dulcimer vs psalterium

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). 

psalterium

noun
  • A psaltery (zither-like musical instrument) 

  • An omasum. 

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