In order to answer your question about Ray Bradbury's Fahrenheit 451, let's take a look at some basic information about the hounds as well as the moment in which Montag becomes a fugitive. As a fireman, he and his brigade make use of the mechanical hounds to seek out those who are guilty of possessing books—an act that is against the law. The creatures have eight insect-like legs and a protruding proboscis—a silver needle which can inject tranquilizers into their prey. In part one of the story, it is revealed that Montag does not get on well with the mechanical hounds. At his brigade's firehouse, he has a frightening encounter:
The Hound half rose in its kennel and looked at him with green-blue neon light flickering in its suddenly activated eye-bulbs. It growled again . . . [h]e saw the silver needle extend upon the air an inch, pull back, extend, pull back.
Montag is terrified but is able to escape up the fire pole. This confrontation soon makes sense as Bradbury reveals that Montag has been slowly building up a collection of books at his house.
Later, Montag's brigade ends up at his house when his wife and her friends turn him in for possessing books. He gets hold of a flame thrower, killing Beatty—his boss—and destroying the mechanical hound. Unfortunately, the latter is able to inject him with tranquilizer before being torched. Believing his friend Faber would be able to help save him, Montag slowly and painfully travels to his house. When he gets there, the two of them discover that another, more advanced hound has been released—one capable of tracking Montag from miles away. In order to mask his scent from the hound, he instructs Faber to burn everything he touched, wipe everything else down with alcohol, turn on the air conditioning, use moth spray, and turn on the lawn sprinklers. Montag's experience as a fireman comes in handy here, as his expert knowledge ends up saving Faber from the hound.
Here is where we get to the answer of your question. In order to foil the hound, Montag asks Faber to fill a suitcase with his old, dirty clothes and to give him a bottle of alcohol. Then, he runs as fast as he can to the river:
He waded in and stripped in darkness to the skin, splashing his body, arms, legs, and head with raw liquor; drank it and snuffed some up his nose. Then he dressed in Faber's old clothes and shoes. He tossed his own clothing into the river and watched it swept away. Then, holding the suitcase, he walked out in the river until there was no bottom and he was swept away in the dark.
Masking his scent allows him to escape, as the hound soon moves inland away from him. In order to ensure that the hound does not pick up his scent again, he floats down the river until he reaches the shore.