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.
The two things being compared in the poem are the fog and cats. The poet uses feline imagery to describe the fog as soft and stealthy, enhancing its mysterious qualities. This comparison enriches the imagery and emotional resonance of the fog in the poem.
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