Chapter 10 HOW ROUND IS YOUR CIRCLE? Round and round the rugged rock the ragged rascal ran. Nursery rhyme Imagine you wish to make a circular or spherical object but can- not simply use compasses to mark one out. Alternatively, you might have already made something round and wish to check the accuracy of your construction. We all know what a circle is, so here we discuss the problem of determining if what is supposed to be circular is really circular, and within what limits. This matters a lot in engineering applications of all sorts but particularly with rotating shafts and their bearings. So let us begin with an experiment for which you need a United