In this a very interesting insertion on Twain's part, for, as critic Daniel Hoffman says, it is incontestably of African origin. Reading hair-balls was/is a voodoo practice.
Here is the text:
"Miss Wat's nigger, Jim, had a hair-ball as big as your fist, which had been took out of the fourth stomach of an ox, and he used to do magic with it. He said there was a spirit inside of it, and it knowed everything."
- Huck Finn
Conjure-balls are generally used as charms to cast spells. Left in the room, hand, or path of someone, it will produce the desired effect. "In some of the states a spell may be put upon a man by burying a "hair-ball" (one of the compact balls of hair often found by butchers in the stomachs of cows or oxen) under his doorstep. This object (powerful, because peculiar) may also be carried about as an amulet to protect one from spells."
The attempt at divination or knowledge of future events may be one way the slave Jim tries to wrest some control over his life.