earl vs empress

earl

noun
  • 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. 

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

empress

noun
  • The wife or widow of an emperor or equated ruler. 

  • The third trump or major arcana card of most tarot decks. 

  • The female monarch (ruler) of an empire. 

  • A female chimpanzee. 

  • A deciduous tree, Paulownia tomentosa 

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