TERMINOLOGY. 



The lateral faces of this solid are Fig. 35. 



generally oblique-angled parallelo- 

 grams : two of them are always of 

 this figure ; the other two may be 

 rhombs. The only equality subsist- 

 ing among the f ices is between each 

 pair of opposite ones.* 



The Figure 35 is supposed to be 

 oblique in the direction O A, so that 

 the terminal plane forms an obtuse 

 angle with the edge H. Whatever 

 may be the fact, for the purposes of description, the solid angle 

 at A, will be called the acute solid angle, that at O, the obtuse solid 

 angle, while those at E and I, will be called the lateral solid angles. 

 The edges D and F, will be called the obtuse terminal edges, B 

 and C the acute terminal edges ; each of which will also be distin- 

 guished by their respective letters, as well in the case of edges as 

 angles ; H and its opposite the obtuse lateral edges, G and its oppo- 

 site the acute lateral edges. 



The doubly oblique Prism has four une- 

 qual axes passing through the pairs of opposite 

 solid angles, as a b, Fig. 36, c d, ef, and g h ; 

 it has also the prismatic axis i k. 



The individuals belonging to this class will 

 differ from each other in the inclination of P 

 on M, Fig. 35, P on T and M on T, and in 

 the ratios of the edges D H andj<\ 



H This class of prisms may be supposed to stand in the same relation to 

 the right oblique-angled Prisms, that the oblique rhombic Prism does to- 

 the right rhombic Prism : we have only to suppose a right oblique- 

 angled Prism to become oblique, and we have a correct idea of the 

 doubly oblique Prism. As in the case of the oblique rhombic Prism, sa 

 also in this solid, we may conceive that the bases may be disposed as re- 

 spects the prism in. a variety of ways. Crystals themselves, however, 

 present us with but two, which are different. In the first of these, they 

 are disposed in such a manner as not to make with any one of the late~ 

 ral faces an angle equal to that which they form with the prismatic 

 axis, or in such a way that the angles they form with two adjacent 



