GEOMETRY. 355 



a segment formed by the diameter of the circle, as 

 C E B, and is equal to half the circle. 



36. A tangent is a straight line drawn so as 

 just to touch a circle without cutting it, as G H. 

 (Fig. 18.) The point A, where it touches the cir- 

 cle, is called the point of contact. And a tangent 

 cannot touch a circle in more points than one. 



37. A sector of a circle is a space comprehended 

 between two radii and an arc, as B I K. (Fig. 19.) 



38. The circumference of every circle, whether 

 great or small, is supposed to be divided into 360 

 equal parts, called degrees ; and every degree into 

 60 parts, called minutes ; and every minute into 

 60 seconds. To measure the inclination of lines 

 to each other, or angles, a circle is described 

 round the angular point as a centre, as I K, Fig. 19. ; 

 and according to the number of degrees, minutes, 

 and seconds, cut off by the sides of the angle, so 

 many degrees, minutes, and seconds, it is said to 

 contain. Degrees are marked by , minutes by ', 

 and seconds by "\ thus an angle of 48 degrees, 

 15 minutes, and 7 seconds, is written in this 

 manner, 48 15' 7". 



39. A solid is any body that has length, breadth, 

 and thickness : a book, for instance, is solid, so is 

 a sheet of paper ; for though its thickness is very 

 small, yet it has some thickness. The boundaries 

 of 3, solid are surfaces. 



40. Similar solids are such as are bounded by an 

 equal number of similar planes. 



41. A prism is a solid, of which the sides are 

 parallelograms, and the two ends or bases are 

 similar polygons, parallel to each other. Prisms 

 are denominated according to the number of angles 

 in the base, triangular prisms, quadrangular, pent- 

 angular, and so on, as Fig. 20, 21, 22, 23. If the 



A A 2 



