noblewoman vs viscountess

noblewoman

noun
  • A woman having a noble rank, especially one belonging to the peerage; a Lady. 

viscountess

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

  • The wife of a viscount. 

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