Ned repeated the message breathlessly.
"Short circuit!" gasped Tom. "Run on storage battery! I'll have to see to that. Take the wheel somebody!"
"Wouldn't it be better to turn about, and run before the wind, so as not to put too great a strain on the machinery?" asked Lieutenant Marbury.
"Perhaps," agreed Tom. "Hold her this way, though, until I see what's wrong!"
Ned and the government man took the wheel, while Tom hurried along the runway leading from the pilot-house to the machinery cabin. The gale was still blowing fiercely.
The young inventor cast a hasty look about the interior of the place as he entered. He sniffed the air suspiciously, and was aware of the odor of burning insulation.
"What happened?" he asked, noting that already the principal motive power was coming from the big storage battery. The shift had been made automatically, when the main motor gave out.
"It's hard to say," was the answer of the chief engineer. "We were running along all right, and we got your word to switch on more power, after the turn. We did that all right, and she was running as smooth as a sewing-machine, when, all of a sudden, she short-circuited, and the storage battery cut in automatically."
"Think you put too heavy a load on the motor?" Tom asked.
"Couldn't have been that. The shunt box would have taken that up, and the circuit-breaker would have worked, saving us a burn-out, and that's what happened—a burn-out. The motor will have to be rewound."
"Well, no use trying to fight this gale with the storage battery," Tom said, after a moment's thought. "We'll run before it. That's the easiest way. Then we'll try to rise above the wind."
He sent the necessary message to the pilot-house. A moment later the shift was made, and once more the Mars was scudding before the storm. Then Tom gave his serious attention to what had happened in the engine room.
As he bent over the burned-out motor, looking at the big shiny connections, he saw something that startled him. With a quick motion Tom Swift picked up a bar of copper. It was hot to the touch—so hot that he dropped it with a cry of pain, though he had let go so quickly that the burn was only momentary.
"What's the matter?" asked Jerry Mound, Tom's engineer.
"Matter!" cried Tom. "A whole lot is the matter! That copper bar is what made the short circuit. It's hot yet from the electric current. How did it fall on the motor connections?"
The engine room force gathered about the young inventor. No one could explain how the copper bar came to be where it was. Certainly no one of Tom's employees had put it there, and it could not have fallen by accident, for the motor connections were protected by a mesh of wire, and a hand would have to be thrust under them to put the bar in place. Tom gave a quick look at his men. He knew he could trust them—every one. But this was a queer happening.
For a moment Tom did not know what to think, and then, as the memory of that warning telegram came to him, he had an idea.
"Were any strangers in this cabin before the start was made?" he asked Mr. Mound.
"Not that I know of," was the answer.
"Well, there may be some here now," Tom said grimly. "Look about."
But a careful search revealed no one. Yet the young inventor was sure the bar of copper, which had done the mischief of short-circuiting the motor, had been put in place deliberately.
In reality there was no danger to the craft, since there was power enough in the storage battery to run it for several hours. But the happening showed Tom he had still to reckon with his enemies.
He looked at the height gauge on the wall of the motor-room, and noted that the Mars was going up. In accordance with Tom's instructions they were sending her above the storm area. Once there, with no gale to fight, they could easily beat their way back to a point above Shopton, and make the best descent possible.
And that was done while, under Tom's direction, his men took the damaged motor apart, with a view to repairing it.
"What was it, Tom?" asked Ned, coming back to join his chum, after George Ventor, the assistant pilot, had taken charge of the wheel.
"I don't exactly know, Ned," was the answer. "But I feel certain that some of my enemies came aboard here and worked this mischief."
"Your enemies came aboard?"
"Yes, and they must be here now. The placing of that copper bar proves it."
"Then let's make a search and find them, Tom. It must be some of those foreign spies."
"Just what I think."
But a more careful search of the craft than the one Tom had casually made revealed the presence of no one. All the crew and helpers were accounted for, and, as they had been in Tom's service for some time, they were beyond suspicion. Yet the fact remained that a seemingly human agency had acted to put the main motor out of commission. Tom could not understand it.
"Well, it sure is queer," observed Ned, as the search came to nothing.
"It's worse than queer," declared Tom, "it's alarming! I don't know when I'll be safe if we have ghosts aboard."
"Ghosts?" repeated Ned.
"Well, when we can't find out who put that bar in place I might as well admit it was a ghost," spoke Tom. "Certainly, if it was done by a man, he didn't jump overboard after doing it, and he isn't here now. It sure is queer!"
Ned agreed with the last statement, at any rate.
In due time the Mars, having fought her way above the storm, came over Shopton, and then, the wind having somewhat died out, she fought her way down, and, after no little trouble, was housed in the hangar.
Tom cautioned his friends and workmen to say nothing to his father about the mysterious happening on board.
"I'll just tell him we had a slight accident, and let it go at that," Tom decided. "No use in causing him worry."
"But what are you going to do about it?" asked Ned.
"I'm going to keep careful watch over the aerial warship, at any rate," declared Tom. "If there's a hidden enemy aboard, I'll starve him out."
Accordingly, a guard, under the direction of Koku, was posted about the big shed, but nothing came of it. No stranger was observed to sneak out of the ship, after it had been deserted by the crew. The mystery seemed deeper than ever.
It took nearly a week to repair the big motor, and, during this time, Tom put some improvements on the airship, and added the finishing touches.
He was getting it ready for the final government test, for the authorities in Washington had sent word that they would have Captain Warner, in addition to Lieutenant Marbury, make the final inspection and write a report.
Meanwhile several little things occurred to annoy Tom. He was besieged with applications from new men who wanted to work, and many of these men seemed to be foreigners. Tom was sure they were either spies of some European nations, or the agents of spies, and they got no further than the outer gate.
But some strangers did manage to sneak into the works, though they were quickly detected and sent about their business. Also, once or twice, small fires were discovered in outbuildings, but they were soon extinguished with little damage. Extra vigilance was the watchword.
"And yet, with all my precautions, they may get me, or damage something," declared Tom. "It is very annoying!"
"It is," agreed Ned, "and we must be doubly on the lookout."
So impressed was Ned with the necessity for caution that he arranged to take his vacation at this time, so as to be on hand to help his chum, if necessary.
The Mars was nearing completion. The repaired motor was better than ever, and everything was in shape for the final test. Mr. Damon was persuaded to go along, and Koku was to be taken, as well as the two government officials.
The night before the trip the guards about the airship shed were doubled, and Tom made two visits to the place before midnight. But there was no alarm.
Consequently, when the Mars started off on her final test, it was thought that all danger from the spies was over.
"She certainly is a beauty," said Captain Warner, as the big craft shot upward. "I shall be interested in seeing how she stands gun fire, though."
"Oh, she'll stand it," declared Lieutenant Marbury. The trip was to consume several days of continuous flying, to test the engines. A large supply of food and ammunition was aboard.
It was after supper of the first day out, and our friends were seated in the main cabin laying out a program for the next day, when sudden yells came from a part of the motor cabin devoted to storage. Koku, who had been sent to get out a barrel of oil, was heard to shout.
"What's up?" asked Tom, starting to his feet. He was answered almost at once by more yells.
"Oh, Master! Come quickly!" cried the giant. "There are many men here. There are stowaways aboard!"
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