5.  And  all  the  first-born  in  the  land  . . .  shall  
  die--The  time,  the  suddenness,  the  dreadful  severity  of  this  coming  
  calamity,  and  the  peculiar  description  of  victims,  among  both  men  and  
  beasts,  on  whom  it  was  to  fall,  would  all  contribute  to  aggravate  its
  character.
  
         
  the  maid-servant  that  is  behind  the  mill--The grinding of  the  
  meal  for  daily  use  in  every  household  is  commonly  done  by  female  slaves  
  and  is  considered  the  lowest  employment.  Two  portable  millstones  are  
  used  for  the  purpose,  of  which  the  uppermost  is  turned  by  a  small  
  wooden  handle,  and  during  the  operation  the  maid  sits  behind  the  
  mill.
JFB.
Picture Study Bible