2 DEFINITIONS. 



A plane angle, or as it is more commonly termed, 

 an angle, is formed by the meeting of any two lines 

 or edges. The angles do e, dog, fig. 1. are formed 

 by the meeting of the lines do, oe, and do, og. 



A solid angle is produced by the meeting of three or 

 more plane angles, as at o, fig, 1. 



Fig. 2. 



The measure, or, as it is sometimes termed, the 

 value of an angle, is the number of degrees, minutes, 

 &c. of which it consists ; these being determined by 

 the portion of a circle which would be intercepted by 

 the two lines forming the angle, supposing the point 

 of their meeting to be in the centre of the circle. 



For the purpose of measuring angles the circle is 

 divided into 360 equal parts, which are called degrees; 

 each degree into 60 equal parts, which are called 

 minutes ; and each minute into 60 seconds ; and these 

 divisions are thus designated; 360% 60', 60", the ' 

 signifying degrees, the ' minutes, the " seconds. 



If | of the circle, or 90, be intercepted by the two 

 lines a o, ob, fig. 2, which meet at an angle a o b in 

 the centre, those lines are perpendicular to each 

 other, and the angle at which they meet is said to 

 measure 90, and is termed a right angle. 



If less than | of the circle be so intercepted, as by 

 the lines ob,oc, the angle b o c will measure less than 

 90, and is said to be acute. If it measure more than 

 90, as it would if the angle were formed by the lines 

 a o, o c, it is called obtuse. 



