GEOMETRY. 351 



12. A superficies, or surface, has length and 

 breadth only. The extremities of a superficies are 

 lines. 



A plane, or plane surface, is that which is 

 every where perfectly flat and even, or which will 

 touch every part of a straight line, in whatever 

 direction it may be laid upon it. The top of 

 a marble slab, for instance, is an example of 

 this, which a straight edge will touch in every 

 point, so that you cannot see light any where 

 between. 



A curved surface is that which will not coincide 

 with a straight line in any part. Curved surfaces 

 may be either convex or concave. 



A convex surface is when the surface rises up in 

 the middle, as, for instance, a part of the outside 

 of a globe. 



A concave surface is when it sinks in the middle, 

 or is hollow, and is the contrary to convex. 



A surface may be bounded either by straight 

 lines, curved lines, or both these. 



13. Every surface bounded by straight lines 

 only, is called a rectilinear figure. 



14. Three is the fewest number of sides that a 

 rectilinear figure can have ; and it is then called a 

 triangle. 



15. Triangles are of different kinds, according 

 to the lengths of their sides. 



An equilateral triangle has all its sides equal, as 

 A B C, Fig. 4. 



An isosceles triangle has two equal sides, as 

 D E F, Fig. 5. 



A scalene triangle has all its sides unequal, as 

 GHI, Fig. 6. 



16. Triangles are also denominated according 

 to the angles they contain. 



