What was the mounted soldier on the moor watching out for ?

The mounted soldier was watching for an escaped convict.

A steep curve of heath-clad land, an outlying spur of the moor, lay in front of us. On the summit, hard and clear like an equestrian statue upon its pedestal, was a mounted soldier, dark and stern, his rifle poised ready over his forearm. He was watching the road along which we travelled.

"What is this, Perkins?" asked Dr. Mortimer.

Our driver half turned in his seat.

"There's a convict escaped from Princetown, sir. He's been out three days now, and the warders watch every road and every station, but they've had no sight of him yet. The farmers about here don't like it, sir, and that's a fact."