double boiler vs fish kettle

double boiler

noun
  • 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. 

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

fish kettle

noun
  • A type of large pan, usually long with rounded corners, used to steam fish. 

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