bolstering vs fortification

bolstering

noun
  • The act by which something is bolstered; support. 

fortification

noun
  • The act of fortifying; the art or science of fortifying places to strengthen defence against an enemy. 

  • A jagged pattern sometimes seen during an attack of migraine. 

  • That which fortifies; especially, a work or works erected to defend a place against attack; a fortified place; a fortress; a fort; a castle. 

  • An increase in effectiveness, as by adding ingredients. 

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