But Roy determined to have the machines closely guarded.  The situation was explained to Miss Prescott, and while she and her small protege took seats in the grand stand Jake was detailed to guard the aeroplanes.  This done, the boys got into the machine and prepared to start for town.  But the girls interfered.

“Aren’t you going to take us along, you impolite youths!” cried Bess.

“Oh, certainly, your company is always charming,” returned Jimsy, with a low bow.

“Of course it is, but you wouldn’t have asked us to come if we had not invited ourselves,” declared Peggy vehemently.

“How can you say so?  Our lives would be a dry desert without the girl aviators to liven things up,” declared Jimsy.

“Jimsy Bancroft, if you are going to get poetical you’ll leave this car,” cried Jess.

“That’s just it,” declared Jimsy, “girls can cry their eyes out over romantic heroes, but when a regular fellow starts to get ‘mushy’ they go up in the air.”

Amidst the chorus of protestations aroused by this ungallant speech Roy started the car.  Swiftly it sped out of the grounds; but not so swiftly that the keen eyes of Lish Kelly did not see it.

He called Herman Le Roy, the Cuban aviator, to him.

“Le Roy, you are not in the altitude contest,” he said, “hop in my car with me and we’ll follow those kids.  They’re up to something.”

The Cuban looked at him and smiled, showing two rows of white teeth under his 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.”