LAWS OF DECREMENT 



339 



THE RIGHT SQUARE PRISM. 



Its elements. 



The inclination of any two adjacent planes at their 

 common edge nr 90. 

 Plane angles 90. 

 Terminal edges equal. 

 Lateral edges equal. 



Its units of comparison. 

 Fig. 346. 



Let the terminal edge be to the lateral edge as 

 mini this will be the unit of comparison for decre- 

 ments on the terminal edges. For simple and mixed 

 decrements on the angles of the terminal planes, 

 the ratio of a b : af becomes the unit, which is 



V<2 m 2 m 

 " ~~ nVo 9 an d f r similar decrements on 



the lateral angles the ratio of a e, being \ the diagonal 

 of the lateral plane, to the terminal edge a d, may be 

 regarded as the unit. But we cannot immediately 

 determine the ratio of the intercepted portions of the 

 lines a e, ad, from the inclination of the secondary 

 plane on the lateral primary plane. It must be 

 deduced from the ratio to the edge a d, of a perpen- 

 dicular a c upon the diagonal mf. 



It is very obvious that this perpendicular possesses 

 the character assigned to those lines from which the 



