lightening vs slackening

lightening

noun
  • The act or result of making something light or lighter. 

  • The removal of cargo (especially crude oil) from a vessel in order to reduce its draft 

  • The sensation caused by the descent of the uterus into the pelvic cavity before the onset of labour 

slackening

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

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