airship vs zeppelin

airship

noun
  • A police air unit, a police helicopter 

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

  • Any aircraft. 

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

verb
  • To transport goods by aircraft. 

zeppelin

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