airpost vs correspondence

airpost

noun
  • Synonym of airmail 

correspondence

noun
  • Postal or other written communications. 

  • A relation. 

  • Reciprocal exchange of civilities, especially conversation between persons by means of letters. 

  • An agreement of situations or objects with an expected outcome. 

  • Newspaper or news stories. 

  • A similarity between physical and spiritual things (e.g. light to wisdom, or warmth to love) 

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