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 some 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!”