Drainage by Gravitation. 27 



matically open on a falling tide, as soon as the water in the 

 river is slightly below that in the drainfand close as soon as 

 the tide rises above the level of the outflowing water. A very 

 small head is sufficient to open and close these doors. 



In large drains the doors are made in pairs, the size of 

 the opening for the pair seldom exceeding 20 feet in width. 

 The doors being self-acting, a greater width than this is un- 

 advisable, the concussion of the doors as they come together 

 on closing with the rapidly increasing head of the rising tide 

 throwing a considerable strain both on the frame-work of the 

 door and the masonry. 



The doors shut against a solid wooden sill at the bottom 

 and a pointed frame at the top, hooded over. In some sluices 

 the doors are made of sufficient height to be above the rise of 

 the highest tide. This plan adds considerably to their weight 

 and cost, and from the great length renders them liable to 

 strain without any corresponding advantage. The angle at 

 which the doors are set is generally about 25 degrees ; it 

 should not be less than 20, or exceed 30 degrees. The heel 

 post works in a masonry hollow quoin on a pivot at the bottom, 

 and is held in place by an anchor-strap at the top. These 

 doors do not open back flush with the wall, as in a lock, but 

 stand off sufficiently to allow the rising tide to get behind and 

 act on the back so as to close them. In order to give stability 

 to the structure, the masonry opening is made of considerable 

 width, and forms a bridge over the drain. 



The length of the piers against which the doors open is 

 about double the width of the opening, and they are in- 

 variably, as are also those on the inside, made with pointed 

 or rounded ends. On the inside of the arch draw-doors are 

 also placed, working in grooves, and made to lift up or down 

 by means of a cast-iron toothed bar, actuated by pinions and 

 gearing so adapted that one man has complete control over 

 the door. In large doors counterbalance weights are sus- 

 pended by chains over pulleys. These doors are kept open 



