68 TERMINOLOGY. . 82. 83.' 



This process is not limited to forms which, like the rhom- 

 bohedron, are originally contained under tetragonal faces ; 

 but it can be extended to such as are originally contained 

 under triangles, and is therefore applicable to pyramids of 

 every description. In this case, however, the given form 

 requires a certain preparation, the nature of which will be 

 explained in its proper place. 



Forms produced in this way, if simple, are the derived 

 forms themselves; if compound, they are, like those in 

 . 80., considered as intermediate or auxiliary forms, and 

 made use of accordingly. 



Of intermediate forms in general, it may here be re- 

 marked, that, by enlarging their homologous faces, till the 

 rest disappear, they may be resolved, and by that means the 

 simple forms which they contain, may be extracted. 



. 82. THIRD PROCESS. 



The third process requires planes to be laid on 

 the terminal edges of the given form, which may 

 likewise be an intermediate one (. SO. 81.) ; their 

 number and inclination being such, that the inter- 

 sections of the faces from both apices produce a 

 plane figure, similar and parallel to the horizontal 

 projection of the given form. The derived form is 

 contained under these planes. 



The number of faces contiguous to every terminal edge, 

 as employed in this process, is either one or two ; more 

 than two faces can never be applied to one terminal edge. 

 This process in some cases affords a determined solution of 

 a problem, which it would be impossible to obtain from a 

 process analogous to that of . 80. 



. 83. FOURTH PROCESS. 



The fourth process requires the consideration of 

 those differences which take place in the situation 



