Ishmael describes the duty of sitting atop the mast-head, or the tip of each long spar extending upward from the deck of the Pequod. Ishmael notes, half-comically, that “mast-head-standing” has a long history: the Babylonians did something similar when they built the tower of Babel, and stone carvings of Napoleon, Admiral Nelson, and George Washington sit atop tall columns in the Western world and America. But Ishmael wishes to describe in greater detail the watch sailors must serve while sitting atop the masts.
As in other parts of the novel, Ishmael here does not seek only to describe the activity of sitting on the mast. Instead, he wishes to situate that activity within a historical context that describes, over time, how men have chosen to sit atop high things and look down upon the world. Fittingly, as his first example, Ishmael takes the Tower of Babel, an attempt by man, in Biblical times, to reach the sky—and which resulted in God striking down the tower as an overreach by men, and ensure that such an effort could never be done again by making men speak different languages. Looked at that way, the Babel story led to the diversity on the Pequod. Looked at another way, the story hints at Ahab's own possible overreach in his monomaniacal quest.
Ishmael says that sailors on the Pequod and similar vessels sit atop the mast for two-hour shifts while it is light out. The top of whaling vessels tends not to have crow’s nests, or small seating areas, but rather have only two slats of wood upon which the sailor is to balance, either seated or standing. Ishmael says that the purpose of the mast-head watch is to spot whales, but that many young men, lulled into calming meditation by the serenity of the South Seas, think only of their own happiness and contentment, and never spot whales at all. Ishmael considers this a natural outcome of the beauty of the scene from the mast-head, although he notes that this phenomenon often angers harpooneers, who are hoping to hunt and kill whales.
Sitting atop the mast has two purposes in the novel. The first, as here, is inherently practical—one has the best view from this vantage, and therefore one might spot whales from here with great efficiency. Second, the top of the mast-head has a kind of philosophical importance on a whale-ship, as here, men are alone, able to view the sea in all its vastness. There are few places on a ship where a man can be completely alone with his thoughts, but the mast-head is one of those places. And in this high place of beauty and contemplation, men lose themselves.