I have been one acquainted with the night.
I have walked out in rain—and back in rain.
I have outwalked the furthest city light.
I have looked down the saddest city lane.
I have passed by the watchman on his beat
And dropped my eyes, unwilling to explain.
I have stood still and stopped the sound of feet
When far away an interrupted cry
The narrator has passed by a “watchman on his beat.” His “beat” is his designated territory as watchman, but could also refer to the continuous, repetitive motion of his feet pacing back and forth. The watchman’s steady “beat” is analogous to the narrator’s continuous walking as both the watchman and the narrator’s actions seem to be endless motions that will not change; the narrator will always walk at night and the watchman will always walk to a steady beat. “Beat” may even evoke the speaker’s own heartbeat and how the sound is mimicked by the watchman’s feet. 
 

Acquainted with the Night