rugby ball vs volleyball

rugby ball

noun
  • A spheroidal ball designed for the sport of rugby. 

volleyball

noun
  • The inflated ball used in such a game. 

  • A game played on a rectangular court between two teams of two to six players which involves striking a ball back and forth over a net. 

How often have the words rugby ball and volleyball occurred in a corpus of books? (source: Google Ngram Viewer )