ed out as Bill came up.
"There is some monkey work going on here. Somebody is foolin' with your
plane. We lock the hangar every night, and someone is always around all
day, but allee samee, as the Chinee says, allee samee, _somebody_ gets
that machine all out of tune as soon as I get it right. And it's no
fool, either. Whoever is tinkering with it understands that type of
flyer down to the ground. He knows just what to discombobolate in order
to make us the most trouble."

Ernest laid a hand on the man's shoulder.

"The thing is, Tom, we will have to look for a motive. Now what earthly
motive can anyone have?"

"Search me!" said Tom. "Whoever is doing it doesn't want to hurt Mr.
Jardin here, because the damage is always to something that will keep
the plane from rising. For instance, yesterday the spark plugs had mud
in 'em. Before that, the exhaust wouldn't work; one time the priming pin
was clean gone; once the dust cap was half off; then the drum control,
warping the wings got on the blink. I tell you, it is enough to drive
anybody crazy! Lately we have took to sleeping in the hangar, but things
happen just the same."

"I am afraid it is a case of poor construction," said Ernest. "There is
no one who would pick on Jardin like that. Why don't they do something
to _my_ plane? Jardin has no enemies. He has invited about every boy in
the whole school to ride with him."

"Certainly I have!" said Jardin. "I guess I more than pay my way around
this place! I have stood treat oftener than any one in the whole school.
It doesn't pay to be an enemy of mine."

Ernest frowned. "It is not a case of treating," he said sternly. "It is
merely that no special fellow here owes you a grudge. So, as they have
no reason to owe me a grudge either, I don't see why I do not come in
for some of the damage, or you, Tom. There are only three planes here.
Why do they pick on Jardin? It beats me! There is something back of this
that I do not understand."

Bill, cautiously studying Frank, said to himself, "There will be trouble
with the other planes to-morrow. The conversation has given Frank an
idea."

"Well," said Jardin mysteriously, "after today I don't care what
happens. Come along, Tom, and see if she is all to the bad today."

Together they walked over to the hangar and wheeled Jardin's plane out
into the field. It could not be made to start. Tom gave a short, hard
laugh.

"I am beaten!" he declared. "The screws are all loose o