cowling vs freemartin

cowling

noun
  • A young or little cow; calf. 

  • A removable protective covering for the engine of an aircraft, motorcycle, etc. 

freemartin

noun
  • A female calf, born as twin with a bull calf, but sexually imperfect (often infertile). 

  • Any female animal born sterile or otherwise infertile. 

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