dampen vs keep under

dampen

verb
  • To become damped or deadened. 

  • To lessen; to dull; to make less intense (said of emotions and non-physical things). 

  • To become damp or moist. 

  • To make damp or moist; to make moderately wet. 

keep under

verb
  • To hold in subjection; hence, to oppress. 

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