mild vs rigid

mild

adj
  • Not overly felt or seriously intended. 

  • Not sharp or bitter; not strong in flavor. 

  • Of only moderate severity; not strict. 

  • Not serious or dangerous. 

  • Moderately warm, especially less cold than expected. 

  • Gentle and not easily angered. 

  • Acting gently and without causing harm. 

noun
  • A relatively low-gravity beer, often with a dark colour; mild ale 

rigid

adj
  • Uncompromising. 

  • Stiff, rather than flexible. 

  • Fixed, rather than moving. 

  • Rigorous and unbending. 

noun
  • An airship whose shape is maintained solely by an internal and/or external rigid structural framework, without using internal gas pressure to stiffen the vehicle (the lifting gas is at atmospheric pressure); typically also equipped with multiple redundant gasbags, unlike other types of airship. 

  • A bicycle with no suspension system. 

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