Should you ever want to draw a circle and have no compasses at hand or should you want to draw a larger circle than you can with your compasses tie a bit of strong thread to a pin, make a loop in the string at whatever length you want the radius - that is half of the diameter of the circle - to be.
This done, drive the pin in at the point where you want the center of the circle, put the point of a lead pencil in the loop and move it around the pin, as shown at A in Fig. 50, keeping the thread taut and a perfect circle, nearly, will result.
Fig. 50A. how to draw a circle with a thread