billycan vs double boiler

billycan

noun
  • A lightweight pot for cooking or boiling water, used in camping. 

double boiler

noun
  • A pair of interlocking pots, the lower one filled with water which when boiling heats the contents of the upper pot. 

  • A large pan containing hot water, into which other smaller pans are set in order to cook food at low heat (below the boiling point) or to keep food warm. 

How often have the words billycan and double boiler occurred in a corpus of books? (source: Google Ngram Viewer )