earl vs viscount

earl

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

  • A British or Irish nobleman next in rank above a viscount and below a marquess; equivalent to a European count. A female using the style is termed a countess. 

viscount

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

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

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