| 摘要 |
<p>The cover comprises a frame (10) for being fixed to said manhole and a lid (50) for covering a passage opening (11) of the frame. An inner edge (14) of the passage opening (11) of the frame and an outer edge (51) of the lid have the form of respective first and second non-circular curves of constant width, geometrically similar to one another. In the curves of constant width, a distance between two parallel geometric lines tangent to opposite points of said inner edge (14) of the passage opening (11) of the frame or outer edge (51) of the lid is constant no matter what the direction of said two parallel lines. The frame (10) and the lid (50) are lighter than a circular frame and o lid having an equivalent diameter.</p> |
