airship vs fire balloon

airship

noun
  • A lighter-than-air aircraft that can be propelled forward through the air as well as steered. 

  • A police air unit, a police helicopter 

  • Any aircraft. 

  • A high ranking official of the RAF, viewed as arrogant and distant. 

verb
  • To transport goods by aircraft. 

fire balloon

noun
  • A sky lantern. 

  • A Japanese weapon of World War II, consisting of a hydrogen balloon with a bomb attached. 

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