What's the matter? Is any one hurt?"

"I was just going in this room to——" began Mark.

"Don't do it! Don't do it!" exclaimed the professor in an excited whisper. "No one must go in that room. I forgot to tell you and Jack about it. No one must enter. It contains a secret!"

"I heard a strange noise and——" Mark began again.

"It could make no noise! It would be impossible for it to make a noise!" the professor exclaimed.

"I heard something," the boy insisted.

"You were dreaming!" said the professor. "Now go back to bed, Mark, and don't let this happen again. Remember, no one must enter that room unless I give permission!"

Somewhat crestfallen at the outcome of his investigations, but realizing that the professor could do what he wanted to aboard his own ship, Mark went back to bed. But he could not sleep. All the rest of the night he was wondering whether Mr. Henderson had some strange creature hidden aboard the Mermaid. He feared lest the old scientist's mind might be affected and, in his wildness he had made some infernal machine that would, in time, blow the whole ship apart.

But tired nature asserted itself at last, and, weary with vain imaginings, Mark fell into a slumber. The next morning he awoke with a start from a dream that he was being devoured by an immense water snake.

He said nothing to the others about his night's adventure, for, as it transpired, no one else had been awakened by his investigations. The professor did not refer to his conversation with Mark.

"There's something queer going on aboard the ship this trip," said Mark to himself. "But I guess it's none of my business. Professor Henderson seems to know what he is doing and I guess I can trust him."

He resolved to think no more of the strange noises and movements, and, for several nights thereafter he was not disturbed by them.

The weather, which, up to this time had been fair, took a sudden turn for the worse about the fourth day after Mark's little night expedition. One evening the sun sank in a mass of dull lead-colored clouds and a sharp wind sprang up.

"We're going to have a storm," said Mr. Henderson. "It's liable to be a bad one, too, from the way the barometer is falling."

He looked at the glass, and scanned the various instruments that told how high up the Mermaid was and how fast she was traveling.

"We're pretty high up in the air," he said, "and scooting along at about fifty miles an hour. We are going against the wind, too, but fortunately it is not blowing hard."

At that moment there sounded from without a peculiar howling sound, as if a siren whistle was being blown.

"'Pears like there's goin' t' be a tumultuous demonstration of sub-maxiliary contortions in th' empherial regions contiguous t' th' upper atmosphere!" exclaimed Washington, entering from the engine room into the conning tower.

"What's the trouble?" asked Mr. Henderson.

"Terrible big black cloud chasin' us from behind!" exclaimed the colored man.

Noting the alarm in Washington's voice the professor glanced from the rear window. What he saw caused him to exclaim:

"It's a cyclone! We must drop down to avoid it!"

He sprang to a lever controlling the gas and yanked it toward him. There was a shrill hissing sound, and a second later the Mermaid began to sink. The boys watching the gages on the wall of the tower, saw that the craft was falling rapidly.

But, with a rush and roar, the terrible wind was upon them. It caught the craft in its fearful grip and heeled it over as a ship careens to the ocean blast.

"It's a storm in the upper regions! We'll find it calm below!" cried the professor above the howling of the gale. He opened the gas outlet wider and the ship fell more rapidly.

"Are you sure we're over the ocean?" asked Mark.

"Positive!" the professor called back. "We have been traveling straight south over the Atlantic for the last week. We will land in the midst of the waters and float safely."

Lower and lower went the Mermaid. The wind was now blowing with the force of a tornado, and, as the craft had to slant in order to descend, it felt the power of the gale more than if it had scudded before it. But, by skilful use of the directing tube, the professor was able to keep the boat from turning over. As they came further down toward the earth the force of the wind was felt less and less, until, as they came within two hundred feet of the water which they saw below them in the gathering dusk, it died out altogether.

"Now we are free from it," said the professor as the Mermaid came down on the waves like an immense swan.

"Are you going ahead or going to stop here?" asked Mark

"We'll keep right on," Mr. Henderson answered. "No telling when the storm may strike down here. We'll go as far as we can to-night."

CHAPTER VII

A QUEER SAIL

NOW that the fear and worriment was over they all began to feel hungry, and, while Mark and Jack took charge of the conning tower Washington got breakfast. The professor seemed preoccupied during the meal, and several times, when Mark spoke to him, he did not reply.

"I wonder if he is worried about something, or is thinking of something which seems to be concealed in the storeroom," the boy thought.

But, after a while, the professor seemed to be more like himself. He was busy over several maps and charts, and then announced the ship would try air-sailing again for a while.

"We can make better time above than we can on the water," he said, "and I am anxious to get to the mysterious island and learn what is in store for us."

Perhaps if the professor had been able to look ahead, and see what was soon going to happen, he would not have been so anxious for it to occur.

It was shortly after dinner when, the gas container having been filled, the ship rose in the air, and began sailing over the ocean, about a mile up. The day was a fine one, and, as they were moving south, it was constantly growing warmer. Down on the water, in fact, it was quite hot, but in the air it was just right.

Like some immense bird the Mermaid went flying through the air. The boys and the professor sat upon the deck in easy chairs. It was like being on the top of some tall "sky-scraper" building which, by some strange power, was being moved forward. Below them the ocean tumbled in long, lazy swells.

Suddenly Mark, who was looking through a telescope at the expanse of water stretched out under them, gave a cry.

"There's a ship! She's on fire!"

"Where?" asked the professor, stretching out his hand for the glass.

"Just to the port of the forward tube. See the smoke!" exclaimed Mark.

Mr. Henderson looked. Through the lens he saw a column of black vapor rising skyward. Mingled with it were red flames.

"Lower the Mermaid!" he cried. "We must save those on board if we can!"

Mark ran to the conning tower, where Washington was, to give the order. The colored man, who was looking ahead, intent on guiding the ship, did not at first hear what Mark called.

"Lower us! Send the Mermaid down!" Mark cried again.

The sudden shout, and the excited voice of Mark so startled Washington, that, fearing some accident had happened, he pulled the lever, controlling the gas supply, with more force than necessary.

There was a loud explosion, followed by a crackling sound, a flash of light, and the Mermaid came to a sudden stop.

"What's the matter?" cried Mark, feeling that something was wrong.

"I don't know!" Washington replied, as he dashed toward the engine room.

The Mermaid, her forward flight checked, hung in the air, suspended, neither rising or falling.

"Why don't we go on down?" the professor asked, hurrying to the tower.

"There has been an explosion— an accident!" exclaimed Mark. "I guess we can't go down!"

"But we must!" Mr. Henderson insisted, seizing the lever which should have produced a downward motion. The handle swung to and fro. It was disconnected from the apparatus it operated.

The ship was now stationary in the air, moving neither forward nor backward, neither rising nor falling. Washington had stopped the air pumps as soon as he learned something was wrong.

When Mr. Henderson saw the useless lever, which had controlled the outlet of gas from the holder, he ran out on deck. One glance told him what had happened. One of the electric wires had become short-circuited,— that is, the insulation had worn off and allowed the current to escape. This had produced a spark, which had exploded the gas which was in the pipe leading from the generator up into the aluminum holder. Fortunately there was an automatic cut-off for the supply of vapor, or the whole tank would have gone up.

As it was, only a small quantity had blown up, but this was enough to break the machinery at the point where the lever in the conning tower joined the pipe. If it had not been for the automatic cut-off all the gas in the holder would have poured out in a great volume, and the ship would have fallen like a shot.

"Can we do nothing to save those on the burning vessel?" asked Mark, pointing to where a cloud of smoke hung over the ocean.

"I fear not, now," answered the professor. "We are in a bad plight ourselves."

"Are we in any danger?" asked Jack.

"Not specially," Mr. Henderson replied. "But we must find a means of lowering ourselves gradually."

"Then it will be too late to save any of those on the ship," observed Mark.

"I'm afraid so," the scientist made reply.

The Mermaid rested some distance above the surface of the waters. She moved slightly to and fro with the wind, and rocked gently. The professor was examining the broken machinery.

"I have a plan!" suddenly cried Mark.

"What is it?" asked Mr. Henderson.

"Can't we bore a hole in the tank, insert a small faucet or tap, and let the gas out that way gradually?" asked the boy. "When we get down we can rescue those in danger of fire, and, later, can repair the break."

"The very thing!" exclaimed Mr. Henderson. "I never thought of that! Here, Washington! Bring me a drill, and a small stop-cock!"

The drill was obtained from the engine room. Working rapidly Mr. Henderson bored a hole in the lower part of the holder. As soon as the metal was penetrated the gas, which was under considerable pressure, rushed from the tank with a hissing sound. At once the Mermaid began to settle rapidly.

But the professor was prepared for this. He thrust the end of the stop-cock into the hole. It was screwed fast and the valve turned. This stopped the flow of gas and checked the descent of the ship. Then, by opening the tap the vapor was allowed to escape gradually, bringing the Flying Mermaid gently to the water.

As the adventurers approached they could see that the vessel was now a mass of flames. The wind was driving the fire toward the forecastle, and the crew had sought refuge aft. But this expedient could not last long, for, already the tongues of fire were licking the sides of the craft and coming nearer and nearer the seemingly doomed men. The vessel was a large one, and heavily laden.

As those in peril caught sight of the Mermaid settling down into the water, apparently from the clouds, their fears gave place to astonishment. So great was this that they ceased their cries of terror. Then, as they saw that the strange craft navigated the ocean, for the engines were started aboard the Mermaid, they began to call for help.

CHAPTER VIII

THE FLYING MERMAID DISABLED

"WE'LL save you!" shouted Mr. Henderson, who was on the deck, while Mark was steering the craft. "Hold on a few minutes longer and we'll be alongside!"

"They're real! They're real!" some of those aboard the burning ship could be heard to shout. Evidently more than one of them had taken the Mermaid for a delusion of their fear-crazed brain.

"They are real persons!" they called again and again. "They are coming to save us!"

Mr. Henderson ran his ship as near the burning craft as he dared. Then he called to the crew to leap into the water and swim to him. He, with Washington, Jack, Bill and Tom, stood ready to haul aboard any who were too weak to help themselves.

In a few minutes all of those left alive on the sailing vessel— fourteen in all— had come safely aboard the Mermaid. The ship was now completely enveloped in flames.

"Are there any more left on her?" asked Mr. Henderson of one who appeared to be a mate of the burning craft.

"Not a soul!" was the answer. "The captain and ten men perished in the flames. The fire broke out a week ago in the lower hold. We fought it as well as we could but it got the best of us. Then it suddenly broke through the decks, almost like an explosion, a little while ago, and the captain and others were lost, and so were our small boats. We managed to get aft but were about to give up when you appeared."

"What ship is it and where are you from?"

"The Good Hope, laden with logwood, hides, jute and other materials from South America," the mate answered. "We were bound for New York."

"It is more like the Last Hope instead of the Good Hope," observed Mr. Henderson in a quiet voice, as he saw the flames mount higher and higher over the ship. A few seconds later the craft seemed rent by an internal explosion. It appeared to break in two parts, and, amid a shower of sparks and a cloud of black smoke, the vessel sank under the water and was seen no more.

The rescued men turned to behold the final end of their ship. They betrayed no particular emotion, and some of them even laughed, which the professor thought, at the time, was rather strange. But there was little opportunity for speculation. The men were in a sad plight. Few of them had more than the clothes they stood in, though each one wore about his waist a belt, and all of them seemed to guard the leather circlets jealously.

The professor and his crew were soon busy supplying remedies for burns, since several of the men were seared by the flames. Then, as it was learned they had eaten nothing for many hours, it having been impossible to use the galley, a meal was prepared and the survivors of the wreck were well fed.

The hunger of the newcomers having been appeased, they showed much curiosity over the strange craft that had so opportunely come to their rescue. Most of the sailors were ignorant men, and the professor had little fear of them learning anything concerning his secrets. He explained briefly about the Mermaid, but said nothing of whither she was bound.

The addition of fourteen men to the rather small accommodations of the Mermaid was a serious matter to consider. The ship was able to hold them all, and even to sail through the air with them, since Mr. Henderson had provided an excess of power. But it was going to be a problem to feed so many, and still save enough provisions, for the long voyage which lay ahead.

However, Mr. Henderson felt his first duty to be toward his fellowmen, even if his voyage must be delayed, or given up for a time, while he got more provisions. There would be no sleeping quarters for the sailors, but when this was explained to them they cheerfully said they would sleep on deck if necessary. In fact some of them had to, but as the weather was warm and clear this was no hardship. A few found quarters in the engine room and other apartments of the Mermaid.

Finding, after an examination, that his ship was in good order save for the broken gas apparatus, Mr. Henderson gave orders to proceed along the surface of the ocean. The sailors wanted to see how it felt to mount into the air, but Mr. Henderson, refused to attempt a flight until he had made complete repairs, and this would take a day or more.

At this there appeared to be some discontent among the survivors, and they muttered to each other as they stood in a group on deck. But the professor and his assistants were too busy with their preparations for fixing the break to notice this.

While the men were gathered in a knot near the after part of the small deck, the mate separated from them, and, coming close to where Mark was standing, unscrewing some of the broken parts of the pipe said, in a low voice.

"Tell the captain to watch out."

"What do you mean?" asked Mark quickly.

"Hush! Not so loud!" the mate exclaimed. "If the men hear me talking to you, or see me, they may kill me. Tell the captain to look out; that's all. Be on guard, and watch the engine room carefully."

"But why— ?" Mark began, when, turning suddenly, the mate left him. It was well he did so, for, at that instant, one of the sailors, who had observed the two conversing, strolled in their direction.

Much alarmed, Mark sought Mr. Henderson and told him what he had heard.

"I suppose the fire may have turned the poor man's head," the scientist said. "I wonder if he thinks the men I rescued would mutiny and take possession of my ship? If they did they would not know how to work it, so what good would it do?"

"Hadn't we better look out?" asked Mark.

"I'm not afraid," replied the professor. "I will be too busy the next few days, repairing the break, to think of anything else. Besides, what would they want to harm us for? Didn't we save their lives?"

Seeing the scientist placed no faith in what the mate had said, Mark went back to his task.

It soon became too dark to work, and it was decided, after supper, to halt the ship until morning as it would be less risky.

Mark did not sleep well, his dreams being disturbed by visions of pirates and black flags. But morning came and nothing had developed. The men seemed to recover their spirits with daybreak, and mast of the crew, after breakfast, greeted Mr. Henderson pleasantly, and asked to be allowed to help fix the ship.

It took the skilled labor of the professor, Washington and the boys to mend the break, and, even at that, it was four days in the repairing. But at last the final bolt was in place, and the Mermaid was able to resume her trips through the air.

"We will rise the first thing in the morning," said the professor to Mark and Jack that night. "I am anxious to see how the ship behaves with a big load aboard."

CHAPTER IX

THE MUTINY

MARK was awakened that night by feeling some one trying to turn him over. At first he thought it was Jack, and sleepily muttered that he wanted to be let alone.

"Sorry I can't oblige ye, my hearty!" exclaimed a rough voice in his ear, "but I got particular orders t' tie you up!"

At that Mark tried to sit up, but he found he could not. He discovered that he was closely bound with many turns of a rope, while in front of his bunk stood one of the rescued sailors.

"There," said the man, with a final tightening of the ropes. "I guess you're safe."

"What's the matter? What does it all mean?" asked Mark, much bewildered.

"It means that we have possession of the ship," the sailor answered, "and, if you're wise you'll not make a fuss. It wouldn't do any good, anyhow, as all your friends are in the same condition."

Then, picking Mark up, as if he was a baby, the man slung him over his shoulder and carried him to the living room. There Mark saw Jack, the professor, Washington, and the others similarly bound.

"Do you realize what you are doing?" asked the professor angrily of his captors. "You are mutinying, and are liable to severe punishment."

"If they ever get us," added one of the men. "We've got the ship now, and we mean to keep her. You'll have to run her or show us how."

"Never!" cried the professor.

"I guess he will when he feels this," said one of the men, as he dragged from a recess two wires. "I happen to know something of electricity, and when he feels these perhaps he'll change his mind. I'll start the dynamo."

The sailor showed that he was acquainted with machinery, for soon the hum of the electric apparatus was heard.

"Now to make him tell!" the man with the wires exclaimed, advancing toward the professor, who turned pale.

"Stop! You must not torture the old man!" cried a voice, and the mate of the Good Hope stepped in front of the sailor with the electrified wires.

"Who's going to stop me?" asked the man.

"I will. It's not necessary," the mate went on quickly. "If we make him weak we may kill him, and he can not tell us what we want to know. One of the boys can tell us how to run the ship."

The mate came quickly over to where Mark lay, and whispered:

"Consent to tell. It is the only way of saving his life. Tell 'em how to raise the craft. Then leave all to me. I will save you all and the ship, too, if I can. But consent."

Mark nodded his head, and the mate cried:

"I knew I could fetch 'em. I have hypnotic power. This boy will raise the ship for us. Loosen his bonds, some of you."

Satisfied that they were now on the way to experiencing a new sensation, the sailors took the ropes off Mark's arms and legs, and he was allowed to rise. With a reassuring nod toward the professor he led the way to the engine room, followed by half the men. He resolved to start the gas machine slowly, so as to make the upward trip last longer, thinking before it had gone far, some way of escape from the mutineers might be found.

While a crowd of the sailors stood near him, Mark operated the machinery in the engine room that started the gas generating, and set the negative gravity apparatus working.

"You'd better not try any tricks on us," said one of the men in an ugly tone of voice.

"I'm not going to," replied Mark. "If you go out on deck you will soon see the ship leaving the water and mounting into the air."

"Some of you go," ordered a man with a big bushy red beard. "See if the ship rises. When she begins to go up sing out. I'm going to stay here and see how the young cub does it so I can work it myself."

Obeying the red-bearded man, who seemed to be a leader, several of the sailors went out on the deck. It was quite dark, but there was a phosphorous glow to the water which made the rolling waves visible.

The gas was being generated, as could be told by the hissing sound. Mark watched the machinery anxiously, for he knew much depended on him, and the professor was not at hand to guide and instruct him. He watched the dial of the gage which registered the gas pressure and saw it slowly moving. In a little while it would be at the point at which the ship ought to rise.

Presently a quiver seemed to run through the Mermaid. Now a shout came from the watchers on deck.

"She's going up!"

The ship was indeed rising. The red-bearded man, who was addressed as Tony, ran from the engine room to the deck. He saw that the ship was now ten feet above the water. Back he came to where Mark stood by the gas machine.

"Lucky for you that you didn't fool us, lad," he said with a leer. "See that you mind me hereafter. Now show me how the shebang works."

When the ship had risen as far as Tony desired he made Mark send it straight ahead. The boy adjusted the air tube to carry the craft toward the south, but Tony, seeing by a compass in which direction they were headed, ordered Mark to steer due east.

"Fix things so they will stay so, too," added Tony. "I don't want to stop until I get a thousand miles away. Then we'll come down, sail to some sunny island, and enjoy life."

Mark locked the steering apparatus so as to keep the Mermaid headed due east.

"Now you can go back to your friends," Tony said. "When I want you I'll send for you."

With a heavy heart Mark rejoined the professor and others. He found them with their bonds removed. But to guard against their escape several men were on watch outside the door.

"What are they doing?" asked the professor eagerly as Mark entered, and the boy told him what had taken place.

"They will ruin my ship and spoil the whole trip," cried the old scientist. "Oh, why did I ever go to the rescue of the scoundrels?"

"Never mind," said Jack. "Perhaps we may yet outwit them."

Morning came at last. The ship was still shooting forward at fast speed, in an easterly direction. The sailors had learned, in their short stay aboard, where the food and stores were kept, and they lost little time in getting breakfast. They sent same in to their captives, including a big pot of hot coffee, and, after partaking of this the professor and his friends felt better.

The mate of the Good Hope came in to help clear away the dishes. As he passed Mark he slipped into the boy's hand a note.

"Don't read it until you are alone," he said in a low voice, as he hurried from the room.

As soon as the other sailors had left, Mark glanced at the slip of paper. It bore these words:

"Open when you hear three raps, then two, then three, and keep silent."

"What is it?" asked Mr. Henderson.

Mark showed him the paper.

"I wonder what it means," the boy said.

"Do you think he is a friend of ours?" the professor asked.

Mark told him of the mate's conversation the night previous.

"I think we can trust him," the scientist went on. "He must intend to pay us a visit when the others are asleep. When we hear the knocks as he specifies we must open the door and let him in."

All that day the captives were kept in the living room. Once or twice Mark was sent for to make some adjustment to the machinery, but the apparatus, for the most part, was automatic, and needed little attention. The professor, as well as the others, were all impatience for the promised visit of the mate. Still they felt he would not come until night.

In fact it was long past midnight before Mark, Jack and the professor, who were anxiously listening, heard the three raps, then two, then three more. Mark quickly opened the door, and the mate stepped inside, holding his finger to his lips as a sign of caution. Old Andy, Washington, Bill and Tom had fallen asleep.

"I have only time for a few words," the mate said. "I am closely watched. Tony mistrusts me. I will save you if I can."

"Why have they repaid my kindness with such actions?" asked Mr. Henderson.

"Because they are desperate men," replied the mate. "They are nothing more than pirates. They mutinied on the other ship, killed the captain and those of the crew who would not join them, and started off to seek their fortunes. I pretended to join them to save my life, but I have only been watching for a chance to escape.

"Because of lax discipline the ship was sent on fire. We tried to put it out but could not. The rest you know."

"I heard them plan to capture this airship, but could do nothing to stop them. Then I resolved to pretend to act with them. They fear pursuit for their other mutiny, and are anxious to get as far away as possible."

"Do you think they will abandon the ship in a little while?" asked the professor hopefully.

"I'm afraid not," answered the mate. "I think they want to get rid of all of you, so they can sail about as they please. Tony is a smart man. He could soon learn to run this ship, he thinks."

"I doubt it," Mr. Henderson answered. "But how are you going to help us?"

"I have not fully made up my plans," the mate answered. "However I wanted you to know I would do my best to save you. Now I must go. Be on the watch and when I can I will let you know what I have decided on. I will hand Mark a note when I bring your meals, just as I did to-day. I think——"

"Hark! What was that?" asked the professor.

There was a noise outside the door, as if some one was listening.

"Put out the lights!" whispered the mate, and Jack switched off the electric incandescents.

A knock sounded on the door and the voice of Tony called:

"Mark! Come here! I want you to look at the gas machine. It has stopped working, and we are falling!"

CHAPTER X

FOOLING THEIR ENEMIES

MARK hurried into the corridor, taking care to close the door after him, so Tony could get no glimpse of the mate who had risked so much to save his friends. But he need not have been alarmed for the leader of the mutineers was too excited over the stopping of the gas apparatus to give any heed to who was in with the captives.

"Do you think you can fix it?" he asked the boy.

"I guess so," Mark replied confidently. "If I can't there is no danger, for we will fall gradually and land in the water."

"But I don't want to do that," Tony objected. "I want to keep on through the air."

Mark did not reply. By this time he was at the gas machine. He soon saw nothing was the matter save that new material must be placed in the retort where the vapor was generated. He refilled it, the gas was manufactured once more, and the ship began to rise.

"I will know how to do it next time," Tony said with a grin. Mark realized that every time he showed the leader of the mutineers something about the ship it was putting the professor and his friends more and more into the power of the scoundrels. But there was no help for it.

The ship was still plunging ahead, and kept about a mile above the earth. As there was no further need of Mark, he was told he could go back to his friends. When he reached the room where they were held prisoners, he found the mate had gone away, promising again to do all he could for them.

The next night, which it seemed would never come, for the day, locked as the captives were in their room, seemed endless, finally closed in. Mark, Jack and the professor were anxious to know whether the mate would pay them another visit. As for Andy, Tom and Bill, while they were interested in the ship, and wanted to be free from the power of the mutineers, they did not lose any sleep over it.

Shortly after midnight, there came again the peculiar knock, and the mate entered the room. He seemed much excited over something, and, as soon as the portal was securely closed he said to Professor Henderson:

"Is there an island any where near here where men could live for a time?"

"What do you mean?" asked the scientist. "Do you want us to desert the ship and leave these scoundrels in charge?"

"Nothing of the sort," replied the mate, who, had said his name was Jack Rodgers. "But first answer my question. A great deal may depend on it."

Seeing Rodgers was in earnest, the professor looked over some maps and charts, and announced that they were within a few hundred miles of a group of islands.

"When would we reach them?" was Rodgers's next question.

Mr. Henderson made a few rapid calculations on a piece of paper.

"At the present rate of sailing," he said, "we should be there about ten o'clock to-morrow. That is, provided the ship does not slacken speed or increase it."

"There is no danger of either of those two things happening," said the mate. "Tony is too afraid of the machinery to do anything to it. So you may safely figure that our speed will continue the same."

"Then I can guarantee, with all reasonable certainty," the professor said, "that about ten o'clock to-morrow we will be less than a mile from the islands. They are a group where friendly natives live, and where many tropical fruits abound. One could scarcely select a better place to be shipwrecked. But I hope the plans of Tony and his friends do not include landing us there."

"No, nothing like that," the mate answered. "Quite the contrary. But I had better be going. I will try and see Mark some time to-morrow. Tony does not mind when I speak to him."

With this Rodgers left the captives, as he heard some of the sailors moving about and did not want to be discovered. The professor and the boys wondered what the mate's plan might be, but they had to be content to wait and see.

The night passed without incident. About nine o'clock the next morning the mate came to the door of the room where the professor and his friends were prisoners. He made no secret of his approach, but knocked boldly.

"Tell Mark I want to see him," he said, as the professor answered. "All of you keep quiet," he added in a whisper. "There may be good news soon."

Mark slipped from the room. He followed the mate to the upper deck which, at that time was deserted as all the sailors were in the dining room eating, which practice they indulged in as often as they could.

"I have a plan to get rid of these rough men," the mate said to Mark. "It may work, and, again it may not. At any rate it is worth trying, It all depends on you with what help I can give you."

"I'm willing to do my share," Mark said, and for the next ten minutes the boy and the mate were in earnest conversation.

It was about thirty-five minutes later when there arose a sudden commotion in the ship. Mark had returned to his friends and the mate had disappeared. The confusion seemed to come from the engine room where Tony had posted some of his men.

"We're falling down! We'll all be killed!" shouted the men. "The ship is falling into the sea!"

"What is the trouble?" asked the professor as he heard the commotion.

"It is part of the mate's plan," said Mark. "He told me to tell you to do nothing. If Tony or any of the other men come to you just refer them to me."

Two minutes later Tony came rushing into the apartment where the captives were held prisoners.

"Here! Come quickly, Mark!" he exclaimed. "Something has gone wrong with the gas machine again, and you must come and fix it before we are all dashed to pieces!"

With every appearance of haste Mark rushed from the apartment, following Tony. The latter led the way to the engine room.

"Can anything be done?" he asked.

Mark took a survey of the machinery.

"It is too late," he said as though much excited. "The ship is falling down toward the sea with terrific force."

It needed but a glance at the height gage to show this. The pointer was revolving rapidly about the face of the dial.

"Will the ship stand the blow?" asked Tony.

"Not at the rate it is falling," replied Mark. "She will go all to pieces when she strikes the water, and she may explode!"

"What are we to do then?" asked the leader of the mutineers.

"We must save ourselves!" cried the mate, running in at this juncture. "Let our prisoners shift for themselves as best they can. Let's all leap into the sea. There we at least have a chance for our lives. But if we stay on this ship we will all be drowned like cats in a bag."

"What do you propose?" asked Tony, his face white with fear.

"When the ship comes near enough the surface of the water to make it safe we should all drop overboard!" the mate exclaimed. "We are near some islands, I understand, and we can thus save our lives by swimming ashore."

This plan seemed to meet with instant favor, and a little later there was a rush for the deck, as each one wished to be the first to escape from the boat they believed to be doomed.

Lower and lower fell the Mermaid. She was like a wounded bird which the shot of the hunter has crippled. Down and down she fluttered.

By this time all the sailors, save the mate were on deck. He and Mark remained in the engine room.

"Don't let her get too low," the mate whispered.

"I'll watch out," Mark replied. "I want to give them a good scare while I'm at it."

The ship was now within fifty feet of the water. There was a cry of terror from the sailors. Some of them leaped over the rail and started to swim ashore, as the ship was by this time close to a group of islands.

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