This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A simple and efficient means of projecting protractor readings to a larger size is shown in Fig. 1. One point of the compass is placed at the center of the protractor and an elastic band is looped between the points. Then the points are spread to the radius desired, and the protractor is read where the elastic band crosses its scale. A light band should be used, and looped as shown in Fig. 2. In this way a circle of any size may be quickly divided, if a pencil mark is made each time the band comes over the proper figure. - Contributed by Thos. L. Parker, Wibaux, Mont.
Ill: The Extension Marks can be Easily Read on the Protractor under the Elastic Band