ON THE USE OF SYMBOLS. 245 



edges is implied, unless their absence be denoted by 

 the characters which will be presently given and ex- 

 plained. This symbol implies that the edge B is 

 replaced by two planes, one of which results from a 

 decrement by three rows in breadth proceeding along 

 the terminal plane, and the other by three rows in 

 breadth proceeding along the lateral plane. The 



symbol B might be sufficient to denote the planes of 

 modification e, but fur the sake of conformity with 



the general system of notation, it should be written 



i 

 B. 



i 



When the lateral edge of a prism is modified by 

 two similar planes, the symbol representing them 

 will be P G P . The G standing single, implies that 

 the symbol refers to a single edge. 



The planes belonging to class d of the modifications 

 of the right rectangular prism, may be readily con- 

 ceived to result from decrements proceeding along 

 either of the planes M or T. If along the plane M 5 

 the symbol would be G /p P G ; but if the decrement 

 be supposed to have proceeded along the plane T, 

 its symbol would be P G' G p . 



The Tetrahedron. 

 Fig. 304. 



Simple and mixed decrements on the angles of the 

 tetrahedron producing planes belonging to class b, 



