Watch him.

He'll break out some day.' There were others more dangerous than he, because older in crime and ready for any desperate outbreak to change the monotony of long sentences.

These men soon divined Dan's mood, and in the mysterious way convicts invent, managed to convey to him before a month was over that plans were being made for a mutiny at the first opportunity.
Thanksgiving Day was one of the few chances for them to speak together as they enjoyed an hour of freedom in the prison yard.

Then all would be settled and the rash attempt made if possible, probably to end in bloodshed and defeat for most, but liberty for a few.

Dan had already planned his own escape and bided his time, growing more and more moody, fierce, and rebellious, as loss of liberty wore upon soul and body; for this sudden change from his free, healthy life to such a narrow, gloomy, and miserable one, could not but have a terrible effect upon one of Dan's temperament and age.
He brooded over his ruined life, gave up all his happy hopes and plans, felt that he could never face dear old Plumfield again, or touch those friendly hands, with the stain of blood upon his own.