third degree vs trial balloon

third degree

noun
  • Intensive rough interrogation in order to extract information or a confession 

trial balloon

noun
  • An idea, suggestion, or prospective action, product, etc., offered to an audience or group in order to test whether it generates acceptance or interest. 

  • A small balloon released into the sky to determine the direction and tendency of winds in the upper air before a manned ascent in a larger balloon; a ballon d'essai. 

How often have the words third degree and trial balloon occurred in a corpus of books? (source: Google Ngram Viewer )