keep watch vs look out

keep watch

verb
  • To guard; to watch over someone or something. 

look out

verb
  • To be vigilant and aware often as an imperative to alert a person to danger. 

  • To find by looking: to hunt out. 

  • To be facing (used with "on"). 

  • Used other than figuratively or idiomatically: see look, out. 

How often have the words keep watch and look out occurred in a corpus of books? (source: Google Ngram Viewer )