by Means of Graj^Mcal Methods. 



271 



graphic projection, was studied and figured by Mr. P. B. 

 Condit. 



The combination is a simple one, base c (00 L), pyramid^ (111) 

 and ditetragonal pyramid s. As fundamental measurement 

 the angle oi jp /\]) over the base, 136° 36^, was chosen, from 

 which <? A J9 = 68° 18^ is obtained. The poles of the pyramid^ 

 are easily located, figure 19, by means of the stereographic scale. 

 Erecting a perpendicular at x' to represent the vertical axis, 

 and plotting the angle 68° 18' from the center, the length of 

 the vertical axis was found by means of scale I^o. 4 to be 



19 



1-Y71, the calculated length being I'TTT. By means of the 

 small circle protractor the distances betw^een the poles of the 

 pyramid, 111 Alll and 111 Alll were found to be 82° 20' and 

 82° 15', the calculated value being 82^ 9'. 



For the determination of the symbol of the ditetragonal 

 pyramid, the angles between two adjacent s faces and from g 

 to 5 were measured, as follows: ^A'^=9° 40', (?A5=25° 30'. 

 About the center a small circle with radius 25° 30', taken from 

 the stereographic scale, was first described, upon which the 

 poles of the pyramid must be located. Since s f\s is 9° 40', 

 four of the poles (those lettered s in figure 19) must be sym- 

 metrically located at 4° 50' on either side of the vertical diam- 



