OF SPACE. S3 



perpendicular to the common section, are perpendicular to 

 the other plane. 



147. Definition. The inclination of a straight line 

 to a plane is the angle, contained by that line, and another 

 straight line drawn from its intersection with the plane to 

 the intersection of a perpendicular let fall from any pomt 

 of the line upon the plane. 



148. Definition. The inclination of two planes is 

 the inclination of two lines, one in each plane, perpendi- 

 cular to the common section. 



149. Definition. Parallel planes are such as never 

 meet, although indefinitely produced. 



150. Definition. A solid angle is made by the meet- 

 ing of two or more plane angles, in different planes. 



151. Definition. Similar solid figures are such as 

 have all parts of their surfaces similar and similarly placed; 

 and all their sections, in similar directions, respectively 

 similar. * 



152. Definition. A pyramid is a solid contained by 

 a plane basis and other planes meeting in a point. 



153. Definition. A prism is a solid contained by 

 planes of which two that are opposite, are equal, similar, 

 and parallel, and all the rest parallelograms. 



154. Definition. A cube is a lolid contained by six 

 equal squares. 



155. Definition. A solid of revolution is that which 

 is described by the revolution of any figure round a fixed 

 axis. 



156. Det^inition. A sphere is described by the re- 

 volution of a semicircle on its diameter as an axis. 



