How the well cover was invented

Human beings invented wells in ancient times, in order to prevent people from falling into the well, they put a cover, which is the well cover. There are square and round wells, and later found that round wells are easy to dig up, round well covers also save materials, better handling, put on the ground to roll on it. Round well cover also has a benefit, that is, how will not leak down, round well cover from any angle will not fall into the well, so the well cover is designed to be round.