introduce vs usher in

introduce

verb
  • To make (something or someone) known by formal announcement or recommendation. 

  • To cause (someone) to be acquainted (with someone else). 

  • To add (something) to a system, a mixture, or a container. 

  • To bring (something) into practice. 

usher in

verb
  • To announce the arrival of something; to herald. 

  • To begin something with preparatory material; to introduce. 

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