A structure of a hinge is provided. The hinge includes a main leave secured to a door frame and a subordinate leave joint with the main leave by a pintle and secured to a door. The main leave has a first helical surface in a lower knuckle and the subordinate leave has a second helical surface in a central knuckle engaged with the first helical surface. Further the first helical surface has a vertical end and a plurality of domes engageable with a vertical end and a plurality of semi-circular recesses. So that when the door is opened, it will gradually and slowly closed on its own gravity.