death knell vs tocsin

death knell

noun
  • The tolling of a bell announcing death. 

  • A sign or omen foretelling the death or destruction of something. 

tocsin

noun
  • A bell used to sound an alarm. 

  • An alarm or other signal sounded by a bell or bells, originally especially with reference to France. 

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