Ugh! it's like moist black velvet." which type of figurative language?

Other students in the class that Scoresby studied with did very well in their examination displaying their level of intelligence.  

On the contrary, Scoresby hardly knew anything.  His did not perform well displaying his stupidity.

Explanation:

As related by Clergyman in the passage “Luck” by Mark Twain, the paragraph “About forty years ago, I was an instructor in the military academy ………..give answers that were miracles of stupidity.  I knew of course that when examined again he would fail and be thrown out”, gives us enough evidence that in comparison to the performance of other students in the class when examined, Scoresby’s performance was very bad.  

According to the clergyman, who was his instructor then in school, Scoresby was not doing well in the class.  The clergyman even quoted his answers as “stupidity”, “stupid head.”   He did not know about Julius Caesar’s history nor was he good at Mathematics.