

ITS MODIFICATIONS. 



Primary form. A rhomboid. 



It is found convenient to designate the edges and 

 angles 'of this figure as follows. 



The angle at A is the superior angle of the plane P ; 

 that at O is the inferior angle ; those at F are the 

 lateral angles ; the edges B are the superior edges ; 

 those at D the inferior edges. 



The solid angle at A may also be called the termi- 

 nal solid angle, or solid angle of the summit. The 

 edges B the terminal edges, or edges of the summit. 

 The solid angles at E, the lateral solid angles, and 

 the edges D, the lateral edges. 



The individuals belonging to this class are usually 

 distinguished from each other by the inclination of P 

 on P'. When P on P' measures more than 90, the 

 rhomboid is called obtuse ; when less, it is called 

 acute. 



The angle P on P', is limited between 180 and 60, 

 but it is obvious it can never reach either of those 

 limits ; for the axis must vanish, before the planes P 

 and P 7 would reach 180, or become one plane, and 

 it must be infinite, before these planes could incline 

 to each other at an angle of 60; in either of which 

 cases the figure would cease to be a rhomboid. 



