256 Pen field — Use of the Stereograjphic Projection for 



with a diameter, and it may then be told by inspection what 

 line of the protractor is tangent to the great circle under con- 

 sideration. In the example cited it is evident that the 60° 

 lines of the protractor are tangent to the great circle arcs 

 passing through o. With such a protractor it is probable 

 that angles could be measured to within a quarter of a ^degree 

 or closer, depending upon the size of the projection. 



Method of measuring spherical angles at the point o by means of a trans- 

 parent protractor, PP. 



Projection upon the plane of a Meridian, — Figures 8 and 9 

 represent the Western and Eastern Hemispheres in stereo- 

 graphic projection on the meridian 20° W., 160° E., from 

 Greenwich. The construction of the meridians and parallels is 

 a very simple matter. A circle of any desired size and two 

 diameters at right angles to one another are first drawn. The 

 points of intersection of the meridians with the equator and 

 of the parallels with the central meridian are next determined, 

 best from a table, while the radii of the several meridians and 

 parallels are likewise taken from tables described in a note, at 

 the close of this article. Attention may be called to the fol- 

 lowing interesting relations which may at times be useful. 

 The center points about which the meridians 10°, 20°, etc., 

 from the periphery are described are the stereographically pro- 

 jected 20°, 40°, etc., points on the equator, measured from the 

 center. The lengths of the radii for describing the parallels 

 10°, 20°, etc., from the poles are the distances from the center 

 of the map to the stereographically projected 20°, 40°, etc., 



