acclivity vs declivity

acclivity

noun
  • A slope or inclination of the earth, as the side of a hill, considered as ascending, in opposition to declivity, or descending; an upward slope; ascent. 

declivity

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

  • A downward bend in a path. 

  • 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. 

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