cistern vs vacuity

cistern

noun
  • A cisterna. 

  • The vessel surrounding the condenser in a steam engine. 

  • In a flush toilet, the container in which the water used for flushing is held; a toilet tank. 

  • A reservoir or tank for holding water, especially for catching and holding rainwater for later use. 

vacuity

noun
  • An empty or inane remark or thing. 

  • Idleness; listlessness. 

  • Emptiness. 

  • Physical emptiness, an absence of matter; vacuum. 

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