Answer:
on silent haunches. and then moves on. Sandburg is extending the metaphor of the fog looking catlike by including this line of it sitting and looking "on silent haunches." Haunches are kind of like a cat's thighs, and they're the parts that enable kitty to jump and pounce as far as she'd like to.
Step-by-step explanation:
The fog comes on little cat feet. It sits looking over harbor and city on silent haunches and then moves on. What does the metaphor of the cat say about fog? The metaphor shows that fog can be compared to a cat in a multitude of ways including its attitude, sound, motion, position, and intention.