helper vs sub-lieutenant

helper

noun
  • One who helps; an aide; assistant; auxiliary. 

  • That which helps; anything serving to assist. 

  • A locomotive that assists a train, usually on steep gradients. 

  • A person who does cleaning and cooking in a family home, or in a market; domestic employee. 

sub-lieutenant

noun
  • A commissioned officer of the navy ranking above an acting sub-lieutenant and below a lieutenant. 

  • A commissioned officer of the Royal Navy ranking above a midshipman and below a lieutenant. 

How often have the words helper and sub-lieutenant occurred in a corpus of books? (source: Google Ngram Viewer )