Tom's excited announcement startled Mr. Damon and the others as much as if the young inventor had informed them that the airship had exploded and was about to dash with them to the earth. The men leaped to their feet, and stared at the lad.

"A stowaway on board!" cried Mr. Damon. "Bless my soul! How did he--"

"Are you sure that message is straight?" asked Mr. Jenks. "Did Eradicate see the man?"

"He says he did," answered Tom. "The man is hidden away on board now--probably among the stores and supplies."

"Bless my tomato sauce!" exploded Mr. Damon. "I hope he doesn't eat them all up!"

"We must get him out at once!" declared Mr. Jenks.

"I knew something would happen on this voyage," came from Mr. Parker. "I predicted it from the first!"

Tom thought considerable, but he did not answer the scientist just then. Another communication was coming to him by wireless. He listened intently.

"Father says," the lad told his companions "that Eradicate only had a glimpse of the man at the last moment. He was looking from the rear store-room window--he's the same man who called on me that time--Rad remembers him very well."

"Bless my shoes! What's to be done?" inquired Mr. Damon, looking around helplessly.

"We must get him out, that's all," decided Mr. Jenks; with vigor. "Get him out and drop him overboard!"

"Drop him overboard!" cried Mr. Parker, in horror.

"Not exactly, but get rid of him," proceeded the diamond seeker. "That man is one of my enemies. He has been sent by the band of diamond makers hidden among the mountains, to spy on me, and, if possible, prevent me from seeking to discover their secret. He tried to work on Tom's Swift's fears, and frighten him from using his airship on this quest. Then, when he failed, the man must have sneaked into the shed, and hidden himself in the ship. We must get rid of him, or he may wreck the Red Cloud!"

"That's so!" cried Tom. "We must try to capture him. I think we had better--" the lad paused, and again listened to the wireless message. "Father says Eradicate saw the man have a gun, so we must be careful," the young inventor translated the dots and dashes.

"Bless my powder horn!" exploded Mr. Damon.

"We shall have to proceed cautiously then," spoke Mr. Jenks. "If he is like any others in the gang he is a desperate man."

"Better sneak up on him then, if we can," proposed Mr. Parker. "There are enough of us to cope with one man, even if he is armed. You have weapons aboard, haven't you?" he inquired of Tom.

"Yes," was the hesitating answer, "but I don't want to use them if I can help it. Not only because of the danger, and a dislike of shedding blood, but because a stray bullet might pierce the gas bag and damage the ship."

"That's so," agreed Mr. Jenks. "Well, I guess if we go at it the right way we can capture him without any shooting. But we must talk more quietly--we ought to have whispered --he may have heard us."

"I don't think so," replied Tom. "The storeroom is far enough off so that he couldn't hear us. Besides, the motor makes such a racket that he couldn't distinguish what we were talking about, even if he heard our voices. So, unless he heard the wireless working, and suspects something from that, he probably doesn't know that we are aware of his presence aboard."

"But why do you think he has remained quiet all this while, Tom?" asked Mr. Damon.

"Probably he wants to wait until the ship is farther out west," suggested Mr. Jenks. "Then he will be nearer his friends, and can get help, if he needs it."

"And do you really believe he would destroy the Red Cloud?" asked Mr. Parker.

"I think that all he is waiting for is a favorable chance," declared the diamond seeker. "He would destroy the craft, and us too, if he could prevent us from discovering the secret of Phantom Mountain, I believe."

"Then we must get ahead of him," decided Tom, quietly. "I have just flashed to dad a message, telling him that we will heed his warning. Now to capture the stowaway!"

"And while we're about it, give him a good scare when we do get him," suggested Mr. Jenks.

"How?" asked Tom.

"Threaten to drop him overboard. Perhaps that will make him tell how he happened to get in our ship, and what are the plans of the gang of diamond makers. We may get valuable information that way."

"I don't believe you can scare such fellows much," was Tom's opinion, but it was agreed to try.

"How are you going to capture him?" asked Mr. Parker. "If he has a gun it won't be any too easy to go in the storeroom, and drag him out."

"We'll have to use a little strategy," decided Tom, and then they discussed several plans. The one finally adopted was that Tom and Mr. Damon should enter the storeroom, casually, as if in search of food to cook for supper. They would discuss various dishes, and Mr. Damon was to express a preference for something in the food line, the box containing which, was well hack in the room. This would give the two a chance to penetrate to the far end of the apartment, without arousing the suspicions of the hidden man, who, doubtless, would be listening to the conversation.

"And as soon as we get sight of him, you and I will jump right at him, Mr. Damon," said Tom. "Jump before he has a chance to use his gun. Mr. Jenks and Mr. Parker will be waiting outside the room, to catch him if he gets away from us. I'll have some ropes ready, and we'll tie him up, and--well, we'll decide later what to do with him."

"All right. I'm ready as soon as you are, Tom," said the eccentric man. "Come ahead."

They went softly to the storeroom, and listened at the door. There was no sound heard save that made by the machinery.

"I wonder if he's really here?" whispered Mr. Damon.

"We'll soon find out," answered Tom. "Let's go in."

They entered, and, in pursuance of their plan, Tom and his friend talked of various foods.

"I think I'd like some of that canned lobster, with French dressing on," spoke the eccentric man.

"That's away in the back end of the room," said Tom, in a loud voice. "It's under a lot of boxes."

"Then I'll help you get it out! Bless my frying pan! but I am very fond of lobster!" exclaimed Mr. Damon, in as natural tones as was possible under the circumstances.

He and Tom moved cautiously back among the boxes and barrels. They were glancing about with eager eyes. Tom switched on an electric light, and, the instant he did so, he was aware of a movement in a little space formed by one box which was placed on top, of two others. The lad saw a dark figure moving, as if to get farther out of sight.

"I've got him!" cried Tom, making a dive for the shadow.

A moment later the young inventor was bowled over, as a dark figure leaped over his head.

"Catch him, Mr. Damon!" he cried.

"Bless my hatband! I--I--" Mr. Damon's voice ended in a grunt. He, too, had been knocked down by the fleeing man.

"Look out, Mr. Jenks!" cried Tom, to warn those on guard at the door of the storeroom.

There was the report of a gun, some excited shouts, and when Tom could scramble to his feet, and rush out, he beheld Mr. Parker calmly sitting on a struggling man, while Mr. Jenks held a gun, that was still smoking.

"We caught him!" cried the scientist.

"Anybody hurt?" asked Tom, anxiously.

"No, I knocked up his gun as he fired," explained Mr. Jenks. "Where are the ropes, Tom?"

The cords were produced and the man, who had now ceased to struggle, was tightly bound. He uttered not a word, but he smiled grimly when Mr. Damon remarked:

"I guess I'll go back in the storeroom, Tom, and see how much food he ate."

"Oh, I guess he didn't take much," declared the lad. "He wasn't there long enough."

"Well, Farley Munson, so it's you, is it?" asked Mr. Jenks, as he surveyed the prisoner.

"Do you know him?" asked Tom, in some surprise.

"He was in with the diamond makers," said Mr. Jenks. "He was one of those who took me to the secret cave. But it will be the last time he ever goes there. How high up are we, Tom?"

"About two miles. Why?"

"I guess that will be far enough to let him fall," went on the diamond seeker. "Come on, Mr. Damon, help me throw him overboard!"

"You--you're not going to throw me over--with the airship two miles high; are you?" gasped the man.

"Will you tell us what we want to know, if we don't?" asked Mr. Jenks.

"What do you want to know?"

"How you got aboard, and what your object was in coming."

"That's easy enough. I had been hanging around the shed for several days, watching a chance to get in. Finally I saw it, when that colored man went to feed his mule, and I slipped in, and hid in the airship. The stores were all in then, and I stowed myself away among the boxes. I had food and water, so I didn't touch any of yours," and he looked at Mr. Damon, who seemed much relieved.

"And what was your object?" demanded Mr. Jenks.

"I wanted to prevent you from going to Phantom Mountain."

"How?"

"By destroying the airship if need be. But I hoped to accomplish it by other means. I would have stopped at nothing, though, to prevent you. You must keep away from there!"

"And if we refuse?" asked Tom.

"Then you'll have to take what comes!"

"But not from you!" exclaimed Mr. Jenks. "We're going to get rid of you."

The man's face showed the alarm he felt.

"Oh, don't worry," said Mr. Jenks, quickly, "we're not going to toss you overboard. We're not as desperate as your crowd. But we're going to get rid of you, and then go on before you can send any word to your confederates. We'll put you off in the most lonesome spot we can find, and I guess you'll be some time getting back to civilization. By that time we'll have the secret of the diamonds."

"You never will!" declared the man, firmly. And he would say nothing more, though by threats and promises Mr. Jenks tried to get from him something about the men in with him, and where the cave of the diamonds was located.

Heavily bound with ropes the man was locked in a small closet, to be kept there until a favorable spot was reached for letting him go. Mr. Jenks' plan, of dropping him down in some place where he would have difficulty in sending on word to his confederates was considered a good one.

Three days later, in crossing over a lonely region, near the Nebraska National Forest, Farley Munson, which was one of the names the spy went by, was dropped off the airship, when it was sent down to within a few feet of the earth.

"It will take you some time to get to a telegraph office," said Mr. Jenks, as a package of food, and a flask of water was tossed down to the stowaway. He shook his fist at those in the airship, and shouted after them:

"You'll never discover the secret of Phantom Mountain!"

"Yes, we will," declared Tom, as he sent the Red Cloud high into the air again.