4 DEFINITIONS. 3 



The lines a o, o b, or a o, o c, or b o, o c, are sometimes 

 said to contain a right, an obtuse, or an acute angle. 



In fig. I, the plane a, and that on which the figure 

 is supposed to rest, are called summits, or bases, or 

 terminal planes, and the planes b and c, with those 

 parallel to them, are termed lateral planes'. 



The edges of the terminal planes, as d, e, m, n, 

 fig. I, are called terminal edges. 



The edges/, g, h, produced by the meeting of the 

 lateral planes, are termed lateral edges. 



The planes of a crystal are said to be similar when 

 their corresponding edges are proportional, and their 

 corresponding angles equal. 



Edges are similar when they are produced by the 

 meeting of planes respectively similar, at equal angles. 



Angles are similar when they are equal and con- 

 tained within similar edges respectively. 



Solid angles are similar when they are composed of 

 equal numbers of plane angles, of which the corre- 

 sponding ones are similar. 



Fig. 3. 



An equilateral triangle, fig. 3, is a figure contained 

 within three equal sides, and containing three equal 



angles. 



Fig. 4. 



An isosceles triangle, fig. 4, has two equal sides, 

 b, which may contain either a right angle, or an 



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