Page:An introduction to linear drawing.djvu/26

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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 unequal, 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.

25. 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.