boiler vs double boiler

boiler

noun
  • A kitchen vessel for steaming, boiling or heating food. 

  • An old woman. 

  • A device consisting of a heat source and a tank for storing hot water, typically for space heating, domestic hot water etc., disregarding the source of heat. 

  • A sunken reef, especially a coral reef, on which the sea breaks heavily. 

  • A person who boils something. 

  • A steam boiler. 

  • An apparatus for heating circulating water or other heat transferring liquid. 

  • Boilerplate. 

  • A tough old chicken only suitable for cooking by boiling. 

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 boiler and double boiler occurred in a corpus of books? (source: Google Ngram Viewer )