contemplate vs knock on wood

contemplate

verb
  • To look at on all sides or in all its aspects; to view or consider with continued attention; to regard with deliberate care; to meditate on; to study, ponder, or consider. 

  • To consider as a possibility. 

knock on wood

verb
  • To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement. 

intj
  • Hopefully; a self-directive to undertake the customary action to ward off bad luck. 

How often have the words contemplate and knock on wood occurred in a corpus of books? (source: Google Ngram Viewer )