86 



MIIN'EEALOGT. 



cleavage planes. A plane angle is produeed by the meeting 

 of any two lines or edges. 



The angles d o e, d o g, fig. 4, are 

 /} ~a /7 formed by the uniting of the lines 

 do, oe, and do, off. 



A solid angle is produced by the 

 meeting of three planes. 



A plane angle is either an acute, a 

 FIG. 4. right, or an obtuse angle; and these 



will be readily understood by the fol- 

 lowing explanations and the accompanying figures : 



First describe a circle, and divide it into 360 degrees ; 



next draw a perpendicular line a b, and a horizontal 



line c d, intersecting each other at the centre, and 



dividing the circle into four equal parts, each containing 

 ninety degrees, fig. 5. 



FIG. 6. 



If the angle be less than ninety degrees, it is an acute 

 angle ; if ninety degrees, a right angle ; if more than ninety, 

 an obtuse angle. Thus, in fig. 6, a and b form a right 

 angle ; b and c an acute angle ; a and c an obtuse angle : 

 or in another and perhaps simpler form, fig. 7, a and b 

 are an acute angle; a and c, a right angle; a and d, an 

 obtuse angle. 



In fig. 4, the plane a, and the plane opposite, on which the 

 object is depicted as resting, are called the summit, or the 

 base, or the terminal plane ; while the planes b and c, with 

 those parallel to them, are termed lateral planes. 



The edges of the terminal planes, as d, e, m, and n, fig. 4, 

 are called terminal edges. 



The edges f, g, and h, produced by the meeting of the 

 lateral planes, are termed lateral edges 



