DEFINITIONS. 



23 



in length, breadth, and height. Thus if we suppose the 

 compound molecule abstracted in an intermediary 

 decrement to belong to a single plate, it must consist 

 of some other numbers of molecules in the directions 

 d, and e, fig. 48.* 



Fig. 48. 







In fig. 48 the compound molecule consists of a single 

 molecule in height, two on the edge d, and three on 

 the edge e, producing the new plane a b c. 



Fig. 49. 



Fig. 49 exhibits an intermediary decrement in 

 which the compound molecule consists of three single 

 molecules in height, four on the edge d^ and two on 

 the edge e, producing the new plane a b c. 



In the simple and mixed decrements upon an angle, 

 as shewn in fig. 43 and 46, the number of molecules 



* It may be remarked that the planes produced by simple and mixed 

 decrements, intersect one or more of the primary planes in lines parallel 

 to one of their edges or diagonals. The term intermediary has been used 

 to express this third class of decrement, because the line at which the 

 secondary plane produced by it, intersects any primary plane, is never 

 parallel to either an edge or diagonal of that plane, but is an intermcdiatt 

 line between the edge and the diagonal, as may be observed by com- 

 paring the figures 42, 43, and 48. 



