lull vs smoothen

lull

verb
  • To become gradually calm; to subside; to cease or abate. 

  • To cause to rest by soothing influences; to compose; to calm 

noun
  • A period of rest or soothing. 

  • A period without waves or wind. 

  • An extended pause between sets of waves. 

  • A period of reduced activity; a respite 

smoothen

verb
  • To become smooth. 

  • To make smooth. 

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