Mother Superior vs anchoress

Mother Superior

noun
  • The head of a convent; an abbess. 

anchoress

noun
  • An anchorwoman. 

  • A female anchorite. A woman who chooses to withdraw from the world to live a solitary life of prayer and contemplation. 

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