Chapter 4 BUILDING THE WORLD'S FIRST RULER Three grains of barley, dry and round, make an inch. Proclamation of King Edward II, 1324 In chapter 2 we considered how to draw a straight line. This turned out to be a little more difficult than one might assume. What we shall do in this chapter is turn this straight edge into a graduated ruler, so that we can actually begin to measure, and in order for this ruler to be useful we should ensure that the graduations are equally spaced. Just as we cannot use a straight edge to draw the first straight line, so we shall not be able to use an existing ruler to mark out the graduations on our