US TERMINOLOGY. . 138. 



ranikls, besides rectangular four-sided prisms, and eight- 

 sided prisms of alternately equal angles, &c. It is therefore 

 not very difficult from one form being known, to trace or 

 find out the system to which it belongs. The only excep- 

 tion occurring here, is the right rectangular four-sided 

 prism, which may belong, as a simple form, or as a com- 

 pound one, to three, or at least to two different systems, 

 if we abstract from the plane perpendicular to the axis. 

 As a simple form, it is the hexahedron, and belongs to the 

 tessular system ; as a compound form, which consists of two 

 simple ones (P os and P -j- ), it is a right rectangular 

 prism, and belongs to the pyramidal system ; and as a com- 

 pound form, consisting of three simple ones (P eo. 

 Pr -f oo. Pr 4- GO.), it is likewise a right rectangular prism, 

 but belongs to the prismatic system. The abstract geome- 

 trical consideration of these forms, yields no characters by 

 which they could be distinguished from one another, though 

 the means will be afterwards (. 159.) pointed out, by which 

 this uncertainty may be removed, and which principally 

 depend upon the connexion of certain forms with each 

 other, and upon several peculiarities occurring along with 

 them in natural bodies. 



If the given form possesses finite dimensions, these are 

 either known, or may be found by immediate measure- 

 ment ; in both cases, therefore, it is possible to obtain those 

 of the fundamental form, and consequently also of the se- 

 ries to which it belongs. 



III. OF COMBINATIONS. 



OF COMBINATIONS IN GENERAL. 



. 138. DEFINITION. 



A compound form is termed a Combination 

 (. 34.). 



