diminution vs slackening

diminution

noun
  • The act or process of making diminutive. 

  • A lessening, decrease or reduction. 

  • a compositional technique where the composer shortens the melody by shortening its note values. 

slackening

noun
  • The act by which something slackens; loss of speed, tautness, etc. 

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