"And between whiles I had to look after the savage who was fireman. He was an improved specimen; he could fire up a vertical boiler. He was there below me, and, upon my word, to look at him was as edifying as seeing a dog in a parody of breeches and a feather hat, walking on his hind-legs. A few months of training had done for that really fine chap. He squinted at the steam-gauge and at the water-gauge with an evident effort of intrepidity—and he had filed teeth too, the poor devil, and the wool of his pate shaved into queer patterns, and three ornamental scars on each of his cheeks. He ought to have been clapping his hands and stamping his feet on the bank, instead of which he was hard at work, a thrall to strange witchcraft, full of improving knowledge. He was useful because he had been instructed; and what he knew was this—that should the water in that transparent thing disappear, the evil spirit inside the boiler would get angry through the greatness of his thirst, and take a terrible vengeance. So he sweated and fired up and watched the glass fearfully (with an impromptu charm, made of rags, tied to his arm, and apiece of polished bone, as big as a watch stuck flatways through his lower lip), while the wooded banks slipped past us slowly…." (2.8)
Marlow may not be a total racist jerk, but he still doesn't consider the native Africans his equal. He sees them instead as animals, calling this fireman "a dog in a parody of breeches and a feather hat, walking on his hind-legs." They don't bother teaching him anything about hydraulics or engineering; they just tell him that an evil spirit will take revenge if the boiler ever becomes empty. This is essentially the same as getting your kid to stay in bed by telling her that there's a monster under it.