lightship vs pharos

lightship

noun
  • A vessel riding at anchor and displaying a light for the guidance of sailors, in a position where a fixed lighthouse structure would be impracticable. 

pharos

noun
  • An ancient lighthouse or beacon to guide sailors. 

  • That which stands out; a shining light. 

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