backflow vs waning

backflow

verb
  • To flow in the wrong or backward direction. 

noun
  • The flow of a fluid (through a pipe etc.) in a direction opposite to that which is normal or intended. 

waning

noun
  • The fact or act of becoming smaller. 

  • The fact or act of becoming less or less intense or present; fading. 

adj
  • Becoming weaker or smaller. 

  • Of the lunar phase: as it shrinks when viewed from the Earth. 

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