legwork vs trial balloon

legwork

noun
  • Skillful or vigorous use of the legs, as in dance or sports. 

  • Work, especially research or preparation, that involves significant walking, travel, or similar effort. 

trial balloon

noun
  • 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. 

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

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