petcock vs sillcock

petcock

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

sillcock

noun
  • A faucet designed to be installed horizontally on the outside wall of a building, typically to attach a hose. 

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