"Are there plenty of boards, planks and boxes around your barn, Fenn?"
asked Frank.

"Lots of 'em."

"Then we'll set traps for our friends the enemy," said Frank. "They'll
walk right into them."

Frank explained his plan more in detail as the boys rowed down stream.
His idea was to build a series of traps all about the barn, covering
every approach. The traps would be made of boxes and boards, so arranged
that when a boy walked on them he would tumble off or slip into a box,
and the racket made would apprise those on watch, in the barn, of the
approach of the enemy. Then they could sally out, and, while the Upside
Down boys were in confusion, could easily disperse them.

"That's fine!" exclaimed Bart. "The very thing! We must get right to work
on it tonight."

That evening the four chums spent in the barn back of Fenn's house. There
was considerable hammering and pounding and fitting together of planks,
boards and boxes.

The next afternoon the four boys worked hard perfecting their
arrangements. There were four entrances to the barn, consisting of large
sliding doors in front and rear, and a small door that gave entrance to
the stable proper. The way to each of these was so arranged that any
persons passing along them would have considerable trouble in reaching
the structure. It was impossible to walk along them and not step on a