hy Means of Grajphical Methods. 



2T^ 



bol of the scalenoliedron, as indicated by the method, is 

 \a \ a \ —\a : c, or 2131. The radius through c, and the chord 

 at right angles to it from a^^ establish the relation on the hori- 

 zontal axes, f« : a —\ct^ and also the length of the base-line Oz. 

 The slope of the face, measured with the stereographic scale, 

 was 80° 35', calculated 80° 32', and this angle plotted from 

 the center intersected the vertical form z at a distance 2'993<?, 

 practically 3c. The syinbol of the scalenohedron is therefore 

 \a \ a \ —^a : Sc, or 4371. The rhombohedron J/ was deter- 

 mined by the measurement rAM=di° 10', giving 0001 a3/= 

 75° 1:7'. This angle, plotted from the center, would intersect 

 at 46', measured from x\ or, instead of extending the drawing 

 to such an extent, the slope may be plotted from <?, as indicated 

 in figure 21, and its intersection with the base-line, eqiml ^x' 0, 

 determined. The svmbol of J/ is therefore 1011. 



21 



To 3C,from z' 



\'i rfK 



1 







\ 



s 



/■ \ 3%^ 



] a 



r' ko^ Z 



i 







y 



at 



It seems worth while to call attention to a procedure em- 

 ployed in solving a complex problem encountered in the study 

 of calcite crystals from Union Springs, N. Y.^ The form to 

 be considered is that of a scalenohedron t\, in the same zone 

 as V and c, figure 21, but just a trifie steeper than v. The 

 angle v on %\ measured 3° 55'; hence since c<^A'y = 23° 31', 

 <^Ai\=l')° 36'. Employing the same method of plotting as 

 indicated in figure 21, the radius through the pole of v^ was 

 found to intersect the divided circle at 21° from m (theory 

 21° 3'), and a chord drawn from a^^ at right angles to it, gave 



* This Journal (4) x, p. 237, 1900. 



