Answer :
From what I can make out, I think the two things being compared are the fog of the harbour, and the city.
Fog is a short poem by Carl Sandburg:
The fog comes
on little cat feet.
It sits looking
over harbor and city
on silent haunches
and then moves on.
Sandburg tries to describe similarities between something alive and not alive, correspondingly between cat and fog. He tries to show that fog moves like a cat.
