employ vs retain

employ

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

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

  • To make busy. 

noun
  • The state of being an employee; employment. 

retain

verb
  • To keep in one's pay or service. 

  • To employ by paying a retainer. 

  • To hold back (a pupil) instead of allowing them to advance to the next class or year. 

  • To keep in possession or use. 

  • To hold secure. 

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