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"For a time I would feel I belonged still to a world of straightforward facts; but the feeling would not last long. Something would turn up to scare it away. Once, I remember, we came upon a man-of-war anchored off the coast. There wasn't even a shed there, and she was shelling the bush. It appears the French had one of their wars going on thereabouts. Her ensign dropped limp like a rag; the muzzles of the long six-inch guns stuck out all over the low hull; the greasy, slimy swell swung her up lazily and let her down, swaying her thin masts. In the empty immensity of earth, sky, and water, there she was, incomprehensible, firing into a continent. Pop, would go one of the six-inch guns; a small flame would dart and vanish, a little white smoke would disappear, a tiny projectile would give a feeble screech—and nothing happened. Nothing could happen. There was a touch of insanity in the proceeding, a sense of lugubrious drollery in the sight; and it was not dissipated by somebody on board assuring me earnestly there was a camp of natives—he called them enemies!—hidden out of sight somewhere." (1.30)
Marlow is going down the rabbit hole here. One of the first acts of madness he sees is a man-of-war firing at a totally empty coastline—which sounds pretty crazy to us, too.
[The Swede]: "'The other day I took up a man who hanged himself on the road. He was a Swede, too.' 'Hanged himself! Why, in God's name?' I cried. He kept on looking out watchfully. 'Who knows? The sun too much for him, or the country perhaps.'" (1.33)
Marlow gets his first taste of danger when he learns that the sun has actually driven the "Swede" to suicide. Is this a clear argument for nature making men go crazy?
"Another report from the cliff made me think suddenly of that ship of war I had seen firing into a continent. It was the same kind of ominous voice; but these men could by no stretch of imagination be called enemies. They were called criminals and the outraged law, like the bursting shells, had come to them, an insoluble mystery from over the sea. All their meager breasts panted together, the violently dilated nostrils quivered, the eyes stared stonily up-hill. They passed me within six inches, without a glance, with that complete, deathlike indifference of unhappy savages." (1.36)
It's a mad, mad, mad world: the chained slaves are so beaten down that they don't even notice their surroundings. They're the opposite of dangerous enemies—in fact, it's the Europeans who are the dangerous enemies.