Stanza 6 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Down Wall, from girder into street noon leaks,
A rip-tooth of the sky's acetylene;

The day is slowly passing by as the sun "steps" over the bridge from east to west.
Now it's noon.
The noon light "leaks" onto Wall Street. It passes through the "girders" – heavy metal beams of buildings – to the street.
Line 22 is really obscure, but it's totally fine if you don't understand it. Just say it out loud and hear how awesome it sounds.
One take on line 22 is that the gaps between the tall buildings where the light pours down looks like a tooth that's been ripped out.
The buildings are like teeth, with one missing in the small open space.
The brilliant light from the skies fills that space like an "acetylene" lamp.
"Acetylene" is a gas that was used in street lights way back in the day. Its light is especially bright.
Another take is that the "sky's acetylene" is like a tooth of light biting into the city like a "rip-tide."

All afternoon the cloud-flown derricks turn . . .
Thy cables breathe the North Atlantic still.

The speaker moves his gaze back to the harbor, where "derricks" – a tall metal structure used for lifting things – are turning.
Derricks are often used to load and unload ships. You might have heard of "oil derricks" used to lift oil out of the ground.
These derricks are so high that they seem to reach into the clouds.
The time has moved from noon to afternoon. The day is passing by….
The speaker addresses the bridge again: "thy" (your).
The bridge's cables are like lungs that breathe in the salty air of the Atlantic. Or maybe they are like the strings of a musical instrument.
In Greek poetry the breath of the wind was compared to a musician playing a harp.
You can read the word "still" as a reference to time – the cables continue to breath the sea – or it could mean "quiet, calm," as in "the cables seem to make the sea calm."