“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 ornithological 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!”