Geographical Maps and Sailing Charts. 



247 



to consider the plane of the projection either as passing through 

 the center of a sphere, figure 1, or as tangent to it, figure 2, 

 but, provided its direction is not changed, the position of the 

 plane may be altered without in any way affecting the char- 

 acter of the projection ; the size becomes greater as the plane 

 is shifted to a position farther from the center. 



Figure 1 is a photographic reproduction of a model con- 

 structed by the writer which needs but little explanation, 



Model for illustrating the principle upon which the stereographic projection 



is based. 



The original skeleton sphere, made of wire, is eighteen inches 

 in diameter, aad the map of the Western Hemisphere, drawn 

 on cardboard, is inserted at the center. Following the custom 

 of geographers, the meridian bounding the map is numbered 

 20° W. (160° E.), and the plane upon which the map is drawn 

 is at right angles to a diameter running from the intersection of 

 the 110th meridian and the equator to the point P. Lines of 

 projection are indicated by threads running from P to the 

 intersections of the meridians with the parallels and equator. 



Figure 2 is a diagram plotted in clinographic projection, the 

 same as is used in drawing crystal figures. The original draw- 



