`if it is a great earthen pot has dropped from an unreachable rafter what figure of speech is contained in the expression above​

f. Simile.

i. This is an adverbial sentence.

ii. Its function is to modify the word "armosphere" previously said, to specify where and what is happening.

Explanation:

The full text is "My doubts seem confirmed by the atmosphere, that now prevails. It is as if a great earthen pot has dropped from an unreachable rafter on which it has been resting for a long time, and all its hidden contents are now *exposed* to general view." Through this text, we can see that the phrase 'It is as if a great earthen pot has dropped from an unreachable rafter ...' represents a comparison with the doubts that the narrator presents. This comparison is made through two elements that have nothing in common, but that the author uses to create a new meaning about one of them. This is done through simile.

With the original text, we also have access to the sentence "My doubts seem confirmed by the atmosphere, that now prevails." which shows that "that now prevails." it is an adverbial sentence, assuming the function of an adverb, modifying the word "armosphere", specifying when it is established.