Tower vs penal colony

Tower

noun
  • Denoting the system of weights used by the Saxon and Norman English kings in their minting of coins. 

name
  • The Tower of London, especially seen as a place of imprisonment or punishment. 

  • A habitational surname.; Alternative form of Towers 

penal colony

noun
  • A colonial territory used (mainly) for the detention and forced labor of deportees, typically where free labor is desperately scarce. 

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