20 



S. L. Penfield — Stereographic Projection. 



measure the angles A and D, figure 12, by means of the gradua- 

 tion on the base line of protractor "No. I, page 7, than by con- 

 structing tangents and measuring the plane angles. 



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Supplementary Protractors. — Equipped with sheets contain- 

 ing the divided circle and scales, figure 3, and with protractors 



