“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.