small, dapperly curled mustache.

"I think, Senor Kelly, you have been up to something yourself."

"Well, you know what I told you. We want that five-hundred-dollar prize,
Carlos, and by the looks of things if we don't do something those kids
are likely to get it."

"They have fine machines," agreed the other.

"Yes; and they are equipped with a balancing device that makes them much
more reliable than ours."

"A balancing device!" exclaimed the Cuban, as the two men got into the
car, a small yellow runabout of racy appearance.

"That's what I said, and it's a good one, too. I read an account of it
in an aviation paper; but the description was too sketchy for me to see
how the thing was worked."

"Those boys must be wonders."

"I'm afraid they are. That's why we've got to be careful of them. But
I've got a plan to fix them, the whole lot of them."

"What is it?"

"I'll tell you as we go along."

As the car rolled past the group of aeroplanes with Jake faithfully
standing guard over them, Kelly hailed him in a suave voice.

"Any idea where the young folks have gone?"

Jake, who had no idea that Kelly had a sinister motive in asking the
question, replied readily enough.

"Yes, they've gone into Millbrook to get another spark plug. Something
happened to one of the plugs of that red machine yonder."

"All right. Thanks."

Kelly drove on.

"Do you know what happened to that plug, Carlos?" he asked, as they
reached the open road and bowled forward at a good speed.

"I've got a pretty good guess. It was not altogether an accident, eh?"

"An accident, well, it was, in a sense. I happened to be near that
machine with a monkey wrench and in some way was careless enough to
let it put that plug out of business."

Both men laughed heartily, as if Kelly's rascally act had been the most
amusing thing in the world.

"You are a genius," declared Le Roy.

"Well, I reckon I know a thing or two," was the modest response;
"besides, I need that money."

"But what is your plan?"

"I'll tell you as we go along. Drive fast, but don't keep so close to
that other car that they can get sight of us."

"Not much fear of that. They had a long start of us and are out of
sight now."

"So much the better. It doesn't interfere with my plans a bit, provided
they take the same road back."

"What do you mean to do?"

"Are you good with a shovel?" was the cryptic reply.

"I don't understand you, I must say."

"You wil