SECTION V. 



DECREMENTS. 



Decrements have been already defined to be either 

 simple, mixed, or intermediary ; and the simple decre- 

 ments have been divided into two classes, according 

 as they take place in breadth, or in height : see Defi- 

 nitions, page 19. 



The manner in which all the classes of decrements 

 operate in the production of new planes, has also 

 been explained. 



Simple and mixed decrements take place either on 

 the edges of crystals, or on the angles, and produce 

 new planes which intersect one at least of the primary 

 planes, in lines parallel to one of its edges or diagonals. 



Fig. 97. 



If either a simple or mixed decrement take place on 

 the edge a b, of any primary form whatever, a new 

 plane is produced, whose intersection c d, with the pri- 

 mary plane along which the decrement may be conceived 

 to proceed, is parallel to the edge a b, from which it 

 may be said to begin or set out. 



Fig. 97 shews the character of the secondary plane 

 produced by a simple or mixed decrement on the edge 

 of a rectangular prism. 



i 2 



