Geometric aid for drawing ellipses - has diamond-shaped body with perforations along lines which indicate centres of required circular arcs

摘要

An instrument, to aid in the drawing of approximate ellipses by a series of circular arcs, is made from a piece of transparent plastics in the shape of a rhombus. A row of equally spaced holes is made along each diagonal and the holes are numbered with the numbers indicated on lines joining corresponding holes in each diagonal. To draw an ellipse with a major axis of 9 cm the rhombus is positioned and a pencil point is placed through the hole on each axis at the intersection of the lines marked (9). These points are then used as centres to draw arcs of circles which form the ellipse.