Two were always busy elsewhere.

Casey saw that he was expected to believe that they were at work in the tunnel, driving it in to a certain contact of which they spoke frequently and at length.
At supper they would mention their footage for that day's work, and Casey would hide a grin of derision.

Casey knew rock as he knew bacon and beans and his sour-dough can.

To make the footage they claimed to be making in that tunnel, they would need to shoot twice a day, with a round of, say, five holes to a shot.
As a matter of fact, two holes a day, one shot at noon and one at night, were the most Casey ever heard fired in the tunnel or elsewhere about the mine.

But he did not tell them any of the things he thought; not even Joe, who had intelligence far above Paw and Hank, ever guessed that Casey listened every day for their shots and could tell, almost to an inch what progress they were actually making in the tunnel.