Eight-year-old Josiah Sehl of Stafford County had a loud job to do yesterday, and he approached it with all his lung.

Josiah was one of dozens of volunteers working the obstacle-course portion of the 19th annual Police Equestrian Competition, held this year at the Fredericksburg Fairgrounds. His task was to scare the bejabbers out of seasoned police horses.

Inside a moon bounce that was one of 10 obstacles set up to challenge horses and riders, Josiah shrieked and sprang on cue.

Many of the 112 horses--trained and tested by years of police service in departments as far away as Toronto--are unfazed by traffic and riots and gunfire.

But Josiah was another matter.

The freckled Cub Scout from Stafford got the job because his mother, Donna Sehl, is a part-time 911 communications officer for the meet's host, the Fredericksburg Police Department.

"My mom told me the rules first," Josiah said as he sipped a Coke during a short break. "I know when to scream and stuff, and jump around."

The bouncing, bellowing boy was just one of the challenges designed to approx-imate real mounted-police situations.

To enter the competition ring, horses had to push a "riot ball," sort of a beach ball on steroids, that blocked the gate.

Once in the ring, mounted officers had to write a parking ticket and put it on a van's windshield. They had to speak to a person on a park bench who suddenly threw off a yellow tarp.

They had to walk their horses over a metal grate. Then horses and riders had to stand calmly beside the slamming sliding door of a beer truck.

Josiah and the moon bounce were next; the horses had to walk around the boy dynamo without stepping away.

Once past that challenge, officers had to stop, dismount, pick up a wallet and remount. Then they stood their horses next to the flashing lights and siren of a police car.

The officers had to maneuver their mounts next to a lidded trash barrel, throw in a paper bag and shut the lid. Finally, they had to weave their horses through a line of burning traffic flares.

Many of the day's challenges run counter to common horse sense.

Success hinged on horses' and riders' composure, training and trust.