Why is the bucket cap made round?
The hole under the lid is round, because the cylinder can bear the pressure of the surrounding land best. What's more, the sewer hole means leaving enough space for one person to pass, and the cross section of a person who climbs down the ladder is basically round. So the circle naturally becomes the shape of the drain hole. A square lid will also be seen on some square holes.
This covers the entrance than large, there are around. Usually, this kind of lid is metallic and very heavy. We can imagine that a square hole two feet wide, 1 to 1.5 inches wide. In order to fall into the covers, the need to lift one end, and then rotate 30 degrees, so that it will not be rung hiders, and then covers a 45 degree angle with the horizon, the focus at this time was sufficient for the transfer of the covers off. The circle has no such scruples. From an engineering point of view, the shape of the manhole cover is entirely dependent on the shape of the hole it is to cover.
Since the diameter of the round lid is constant, this does not cause the rotation to fall at any rate, but the square does not vary in distance and may fall off after rotation
