guidepost vs heliograph

guidepost

noun

  • A signpost. 

  • Anything that provides guidance; a guideline. 

heliograph

noun

  • A device for photographing the sun. 

  • A heliogram. 

  • An apparatus for signalling by means of a moveable mirror which reflects flashes of sunlight. 

  • An instrument for measuring the intensity of sunlight. 

verb

  • To send a message by heliograph. 

  • To send a heliograph. 

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