American football vs gridiron football

American football

noun
  • A game similar to rugby football played on a field of 100 yards long and 53 1/3 yards wide (with two 10 yard-long endzones) in which two teams of 11 players attempt to get an ovoid ball into each other's territory. 

gridiron football

noun
  • collectively, Canadian football and American football 

How often have the words American football and gridiron football occurred in a corpus of books? (source: Google Ngram Viewer )