bowler vs derby hat

bowler

noun
  • A bowler hat; a round black hat formerly popular among British businessmen. 

  • One who engages in the sport of bowling. 

  • The pitcher. 

  • A player selected mainly for his bowling ability. 

  • The player currently bowling. 

derby hat

How often have the words bowler and derby hat occurred in a corpus of books? (source: Google Ngram Viewer )