Ishmael states that the crew has become silent, awaiting the final encounter with Moby Dick. Fedallah seems never to sleep, and neither does Ahab, whose hat is pulled down so low of his eyes that no one can tell if those eyes are open or closed. Ahab tells the mates that he does not trust them to sight the whale correctly, since they might avoid the whale and therefore seek to end the voyage prematurely—thus Ahab has himself hoisted up the main mast in order to spot Moby Dick himself. When Ahab reaches the zenith, however, a sea-hawk flies by and takes off his hat, dumping it into the sea far away—and some crewmembers take this for an ill omen of the encounter with the white whale.
Another omen, this time one that all crewmembers can easily interpret. If the hat is a symbol for one’s own head, and if the head is the most important part of the body, then to lose one’s hat is to lose one’s head, and perhaps one’s life. It is no coincidence that Ahab will be killed, soon, by a rope around his neck—his head will be cut off from the rest of his body, and he will be flung into the ocean deep to drown, in keeping with Fedallah’s prophecy. A prophecy that Ahab misinterpreted. Also note that Ahab at this point is no longer trying to fool anyone, telling the mates that he will sight the whale because he doesn't trust them to actually do it. Yet his monomania compels the crew to continue to follow him.