cittern vs guqin

cittern

noun
  • A stringed instrument (chordophone), played with a plectrum (a pick), and most commonly possessing four wire strings and chromatic frets. 

guqin

noun
  • A plucked zither-like stringed instrument (chordophone), traditionally featuring seven unfretted strings, originating in ancient China. 

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