expatiate vs expound

expatiate

verb
  • To write or speak at length; to be copious in argument or discussion. 

  • To range at large, or without restraint. 

expound

verb
  • To make a statement, especially at length. 

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

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