350 GEOMETRY. • 



letter at the vertex, and the other two at the end of 

 each leg, as the angle ABC. 



8. When one line stands upon another, so as 

 not to lean more to one side than to another, both 

 the angles which it makes with the other are called 

 right-angles^ as the angles ABC and A B D, Fig. 3, 

 and all right-angles are equal to each other, being 

 all equal to 90° ; and the line A B is said to he per- 

 pendicular to C D. 



Beginners are very apt to confound the terms 

 perpe?idiculary SLXid plumb or vertical line. A line is 

 vertical when it is at right-angles to the plane of 

 the horizon, or level surface of the earth, or to the 

 surface of water, which is always level. The sides 

 of a house are vertical. But a line may be per- 

 pendicular to another, whether it stands upright or 

 inclines to the ground, or even if it lies flat upon it, 

 provided only that it makes the two angles formed 

 by meeting with the other line equal to each other ; 

 as for instance, if the angles ABC, and A B D be 

 equal, the line AB is perpendicular to CD, what- 

 ever may be its position in other respects. 



9. When one line, BE (Fig. 3), stands upon 

 another, C D, so as to incline, the angle E B C, 

 which is greater than a right-angle, is called an 

 obtuse angle ; and that which is less than a right- 

 angle, is called an acute angle, as the angle E B D. 



10. Two angles which have one leg in common, 

 as the angles ABC, and ABE, are called co/i/i- 

 guous angles, or adjoining angles j those which 

 are produced by the crossing of two lines, as the 

 angles E B D and CBF, formed by C D and E F, 

 crossing each other, are called opposite or vertical 

 angles. 



11. A^^'?^r^ is a bounded space, and is either a 

 surface or a solid. 



