argonaut vs soldier of fortune

argonaut

noun
  • An adventurer on a dangerous but rewarding quest. 

  • Any of several species of shelled octopods of the family Argonautidae (of which only the genus Argonauta is not extinct). 

soldier of fortune

noun
  • A person willing to work for or ally himself with any person or institution who will take him on. 

  • A non-commissioned soldier willing to serve any state or person who will hire him. 

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