employ vs mastery

employ

noun
  • The state of being an employee; employment. 

verb
  • To hire (somebody for work or a job). 

  • To use (somebody for a job, or something for a task). 

  • To make busy. 

mastery

noun
  • The act or process of mastering; the state of having mastered; expertise. 

  • Superiority in war or competition; victory; triumph; preeminence. 

  • The position or authority of a master; dominion; command; supremacy; superiority. 

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