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In this poem the poet describes the fog as a living being. The poet says that fog comes in the city silently without making any noise just like a cat .It also inaudibly engulfs the city and its dwellers. At last it runs away on unknowingly .

carl sandburg tried to describe the fog as in peaceful environment and compared fog to cat as it comes silently just like fog . fog which came and is inveitable to environment silently sits and watch the whole city activities just like a cat as cat animal show its moment silently and in peaceful manner same as in case of fog. and it also vanishes without sound.

Basically , the deep meaning of fog poem is :

like fog and cat comes silently to city and harbour respectively , such same like that ptoblems in mortal human life are inveitable and comes silently . so humans should not loose the ray of hope.

the poetic device used in poem is metaphor , and transferred epithet

theme of poem...

attitude of man towards the nature and how change is not invitable part of natural process.

Hope it helps you... :-)