surmountable vs unimpregnable

surmountable

adj
  • Able to be surmounted or overcome; defeatable. 

unimpregnable

adj
  • Not impregnable; capable of being pierced or assailed. 

  • Incapable of being impregnated; impermeable or infertile. 

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