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OF SPACE. 15 



62. Definition. A plane is a surface, in which if any 

 two points be joined by a straight line, the whole of the 

 straight line will be in the surface. 



63. Definition. An angle is the 

 incHnation of two Hues to each other. 



Scholium. An angle is sometimes denoted by this mark /_-, and 

 is described by three letters placed near the lines, the middle letter 

 at the angular point. 



64. Definition. When a straight 

 line standing on another straight line 

 makes the adjacent angles equal, 

 they are called right angles. 



65. Definition. A straight line between two right 

 angles is called a perpendicular to the line on which it 

 stands. 



^, Definition. Whenaplane 

 surface is contained by a circum- 

 ference, such that all straight lines 

 drawn to it from a certain point in 

 the plane are equal, the surface is a circle. 



67. Definition. The point, equally distant from the 

 circumference, is called the centre. 



^, Definition. Any straight line, drawn from the 

 centre to the circumference, is called a radius. 



69. Defini J ION. The term circle also often implies 

 the circumference, and not the circular surface. 



70. Definition. A portion of the circumference of 

 a circle is called an arc. 



71. Definition. A straight line, joining the ex- 

 tremities of an arc, is its chord. 



