gusle vs gusli

gusle

noun
  • A single-stringed lute-like stringed instrument with a bowl-shaped body, held vertically in the lap and played a bow, originating among the Slavic peoples in the Balkans, especially in the Dinarides region. 

gusli

noun
  • A plucked psaltery-like string instrument, usually played on the lap and sometimes created with table legs so that the musician can play it seated next to the instrument, originating in ancient Russian music. 

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