“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.