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.