Of a sudden he had his hands full trying to keep on the horse's back.

The steed was galloping along with a peculiar motion.
"Whoa! whoa, Jim!" yelled Andy, but Jim paid no attention.

He was off for a run and did not care what happened.
The blanket had not been securely fastened and before long it commenced to slip towards the horse's tail.

Andy tried to haul it back.

His efforts were but partly successful, and with an end of the blanket trailing around one of his hind legs, the steed became more unmanageable than ever.
On and on went horse and rider, until, in the distance, Putnam Hall loomed up.