by foot vs on foot

by foot

adv
  • walking, jogging or running but not in a vehicle or on the back of an animal 

on foot

prep
  • In progress or in preparation; active, in operation. 

  • On one's feet; walking, jogging or running but not in a vehicle or on the back of an animal. 

How often have the words by foot and on foot occurred in a corpus of books? (source: Google Ngram Viewer )