guqin vs hammered dulcimer

guqin

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

hammered dulcimer

noun
  • A type of dulcimer whose metal strings are stretched over a trapezoidal sounding board that has two bridges, one bass and one treble, set on a stand and played with small mallet-like hammers. 

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