With a sharp jerk the horse was pulled to a halt.

Andy heard the two men on the seat jump to the ground. He knew that
their motive was robbery. He knew further that this was another plot of
bad Jim Tapp, the friend and associate of criminals.

In another minute the men would open the wagon doors, pull him out,
perhaps assault him, take the registered mail and fly.

Andy had only a second to act in. He theorized that the wagon, following
the alley, was now probably halted in some secluded side lane.

To escape the clutches of the would-be robbers was everything. Andy,
having no weapon of defence, was no match for them.

"If the rig once reaches the crowded streets, I'm safe," thought Andy.

Then he carried out a speedy programme. Forming his lips in a pucker, as
he had seen Ripley do, Andy uttered two sharp whistles, then a clear,
resounding hiss.

"Thunder!" yelled a voice outside.

"Ouch!" echoed a second.

The horse had given one wild, prodigious bound at hearing the familiar
signal.

The vehicle must have grazed one of the thieves. Its front wheels
knocked the other down.

"My! I'm in for it," instantly decided Andy.

For, swayed from side to side, he realized that the circus wagon was
dashing forward at runaway speed.

CHAPTER XVII

A WILD JOURNEY

Andy Wildwood found himself in a box, in more ways than one.

Judging from the sounds he had heard, the men bent on securing the
registered mail pouch had been baffled. The old circus horse had started
on a sudden and surprisingly swift sprint. From the feeling of turns,
jerks and swings, Andy decided that within four minutes the rig had left
the post-office fully half-a-mile to the rear.

"I've started the horse all right," said Andy. "Old Ripley's signal has
acted like a charm. How to stop the animal, though. That is the present
question?"

Andy ran at the two rear doors of the wagon. He steadied himself, arms
extended so as to touch either side of the box. Then he gave the doors a
tremendous kick with the sole of his shoe.

The doors did not budge. He felt over their inner surfaces where they
came together. The lock was set in the wood. They could be opened only
from the outside.

The wagon box had one aperture, Andy discovered. This was a small
ventilating grating up in one corner above the seat.

He sprang up on the newspaper bag. This brought his eyes on a level with
the grating. It was about four by six inches, with slanting sl