explain vs expound

explain

verb
  • To make plain, manifest, or intelligible; to clear of obscurity; to illustrate the meaning of. 

  • To give a valid excuse for past behavior. 

  • To make something plain or intelligible. 

expound

verb
  • To set out the meaning of; to explain or discuss at length 

  • To make a statement, especially at length. 

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