cat-o'-nine-tails vs riding crop

cat-o'-nine-tails

noun
  • A similarly constructed leather nine-tail whip, as used in British penal colonies and certain armies. 

  • A scourge (multi-tail whip) having nine, often knotted, whipcords, formerly used for flogging as naval punishment. 

riding crop

noun
  • A horsewhip, short whip used to goad a mount. 

How often have the words cat-o'-nine-tails and riding crop occurred in a corpus of books? (source: Google Ngram Viewer )