He was just leaving the road when he heard a great halloo behind him.
The real officer must have appeared on his heels, and the sentry's doubts had been stirred.

A whistle was blown, and, looking back, Peter saw lanterns waving in the gale.

They were coming out to look for the duplicate.
He stood still for a second, and noticed the lights spreading out south of the road.

He was just about to dive off it on the north side when he was aware of a difficulty.

On that side a steep bank fell to a ditch, and the bank beyond bounded a big flood.