declivity vs plummeting

declivity

noun
  • A downward bend in a path. 

  • The downward slope of a hill; the downward slope of a curve. 

  • An inward curve of the exoskeleton of an insect, such as between body segments; a segment of an insect's body where the exoskeleton curves inward. 

plummeting

noun
  • A violent or dramatic fall. 

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