Nor did he guess that Casey Ryan with his mouth shut was more unsafe than "giant powder" laid out in the sun until it sweated destruction.
Persistent effort, directed by an idea based solely upon an abstract theory, must be driven by a trained intelligence.

In this case the abstract theory that every prisoner must be watched must support itself unaided by Casey's behavior.

Not even Joe's intelligence was trained to a degree where the theory in itself was sufficient to hold him to the continuous effort of watching Casey.
Wherefore Paw, Hank and Joe presently slipped into the habit of leaving Casey alone for an hour or so; being careful to keep the guns out of his reach, and returning to the dugout at unexpected intervals to make sure that all was well.
Casey Ryan knew his pots and pans, and how to make them fill his days if need be.

With savory suppers and his care-free, Casey Ryan grin, he presently lulled them into accepting him as a handy man around camp, and into forgetting that he was at least a potential enemy.

Afoot and alone in that unfriendly land, with his left hand smashed and carried in a sling, and on his tongue an Irish joke that implied content with his captivity, Casey Ryan would not have looked dangerous to more intelligent men than these three.
They should have looked one night under the bedding in Casey's bunk.
More important still would have been the safeguarding of their "giant powder" and caps and fuse.