Why in all over the world man hole top cap is round?

Question

All widgets are round. Everything round has a hole in the middle. Some things that are round have a handle. Therefore..
1. All widgets have a hole in the middle
2. Everything with a handle is a widget
3. Neither of the above is true
4. Both the above are true

Answer 1

1. A round manhole cover cannot fall through its circular opening, whereas a square manhole cover might fall in if it were inserted diagonally in the hole. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.
2. Round tubes are the strongest and most material-efficient shape against the compression of the earth around them.
3. A round manhole cover of a given diameter has a smaller surface area than a square cover of the same width, thus less material is needed to cast the manhole cover, meaning lower cost.
4. The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.
5. Circular covers do not need to be rotated to align with the manhole.
6. A round manhole cover can be more easily moved by being rolled.
7. A round manhole cover can be easily locked in place with a quarter turn (as is done in countries like France), which makes them hard to open without a special tool. Lockable covers do not have to be made as heavy, because traffic passing over them cannot lift them up by suction.