Some Tools With Which to Measure and Plan.
By the time you have come to this chapter in your book, you will have drawn a great many pictures of objects. In doing this you have depended on your eyes and hand alone. You have not used a ruler to measure with, nor any tool that would tell you the exact length of a line or the exact size of any shape.
But sometimes it is necessary that a line or shape should be of exact length or size. On this page are shown some very simple tools which you can make yourself, and which you will find useful in carrying out the lessons in this chapter on Measuring and Planning. Figure I is a "circle maker." It can be used in place of a compass. To make it, take a strip of cardboard seven inches long and one inch wide. Bisect its short edges and rule a line connecting these points. Upon this line, mark off, by measuring with a ruler, inch, half-inch, and quarter-inch spaces. Through these points draw lines, and pierce holes with a pin where they cross the center line. A pin placed through the first hole will act as a pivot. Push a sharp pencil through one of the other holes, just far enough to allow the lead to make a mark. The pin marks the center, and the pencil swings around it, as shown in the sketch at the top of [page 68]. The line drawn by the pencil is the circumference of the circle. The distance between the center and the circumference is the radius of a circle. We speak of one radius and of two or more radii of a circle.
Figure II is a little tool that will help you to draw square corners. Mark with a ruler upon an end and one side of the back of an envelope, the spaces for inches, and their divisions into halves and quarters. "Square corner" is another name for right angle. You will often wish to use this measure, called a test square, in squaring corners, and in drawing lines at right angles to each other. A No. 9 envelope will be a good size to use, as the long edge will serve as a ruler. You can make the drawings in this chapter with a ruler and compass, or you can use these simple tools, made by yourself.
Dividing a Circular Space.
There are many ways in which a circular shape may be divided and decorated. Sketch B shows two circles drawn around the same center, with different radii. Such circles are called concentric. Sketch C shows the circle divided into fourths. To do this, place the angle of your test square at the center of the circle and rule two radii. Repeat to secure four right angles at the center of the circle.