II
THE SYSTEM OF CO-ORDINATES
ON the basis of the physical interpretation of distance which has been indicated, we are also in a position to establish the distance between two points on a rigid body by means of measurements. For this purpose we require a "distance" (rod
) which is to be used once and for all, and which we employ as a standard measure. If, now,
and
are two points on a rigid body, we can construct the line joining them according to the rules of geometry; then, starting from
, we can mark off the distance