What are two literary devices and their lines used in Dante's Inferno Canto 20?

In Dante's Inferno Canto XX, Dante uses many analogies, direct comparisons between two things. Because he is traveling in hell, Dante must make realistic, earthly connections so the reader can relate to the supernatural action of the poem.  Here is an example:

Even as the dolphins, when they make a sign
To mariners by arching of the back,
That they should counsel take to save their vessel,

Thus sometimes, to alleviate his pain,
One of the sinners would display his back,
And in less time conceal it than it lightens.

Here, Dante is comparing the way dolphins arch their backs to the way the sinners "display(s) his back" to ease the pain of suffering.

Dante also relies on much visual imagery to convey the supernatural sights he sees.  Remember, he is our tour guide into the abyss, and his visual imagery reminds us constantly of the suffering of others (in bold):

and both of them
Fell in the middle of the boiling pond.

A sudden intercessor was the heat;
But ne'ertheless of rising there was naught,
To such degree they had their wings belimed.