slacken vs taper off

slacken

verb
  • To gradually decrease in intensity or tautness; to become slack. 

  • To make slack, less taut, or less intense. 

  • To deprive of cohesion by combining chemically with water; to slake. 

taper off

verb
  • To diminish or lessen gradually; to become smaller, slower, quieter, etc. 

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