Compass or Compasses?

My thoughts.
According to the Oxford English Dictionary the history of the word ‘compass’ presents many points of uncertainty; and the history of the various senses in which the noun is now used is also obscure. But as regards the mathematical instrument which is the subject of the question, O.E.D. is quite explicit. Its definition says:
Compass. ‘An instrument for taking measurements and describing circles, consisting (in its simplest form) of two straight and equal legs connected at one end by a movable joint.’

There are also several quotes using the word ‘compas’ from c 1340 onwards in out present ‘pair of compasses’ sense of the word.

In my view a Compass is an instrument to find your position in relation to the North Pole. A pair of compasses i.e. possessing two legs. are an instrument for scribing circles or parts thereof