petcock vs stopcock

petcock

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

stopcock

noun
  • A valve, tap or faucet which regulates the flow of liquid or gas through a pipe. 

  • A main shutoff for water to a home from a municipal supply. Usually these valves exist in pairs, one outside the property boundary and one inside the property boundary. 

verb
  • To control the gas pressure in (an oil well) by keeping the well closed in and the gas confined except during set intervals when the oil is flowed or pumped. 

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