How we cite our quotes: (Chapter.Paragraph)
Quote #1
In summer, too…sitting here, one is often reminded of the sea. …the vastness and the lonesomeness are so oceanic, and the silence and sameness too, that the first peep of a strange house, rising beyond the trees, is for all the world like spying on the Barbary coast, an unknown sail. (1.18)
The narrator here (who is sort of Melville himself) compares sitting in his house in the mountains to sailing. Both are isolated and mysterious and exciting. Isolation here is kind of fun; it's a way to imagine an adventure story. You need to be cut off from civilization to have an adventure—though that cutting off can be done in imagination more easily than by actually going out on the ocean.
Quote #2
I saw, through the open window, a lonely girl, sewing at a lonely window. (1.41)
Melville says "lonely" twice, so you'll know that this person is lonely. It also perhaps suggests that the narrator is lonely too; Mariana is actually a figment of the narrator's imagination, so her loneliness is his loneliness. Lonely people think lonely thoughts about lonely people thinking lonely thoughts. With all those lonelies, it starts to look almost crowded in there.
Quote #3
Of a Sunday, Wall-street is deserted as Petra; and every night of every day it is an emptiness. (2.90)
Petra is an ancient Middle Eastern city. Wall Street is then empty and imposing. This is the case on Sundays and nights, when there is no business being conducted—but the suggestion, perhaps, is that Wall Street is always eerily echoey. Business is a lonely, abandoned endeavor.