inconstancy vs instability

inconstancy

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

instability

noun
  • The quality of being unstable. 

  • A state that is not in equilibrium, or in which a small change has a large irreversible effect. 

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