《梦溪笔谈·书画》
画家绘制圆形自有办法，只要将肩头靠近墙壁，尽量伸长臂膀挥手画圆，自然像用圆规画出的一样。
画家为之自有法，但以肩倚壁，尽臂挥之，自然中规。
There is a technique for drawing circles, and as long as you move your shoulder close to the wall and extend your arm as far as possible to swing and draw a circle, it will naturally be like drawing it with a compass.