assart vs empty space

assart

verb
  • To clear forest land for agriculture; remove stumps. 

noun
  • Forest land cleared for agriculture. 

empty space

noun
  • Vacuum; a space containing nothing at all 

  • An unoccupied space where something may be stored 

  • A feeling of longing for someone or something that is gone; a sense of a thing's absence. 

  • An unoccupied area or volume. 

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