7Q 



DECREMENTS, 



Fig. 102. 



Fig. 102 contains a similar decrement on the angle 

 of a tetrahedron. 



Fig. JOS. 



Fig. 103 shews the effect of a similar decrement on 

 the angle of an octahedron. 



In the illustrations hitherto given of the nature of 

 decrements, and of the characters of the secondary 

 planes produced by them, we have considered the 

 effect produced upon only a single edge or angle of 

 the primary form. But as decrements generally take 

 place equally on all the similar edges and angles of 

 any primary form, it will be necessary now to enquire 

 into the manner in which these similar edges and 

 angles are affected, when they are all operated upon 

 at the same time, by any given decrement. 



From the definition already given in page 3, of the 

 nature of similar edges and angles, it will appear that 

 in the rectangular prism, those edges only are similar 



