bombard vs fire on

bombard

noun
  • A bombardment. 

  • A bombardon. 

  • a medieval primitive cannon, used chiefly in sieges for throwing heavy stone balls. 

verb
  • To direct at a substance an intense stream of high-energy particles, usually sub-atomic or made of at most a few atoms. 

  • To attack something or someone by directing objects at them. 

  • To continuously attack something with bombs, artillery shells or other missiles or projectiles. 

  • To continuously send or direct (at someone) 

fire on

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