skirl vs transverse flute

skirl

noun
  • A shrill sound, as of bagpipes. 

verb
  • To make a shrill sound, as of bagpipes. 

transverse flute

noun
  • Any type of flute that is held sideways when played. 

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