Stereographic and Gnomonic Projections. 215 



drawing the great circle SES', figure 1. The angles of the 

 triangles, figure 13, are based upon the data chosen for con- 

 structing figures 1 and 7 ; namely, the first meridian 20° from 

 S, and E 10° from the center of the projections : For any 

 desired variation from these data the angles could be calcu- 

 lated readily. The writer has found triangles made from 

 heavy bristol-board in every respect serviceable for pencil 

 work, and if cut, for example, as shown in figure 13-7, so 

 that only two extremities of the base line touch the T-square, 

 they may first be made approximately right and then accu- 

 rately adjusted by talcing off a little from one end or the other 

 by rubbing against sandpaper. Referring to figure 8 ; having 

 determined the direction c ^ ?n'", and knowing the ortkorhom- 

 bic symmetry of the crystal, the remaining lines could all be 

 determined by means of triangle 1. From figure 8 ; and by 

 use of the triangles II and III, figure 9 could be constructed 

 by finding only one direction, for example, d a m. The tri- 

 angles thus serve as time-savers to any one engaged in this 

 kind of crystal drawing, and likewise insure increased accu- 

 racy in the work. 



Mineralogical Laboratory of the Sheffield Scientific School 

 of Yale University, New Haven, Conn., December, 1905. 



