nobleman vs viscount

nobleman

noun
  • A peer; an aristocrat; ranks range from baron to king to emperor. 

viscount

noun
  • A member of the peerage, above a baron but below a count or earl. 

  • Any of various nymphalid butterflies of the genus Tanaecia. Other butterflies in this genus are called earls and counts. 

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