Ishmael notes that the whale can hold its breath for over an hour, and that air is taken into the whale’s body only through the blowhole, or “spiracle.” Ishmael also describes the scientific debate about the nature of the process by which water is expelled from this blowhole: namely, some believe that it is only a harmless mist, but others claim that the mist is “acrid,” that it can “burn the face” of a sailor or blind anyone who comes too close. Ishmael states that, although he has observed whales spouting many times, and the process seems a simple one, it is often the most simple processes that become the most difficult to understand with scientific certainty.
The whale’s spout has, until this point in the novel, not been mentioned, although it is one of the most recognizable features of a whale. And even this feature is a source of uncertainty, with people having very different ideas of the nature—and danger—and what emerges from the blowhole. Ishmael's comment about the simple processes being difficult to understand scientifically matches with his general outlook in the novel and the way that nearly everything functions in Moby Dick, with myriad possible and conflicting interpretations always existing, like a mist, around nearly all events.