first mate vs first officer

first mate

noun
  • An officer on a merchant ship next in rank to the captain and responsible to the captain for the safety and security of the ship. 

first officer

noun
  • The executive officer, second in command of a naval vessel. 

  • Air Transport Auxiliary rank corresponding to Flight Lieutenant in the Royal Air Force. 

  • Women's Royal Naval Service rank corresponding to Lieutenant-Commander in the Royal Navy. 

  • The copilot of an aircraft. 

  • The chief mate of a merchant ship. 

  • The most senior civil servant in the United States Foreign Service, with responsibility for running an embassy under the authority of and reporting to the ambassador. 

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