Early printing presses actually "pressed" type (letters that were carved in wood or metal) onto a sheet of paper -- hence the word "impression." The printing press above almost looks like one of those industrial irons at a dry-cleaning shop: you press the handle down, and the heavy paper-size slab goes down, pressing type onto paper. If you look at an old book, and run your hands across the page, you can feel the indentations made by the type as it was pressed into the page.

This is an example of what Lakoff and Johnson are talking about when they say that language and thought are structured metaphorically. When we talk about "impressions," we aren't CONSCIOUSLY thinking about printing presses -- you may not have even known much about printing presses until now. But we ARE using the word the way it was used when it applied to printing presses -- notice how hard it is, we believe, to get rid of first impressions, just as you cannot erase print from the page of a book (or, not easily, not without tearing it).

In the phrase "aerial impressions," Cuvier is imagining that bats' faces have a membrane on them which is JUST LIKE a piece of paper, and that the air prints images onto that piece of paper so that the bat knows where it is.