flying buttress vs footstep

flying buttress

noun
  • A buttress that stands apart from the structure that it supports, and is connected to it by an arch (flyer). 

footstep

noun
  • The mark or impression left by a foot; a track. 

  • A step, as in a stair. 

  • The sound made by walking, running etc. 

  • The act of taking a step. 

  • The distance between one foot and the next when walking; a pace. 

  • The indications or waypoints of a course or direction taken. 

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