162 TERMINOLOGY. . 145. 



belonging to different series (. 1 1G), provided they are in a 

 parallel position. Any two rhombohedrons in a parallel 

 position intersect each other in horizontal edges, and their 

 dimensions therefore cannot be determined solely from a 

 combination of this kind. 



The reason why the edges of combination here are ho- 

 rizontal is evident, because the horizontal projections being 

 supposed equal, those faces of two rhombohedrons in a 

 parallel position, which are contiguous to the same apex,* 

 must intersect each other in the line GG (. 143.); and 

 this line itself is horizontally situated in the upright com- 

 bination. 



3. Let n' be = w, R + n' therefore = R cc. This 

 form appears as a face perpendicular to the axis of the 

 rhombohedron R + n (. 111.), and the resulting edges of 

 combination must evidently be horizontal, whatever value 

 n may affect. Example, R co (o) and R (P) in rhom- 

 bohedral Alum-haloide. Vol. II. Fig. 111. 



4. Let n' be = + co. Upon this supposition, R + n' 

 is = 11 + co, or a regular six-sided prism, the limit of the se- 

 ries of rhombohedrons (. 111.). The edges of combination 

 produced by its alternating faces with those of the upper 

 apex of R -4- n are horizontal ; so are the similarly situat- 

 ed ones between the rest of its faces and the faces of the 

 rhombohedron contiguous to the lower apex ; and their 

 situation is independent of the dimensions of R + n (2.). 

 There are also inclined edges of combination to be found 

 here, which depend upon R + n. If we consider the re- 

 gular six-sided prism as a rhombohedron of an infinite axis 

 (. 111.), they are produced by the intersection of those of 

 its faces which are contiguous to the lower apex, and 

 those faces of R+ n which are contiguous to the upper apex 



* It would lead us too far to consider more at large the 

 intersections of faces contiguous to opposite apices, and 

 the edges of combination which they produce. A few ex- 

 amples contained in the subsequent part of the work, will 

 shew their application. 



