compass vs pair of compasses

compass

noun
  • A pair of compasses (a device used to draw an arc or circle). 

  • A magnetic or electronic device used to determine the cardinal directions (usually magnetic or true north). 

  • The range of notes of a musical instrument or voice. 

  • Moderate bounds, limits of truth; moderation; due limits; used with within. 

verb
  • To surround; to encircle; to environ; to stretch round. 

  • To go about or round entirely; to traverse. 

pair of compasses

noun
  • A tool for drawing accurate circles or arches, usually consisting of two arms joined at one end in such a way that the arms can be opened and closed; one of the arms is pointed at its free end while the other holds a pencil, pencil lead or some other implement for making marks. 

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