firmness vs mettle

firmness

noun
  • strictness; mental strength 

  • security; steadfastness; good grip 

  • physical durability; rigidness (of material) 

mettle

noun
  • A quality of endurance and courage. 

  • Good temperament and character. 

adj
  • Spirited, vigorous, stout-hearted. 

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