Instead of a firehouse dog, like a dalmatian, each firehouse now has a mechanical hound. Describe it.

The hound is like a robotic dog.  Like a dog, the hound
can track people by smell.  It is programmed by the firemen to track someone based on
their body chemistry.  The hound is used to track down fugitives.  It is equipped with a
long needle that it can use to inject its victims with to immobilize
them. 

Bradburn describes the hound in the following
passage. 

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"...brass and the copper and the steel of the
faintly trembling beast. Light flickered on bits of ruby glass and on sensitive
capillary hairs in the nylon-brushed nostrils of the creature that quivered gently,
gently, gently, its eight legs spidered under it on rubber-padded
paws."

Montag has several
encounters with the hound.  The time readers learn of the hound is when it growls at
Montag in the firehouse.  Later after he burns Beatty, the hound attacks him and injects
his leg.  He is able to escape, the firemen send another hound after him.  Faber, and
later Granger, help him to change his scent to lose the hound.