Hertfordshire vs Letchworth

Hertfordshire

name
  • An inland county of south-east England bordered by Cambridgeshire, Essex, Greater London, Buckinghamshire and Bedfordshire. 

Letchworth

name
  • A town in North Hertfordshire district, Hertfordshire, England. Synonym: Letchworth Garden City. 

  • A surname. 

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