destiny vs mainspring

destiny

noun
  • The fixed order of things; invincible necessity; an irresistible power or agency conceived of as determining the future, whether in general or of an individual. 

  • That which is inevitable in the fullness of time. 

  • One's eventual fate (not necessarily inevitable or predestined). 

  • That to which any person or thing is destined; a predetermined state; a condition predestined by the Divine or by human will. 

mainspring

noun
  • The most important reason for, or element of, something. 

  • The principal spring of a clockwork mechanism, which drives it by uncoiling. 

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