After the three introductory stanzas, which help set the New York scene, the poem does exactly what its title suggests: delivers an apostrophe to the Brooklyn Bridge. To the poet, the bridge is as vibrant as a living being, almost a deity—and, in fact, the poem personifies the bridge on several occasions. (For example, it imagines the bridge as a "breath[ing]" creature, with "arms," that can "bestow" accolades and grant "reprieve[s].")
In fact, Crane borrows some effects that, traditionally, poets have often used to apostrophize gods. For example, he invokes the bridge with the exclamation "O" ("O harp and altar," "O Sleepless"). He also addresses the bridge with the archaic second-person pronouns "thee," "thou," "thy," and "thine," as though it were not only alive but worthy of special reverence. Even the capitalized word "Sleepless" is a kind of heroic epithet or reverent name for the bridge.
The tone of the apostrophe is not only reverent but passionate. Crane describes the bridge in lush, tender, somewhat hyperbolic terms ("And we have seen night lifted in thine arms") and breaks out in enthusiastic exclamations ("Implicitly thy freedom staying thee!"; "How could mere toil align thy choiring strings!"). Together, these effects frame the bridge as not only a beautiful monument but also a kind of religious icon, an emblem of modern hopes and aspirations.