DECREMENTS, 



decrement may be conceived to proceed, is parallel 

 to the diagonal of that plane. 



Fig. 100. 



Fig. 100 exhibits a plane produced on the angle of 

 a rectangular prism, by a simple or mixed decrement; 

 the edge c d, of the secondary plane, being parallel to 

 the diagonal a b, of the primary form. 



Fig. 101, 



Intermediary decrements may be said to take place 

 only on the solid angles of crystals, by the omission of 

 unequal members of molecules in the direction of the 

 edges which meet at such solid angle. And a plane 

 is thus produced, none of whose lines of intersection, 

 a by c b, ac, with the primary planes, are parallel to 

 any edge, or diagonal of those planes. 



Fig. 101 shews the position of a plane produced by 

 an intermediary decrement on the angle of a rectan- 

 gular prism. 



