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’ 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.

suddenly, from the engine room the mate rushed.

“jump! jump for your lives!” he exclaimed. “the ship is about to blow up!”