damp vs dampen

damp

verb
  • To suppress vibrations (mechanical) or oscillations (electrical) by converting energy to heat (or some other form of energy). 

noun
  • A gaseous product, formed in coal mines, old wells, pits, etc. 

  • Moisture; humidity; dampness. 

adj
  • In a state between dry and wet; moderately wet; moist. 

  • Despondent; dispirited, downcast. 

dampen

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

  • To become damped or deadened. 

  • To become damp or moist. 

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

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