—Make a loop in one end of a thread as before and tie the other end tightly to a large pin; wind the thread around the pin until all of it is on except the loop; push the pin through the paper on which you want to draw the spiral and into the drawing board as shown at [B].
Next put the point of the pencil in the loop and move it around the pin just as you did in making the circle and you will find that you have drawn a very pretty geometrical spiral which is known as the spiral of Archimedes. It is so called because Archimedes was the first to explain that it was caused by a point moving with uniform angular speed and receding from the center at a constant rate.
Fig. 50b. how to draw a spiral with a thread
How to Draw an Ellipse.
—An ellipse can be drawn in the same way as a circle, that is, by means of a string; but instead of one pin you will need two and each pin is driven in at the foci of the ellipse you are to draw as shown at [C]. Simply make a loop of the string, slip it over the pins, put the pencil point in the loop and move it around the pins when an ellipse will be formed.
How to Make and Use a Pantagraph.
—A pantagraph is a simple mechanical linkage for enlarging, copying or reducing the size of a picture. It is shown in [Fig. 51].
To make one of these instruments get four strips of wood about ¹⁄₈ inch thick, ¹⁄₂ an inch wide, and 18 or 20 inches long. Now drill ¹⁄₁₆ inch holes ¹⁄₄ inch apart in each stick the whole length of it. In the ends of three of the sticks make a hole the size of a lead pencil.
