What does this mean: "dead wood we could cut up in the night for next day's stemaning"?

"When you have to attend to things of that sort, to the mere incidents of the surface, the reality -- the reality, I tell you -- fades. The inner truth is hidden -- luckily, luckily. But I felt it all the same; I felt often its mysterious stillness watching me at my monkey tricks, just as it watches you fellows performing on your respective tight-ropes for -- what is it? half-a-crown a tumble -- " (Heart Of Darkness by Joseph Conrad)