Bringing two different things together with an explicit word, such as "like", "just as" or "seems". Unlike metaphor, comparison makes it plain that they are two things.
In Purgatory
This mountain is such that it is always hard at the start, from below; and the more one climbs, the less it hurts. Therefore, when it seems so gentle to you that climbing will be light, like going downstream in a boat with the current.
The relief of the climb is measured by an image of another movement: climbing becomes "light as, with the current, descending a boat." The reader understands the end of the effort through what he has already felt when letting himself be carried.
Cantos I to IV: The shore and Ante-Purgatory, Virgil, to Dante, answering how much is left to climb
because to everyone an iron thread pierces and sews the eyelids shut, as is done to the wild hawk because it will not keep still. It seemed to me, as I walked, that I was committing an outrage, seeing the others without being seen; so I turned to my wise counselor.
Cantos XIII to XVI: Envy and wrath, Dante, narrator, before the envious with sewn eyes
that infinite and ineffable good that is up there runs to love as the ray comes to a shining body. It gives as much as it finds of ardor; so that, however much charity extends, the eternal worth grows over it.
Cantos XIII to XVI: Envy and wrath, Virgil, to Dante, explaining why divine good grows when shared
you did as one who goes at night, who carries the light behind and does not profit by it himself, but after himself makes the people learned, when you said: the age renews itself; justice returns and the first human time, and a new progeny descends from heaven.
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