airmail vs second class

airmail

noun
  • The items of mail so carried. 

  • The system of conveying mail using aircraft. 

verb
  • To send mail by air. 

  • To (unintentionally) throw the ball well over a fielder's head where that fielder is unable to make a play on the ball. 

second class

noun
  • A class of postage with delivery times slower than first class. 

  • A class of travel, inferior to first class. 

  • The second-lowest rank in the Boy Scouts of America, between tenderfoot and first class. 

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