ball lightning vs comet

ball lightning

noun
  • A short-lived glowing ball sometimes observed to float in the air and thought to consist of ionized gas associated with thunderstorms. 

comet

noun
  • A small Solar System body consisting mainly of volatile ice, dust and particles of rock whose very eccentric solar orbit periodically brings it close enough to the Sun that the ice vaporises to form an atmosphere, or coma, which may be blown by the solar wind to produce a visible tail. 

  • Any of several species of hummingbird found in the Andes. 

  • A celestial phenomenon with the appearance of such a body. 

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