Why is the lid of the drain round?
As the manhole cover is made of cast iron, the circle is relatively easy to produce, and it is not easy to produce air holes, which leads to quality problems.
Square round manhole cover, its diagonal length is 1.414 times the length of the side, if you take up the cover stand up in diagonal direction, it is easy to fall down, leading to accidents;
Round well covers each diameter equal length, how to rotate can fit, and won't produce because of the length of the side is not diagonal, lead to the manhole cover fall;
