lessen vs modify

lessen

verb
  • To make less; to diminish; to reduce. 

  • To become less. 

conj
  • unless. 

modify

verb
  • To set bounds to; to moderate. 

  • To change part of. 

  • To qualify the meaning of. 

  • To be or become modified. 

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