In Chapter 21, Duncan stands at the ready to shoot a strange man Hawkeye is approaching. The scene is characterized by dramatic irony:

[...]Duncan turned his eyes just in time to perceive that a hundred dark forms were plunging, in a body, into the troubled little sheet. Grasping his rifle, his looks were again bent on the Indian near him. Instead of taking the alarm, the unconscious savage stretched forward his neck, as if he also watched the movements about the gloomy lake, with a sort of silly curiosity. In the meantime, the uplifted hand of Hawkeye was above him. But, without any apparent reason, it was withdrawn, and its owner indulged in another long, though still silent, fit of merriment.

Duncan does not understand why Hawkeye is laughing, nor why he seems unconcerned about the stranger. What Hawkeye, Uncas, and Chingachgook have all realized is that the strange man is David Gamut, the musician, and that the "dark forms" plunging into the waters are beavers he is trying to convert to Christianity. Hawkeye is laughing at the absurdity of David's proselytizing. It almost looks as though he is baptizing the beavers. Hawkeye also seems to be laughing because Duncan, who has yet to recognize David, is taking the encounter so seriously.

In this instance, dramatic irony provides comic relief in the midst of a suspenseful hunt for Cora and Alice. David and Duncan each appear woefully naive, which is all the funnier because they are each trying so hard at their respective jobs. David feels so intensely called to spread Christianity with his hymns that he can't distinguish between real potential Christians and beavers. Duncan, meanwhile, is so devoted to his role as a soldier that he is primed to shoot at someone so obviously harmless as David. This scene demonstrates that the forest is full of as many fools as fearsome enemies.