hearken vs overhear

hearken

verb
  • To hear (something) with attention; to have regard to (something). 

  • To listen; to attend or give heed to what is uttered; to hear with attention, compliance, or obedience. 

overhear

verb
  • To hear something that was not meant for one's ears. 

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