. 142. 143. OF COMBINATIONS. 153 



been done with certain irregularities of simple forms (. 45.), 

 and reduce the combinations themselves to their peculiar 

 symmetry. 



. 142. EDGES OF COMBINATION. 



The edges, in which the faces of two different 

 forms contained in a combination, meet or intersect 

 each other, are termed Edges of Combination. 



Compound forms contain sometimes a great many edges 

 of combination : in this case they are determined and dis- 

 tinguished from each other by attending to the simple 

 forms, between the faces of which they are situated, or 

 whose intersections they represent. What has been said 

 in . 29. 32. of edges in general, applies likewise to edges 

 of combination. 



It sometimes happens that the edges of the simple forms 

 disappear entirely in a compound form, so that every edge 

 to be met with is an edge of combination. 



. 143. DEVELOFEMENT OF COMBINATIONS. 



The developement of a combination consists in 

 determining, 1 . the kind of all the simple forms con- 

 tained in it ; 2. their peculiar position ; and 3. their 

 relations to each other. 



It is not difficult to recognise the kind of simple forms 

 contained in a combination. For this purpose, enlarge one 

 set of homologous faces which it contains after the other, 

 till the rest disappear ; and the kind of the form will be- 

 come evident, from their number and disposition. Each 

 of these forms is obtained at the same time in their peculiar 

 position. 



The determination of the mutual relations of the forms 

 is in many cases somewhat more circumstantial. They 

 might be derived immediately from their dimensions, 



