difficulty vs mildness

difficulty

noun
  • Physical danger from the environment, especially with risk of drowning 

  • An obstacle that hinders achievement of a goal. 

  • The state of being difficult, or hard to do. 

  • An objection. 

  • An awkward situation or quarrel. 

  • That which cannot be easily understood or believed. 

mildness

noun
  • The quality of being mild; gentleness. 

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