decemvirate vs heptarchy

decemvirate

noun
  • A group of ten people, especially (politics) a council of ten men sharing office or power and particularly (historical) the groups of ten men who reformed and codified Roman law circa 450 BCE. 

heptarchy

noun
  • A government of seven people. 

  • The realm so ruled. 

  • A group of seven states, especially (historical) those in Anglo-Saxon Britain. 

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