fire hydrant vs petcock

fire hydrant

noun
  • A device used by firefighters to obtain water from the main supply, consisting of a barrel, connections for hose and a valve assembly. 

petcock

noun
  • A small valve, spout, or faucet operated by hand, usually used to release pressure or drain fluid. 

How often have the words fire hydrant and petcock occurred in a corpus of books? (source: Google Ngram Viewer )