LAWS OF DECREMENT. 



341 



THE RIGHT RECTANGULAR PRISM. 



Its elements. 



The inclination of any two adjacent planes at their 

 common edge ~ 90. 

 Plane angles zz 90. 

 Parallel edges are always equal. 

 Three adjacent edges always unequal. 

 This inequality cannot be determined but by 

 the means of secondary planes. 



Its units of comparison. 

 Fig. 347. 





Let us suppose the edge o a : o b :: m : n 



o a : o c :: m : o 



will evidently be the unit of comparison for deter- 

 o 



mining the laws of decrement producing the planes 

 of class b. 



, for those of class c. 

 o 



, for those of class d, whose symbol is G p P G. 

 n 



" P G G p 



m 



It has been shewn in p. 267, that the planes com- 

 prehended under class #, might be produced by four 



