dulcimer vs psaltery

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). 

psaltery

noun
  • A zither-like musical instrument consisting of a soundboard with multiple strings, played by plucking the strings with the fingers or a plectrum. 

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