Jack Ketch vs firing squad

Jack Ketch

noun
  • the gallows 

  • death 

  • a public executioner 

  • the devil 

firing squad

noun
  • A group of soldiers or paramilitaries detailed to execute someone, usually by rifle shot, or to discharge their weapons ceremonially at a funeral. 

How often have the words Jack Ketch and firing squad occurred in a corpus of books? (source: Google Ngram Viewer )