prolong vs retard

prolong

verb
  • To put off to a distant time; to postpone. 

  • To lengthen in time; to extend the duration of 

  • To become longer; lengthen. 

  • To extend in space or length. 

retard

verb
  • To put off; to postpone. 

  • To decelerate; to slow down. 

  • To keep delaying; to continue to hinder; to prevent from progressing. 

noun
  • Retardation; delay. 

  • A slowing down of the tempo; a ritardando. 

  • A person or being who is extremely stupid or slow to learn. 

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