LAWS OF DECREMENT. 



325 



THE OCTAHEDRON WITH A SQUARE BXSE. 



Its elements. 



The inclination of the planes at the terminal edges 

 will differ in different minerals, and may be 

 represented generally by /,. 

 Inclination of the planes at the edges of the 

 base will also vary in different minerals, and 

 may be called J 2 . 



Plane angles at the ?ummit will also vary, but 

 they may be denoted generally by A l9 and 

 may be thus deduced, 



A R. cos. 45 



cos. \A V - 



It is by means of the upper spherical 

 triangle marked on the fig. that we are 

 enabled to determine the angle A,. 



For if d k represent f the axis, d e a per- 

 pendicular on the base, and h k j a diagonal 

 of the base, the angles of the spherical tri- 

 angle will be 90, 45, and* /,. And thence 

 the angle e d h, which we have called \ l A t 

 is deduced. 



