What is the mechanical hound and what is its purpose?

The Mechanical Hound is first described early in the first part of the book. The hound is a life-like piece of technology that sniffs out illegal books, hunts down criminals, and incapacitates criminals with its needle, that extends from its nose, filled with procaine or morphine. Its described as living, but not living because it is so real looking and acting that it seems like it's alive. It is made of copper, brass, and steel. It has ruby glass and capillary hairs in nylon-brushed nostrils. If strays from looking like a real dog though in that it has 8 insect-like legs. It is a malevolent creature to Montag though as it growls at him as he passes. Its eyes are green-blue neon, multi-faceted light bulbs. When it "sleeps" it makes noises like an electrical sizzle or frying sound. Its computers can be set to detect many levels of chemical components.