cask vs gravity bong

cask

noun
  • A large barrel for the storage of liquid, especially of alcoholic drinks. 

verb
  • To put into a cask. 

gravity bong

noun
  • A homemade device used to smoke marijuana, consisting of a plastic jug with the bottom cut out and immersed in a vat of water, which is then raised to create a vacuum beneath its bowl of lit marijuana. 

  • Synonym of waterfall bong 

How often have the words cask and gravity bong occurred in a corpus of books? (source: Google Ngram Viewer )