firmness vs inconstancy

firmness

noun
  • strictness; mental strength 

  • security; steadfastness; good grip 

  • physical durability; rigidness (of material) 

inconstancy

noun
  • Lack of constancy; lack of consistency in thought, emotion or action. 

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