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Between us there was, as I have already said somewhere, the bond of the sea. Besides holding our hearts together through long periods of separation, it had the effect of making us tolerant of each other's yarns—and even convictions. (1.4)
Well, when you're stuck on a ship together for months at a time, there's plenty of opportunity to explore each other's psyches. And plenty of time for really long stories.
It had known and served all the men of whom the nation is proud, from Sir Francis Drake to Sir John Franklin, knights all, titled and untitled - the great knights-errant of the sea. It had borne all the ships whose names are like jewels flashing in the night of time, from the Golden Hind returning with her rotund flanks full of treasure, to be visited by the Queen's Highness and thus pass out of the gigantic tale, to the Erebus and Terror, bound on other conquests - and that never returned. It had known the ships and the men. They had sailed from Deptford, from Greenwich, from Erith - the adventurers and the settlers; kings' ships and the ships of men on 'Change; captains, admirals, the dark 'interlopers' of the Eastern trade, and the commissioned 'generals' of East India fleets. Hunters for gold or pursuers of fame, they all had gone out on that stream, bearing the sword, and often the torch, messengers of the might within the land, bearers of a spark from the sacred fire. What greatness had not floated on the ebb of that river into the mystery of an unknown earth! . . . The dreams of men, the seed of commonwealths, the germs of empires. (1.6)
Marlow lists off a bunch of famous figures who explored the Thames River and the sea. The first half of his list catalogs royally commissioned explorers and plunderers, while the latter half are businessmen who developed trade with foreign countries. Marlow seems to view all of them as pioneers who struck out to bring civilization to dark unknown lands—but we're not so sure he really feels that way.
He was a seaman, but he was a wanderer, too, while most seamen lead, if one may so express it, a sedentary life. Their minds are of the stay-at-home order, and their home is always with them—the ship; and so is their country—the sea. One ship is very much like another, and the sea is always the same. In the immutability of their surroundings the foreign shores, the foreign faces, the changing immensity of life, glide past, veiled not by a sense of mystery but by a slightly disdainful ignorance; for there is nothing mysterious to a seaman unless it be the sea itself, which is the mistress of his existence and as inscrutable as Destiny. For the rest, after his hours of work, a casual stroll or a casual spree on shore suffices to unfold for him the secret of a whole continent, and generally he finds the secret not worth knowing. The yarns of seamen have a direct simplicity, the whole meaning of which lies within the shell of a cracked nut. But Marlow was not typical (if his propensity to spin yarns be excepted), and to him the meaning of an episode was not inside like a kernel but outside, enveloping the tale which brought it out only as a glow brings out a haze, in the likeness of one of these misty halos that sometimes are made visible by the spectral illumination of moonshine. (1.9)
What's weird is that Conrad describes seamen as actually kind of sedentary, sitting aboard their ship wherever it takes them rather than actively going out to explore. Marlow, on the other hand, is an explorer in the truest sense of the word. He hasn't come to impose assumptions, but to find meaning and truth—even if he doesn't like what he finds.