In the story "The Pit and the Pendulum," how does Poe use imagery in the first paragraph?

The narrator employs
auditory imagery that describes something heardwhen he
says,

the sound of the inquisitorial voices seemed merged
in one dreamy indeterminate hum. It conveyed to my soul the idea of
revolutionperhaps from its association in fancy with the burr of a
mill-wheel.

This is particularly interesting in that he
connects something that one might hear with a description of something one might see, using
visual imagery. He describes movement, the revolution or turning of
a mill-wheel, in order to describe the way his inquisitors' voices sound. This combination of
two different kinds of imagery, both visual and auditory, is called
synesthesia.

More visual
imagery follows when the narrator describes the appearance of his judges whose
voices he's just described:

I saw the lips of the
black-robed judges. They appeared to me whitewhiter than the sheet upon which I trace these
wordsand thin even to grotesqueness; thin with the intensity of their expression of
firmness

They...