cittern vs dulcimer

cittern

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

dulcimer

noun
  • A stringed instrument, with strings stretched across a sounding board, usually trapezoidal, played by plucking on the strings (traditionally with a quill) or by tapping on them (in the case of the hammer dulcimers). 

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