"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!"

CHAPTER X

FOOLING THEIR ENEMIES

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's 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 pa.s.sed 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.