baroness vs viscountess

baroness

noun
  • The wife of a baron. 

  • A woman holding a baronial title in her own right; a female ruler of a barony. 

viscountess

noun
  • The wife of a viscount. 

  • A woman holding the rank of viscount in her own right. 

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