vault of heaven vs welkin

vault of heaven

welkin

noun
  • The upper atmosphere occupied by clouds, flying birds, etc. 

  • The place above the Earth where God or other deities live; heaven. 

  • The sky which appears to an observer on the Earth as a dome in which celestial bodies are visible; the firmament. 

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