firkin vs milk churn

firkin

noun
  • A small wooden vessel or cask of indeterminate size, used for butter, lard, etc. 

  • A varying measure of capacity, usually being a quarter of a barrel; specifically, a measure equal to nine imperial gallons. 

  • A weight measure for butter, equalling 56 pounds. 

milk churn

noun
  • A tall, conical or cylindrical container for the transportation of milk. 

How often have the words firkin and milk churn occurred in a corpus of books? (source: Google Ngram Viewer )