constancy vs firmness

constancy

noun
  • The quality of being constant; steadiness or faithfulness in action, affections, purpose, etc. 

  • An unchanging quality or characteristic of a person or thing. 

firmness

noun
  • strictness; mental strength 

  • security; steadfastness; good grip 

  • physical durability; rigidness (of material) 

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