. 34'. 35. OF FORMS IN GENERAL. 31 



A solid angle formed by the intersection, or consisting 

 of three, four, five, &c. faces, is said to be a solid angle of 

 three faces, a solid angle of four faces, &c. A solid angle 

 is equiangular ', if the plane angles contiguous to it are equal ; 

 it is unequiangitlar, if they are different from each other. A 

 solid angle, resulting from the junction of two, three, &c. 

 different kinds of edges, is said to be digrammic, trigrammic, 

 &c. ; and a solid angle having all its edges equal, or which 

 possesses only one kind of edge, is, in opposition to the 

 latter, termed a monogrammic solid angle.* 



. 34. SIMPLE AND COMPOUND FOEMS. 



A form contained under homologous faces 

 (. 31.) is termed a Simple Form ; one that is con- 

 tained under faces which are not homologous, a 

 Compound Form. 



We have examples of the former, in the Hexahedron, the 

 Octahedron, as Geometry considers those solids, Fig. 1. 2., 

 and in several others besides. Of the latter in the same, 

 if their angles or edges, or both, are replaced by faces not 

 belonging to their own form, Figs. 3. 4., or in general, if the 

 form is limited by more and other faces, than is required 

 for a simple form. 



. 35, THE COMPOUND FORMS CONSIST OF THE 

 SIMPLE. 



A compound form consists of two or more simple 

 ones. Those faces of the compound, which are 



* MonogrammtCi single-edged or one-edged, from 

 single, and y^PP** a line ; digramtnic, double-edged or two- 

 edged, from $/? , double, and <ygct/u.{A$i ; trigrammic, triple- 

 edged or three-edged, from r^j, triple, and y^a^rj ; referring 

 to the number of different kinds in those lines or edgesy 

 which terminate in the solid angle. 



