圆形最大的特点是半径长度相等。因此当把圆形的井盖放到井口时，井盖就不会掉到井底。如果井盖是矩形，井盖的宽度就不是一定的。矩形的对角线长度大于任何一条边，井盖都有可能掉进井底。
The biggest characteristic of a circle is that its radius and length are equal. Therefore, when the round well cover is placed at the wellhead, the well cover will not fall to the bottom of the well. If the manhole cover is rectangular, the width of the manhole cover is not fixed. If the diagonal length of the rectangle is greater than any side, the manhole cover may fall into the bottom of the well.
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二、不易受损
2、 Not easily damaged
圆形受力均匀，能承受来自不确定方向的负载。当车轮开过，井盖依然平整。如果是其他有棱角的多边形，不但容易翘起造成安全事故，而且棱角也容易磨损。
It can not bear uniform load direction. When the wheels pass, the manhole cover is still flat. If it is other polygon with edges and corners, it is not only easy to cause safety accidents, but also easy to wear.
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三、易于搬运
3、 Easy to carry
圆形井盖便于搬运。由于井盖是沉重的大铁块，如果有井盖损坏或是需要更换，维修工人只需要滚动井盖就可以将其移动到一边。
The round manhole cover is easy to handle. Because the manhole cover is a heavy piece of iron, if the manhole cover is damaged or needs to be replaced, the maintenance workers only need to roll the manhole cover to move it to one side.