354 GEOMETRY. 



When they have a greater number of sides, they 

 are called polygons of 13 sides, of 14 sides, and 

 so on. 



27. Base of a figure is the side on which it is 

 supposed to stand erect, as A B and C D. (Fig. 16.) 



28. Altitude of a figure is its perpendicular 

 height from the base to the highest part, as E F. 

 (Fig. 16.) 



29. Area of a plane figure, or other surface, 

 means the quantity of space contained within its 

 boundaries, expressed in square feet, yards, or any 

 other superficial measure. 



30. Similar Jigures are such as have the same 

 angles, and whose sides are in the same proportion, 

 as Fig. 17. 



31. Equal figures are such as have the same 

 area or contents. 



32. A circle is a plane figure, bounded by a 

 curve line returning into itself, called its circum- 

 ference, ABCD, (Fig. 18.) every where equally 



distant from a point E within the circle, which is 

 called the centre. 



33. The radius of a circle is a straight line drawn 

 from the centre to the circumference, as E F. 

 (Fig. 18.) The radius is the opening of the com- 

 pass when a circle is described \ and consequently 

 all the radii of a circle must be equal to each other. 



34. A diameter of a circle is a straight line 

 drawn from one side of the circumference to the 

 other through the centre, as C B. (Fig. 18.) Every 

 diameter divides the circle into two equal parts. 



35. A segment of a circle is a part of a circle cut 

 off by a straight line drawn across it. This straight 

 line is called the chord. A segment may be either 

 equal to, greater, or less than a semicirckf which is 



