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Close the space between the sides of an angle with a right line, and you make a triangle, a figure which has three angles and three sides.

The *base* is the side on which the triangle is supposed to rest.

The *apex* of a triangle is the point opposite to the *base*.

The *height* of a triangle is a perpendicular drawn from the *apex* to the *base*. In the figure it is shown by the dotted line.

A triangle is called *Isoceles* when *two* sides are equal. If all *three* of the sides are equal, it is *Equilateral*, (which word means *equal-sided*;) and if all the sides are *un*equal, it is called *Scalene*.

24. *Raise a perpendicular on a horizontal.* (fig.9.)

This will produce *right angles*, as we have before remarked. To ascertain if the angle be exact, take a piece of what is called bonnet paper or thin pasteboard, cut it round and then cut the round piece into quarters. Each quarter will have two sides at right angles, and by inserting the *apex* into the opening of the angle drawn by the pupil, any incorrectness will be detected. A small brass or iron *square* will serve the same purpose, but does not satisfactorily show that a right angle is equal to a quarter of a circle, which is also called a *quadrant*.

35. *Cross a right line with a perpendicular.* (fig.10.)

The right line should be drawn in various directions, to show the pupil that a perpendicular may be raised on any right line, whether horizontal or oblique.