《论衡·卷十一·说日篇》
用什么证明呢？拿在屋下树棵竿子来证明。房屋高三丈，竿子在屋梁下面，把它笔直地立起，竿尖顶着屋梁，下面抵着地，这样屋梁离地就刚好三丈。
以植竿於屋下，夫屋高三丈，竿於屋栋之下，正而树之，上扣栋，下抵地，是以屋栋去地三丈。
What is the proof? It is proved by putting a pole in the shade of a tree. The house is three feet high, and the pole is below the beam of the house, standing upright, with the tip of the pole against the beam of the house and the bottom against the ground, so that the beam of the house is exactly three feet above the ground.