356 Mr. Marratt on Siuice-Doorsond F/oo(1-Gates. [May, 



vertical lines, the whole length of the door, and find the centre 

 of pressure of the door, corresponding to the situation the door 

 IS to have in the water ; that is, according to the depth to which 

 it is to be immersed ; lay off on these vertical lines the distance 

 of the centre of pressure from the upper end, and draw a line 

 across the door through the centres of pressure ; this line may- 

 be called the line of pressure. Place the axes, or centre lines, 

 of the gudgeons so as to coincide with this line, and the gate 

 will turn upon the gudgeons, and keep up or let out the water 

 as occasion may require. 



We may observe first, that such a door can be opened with 

 the smallest force possible ; that it will retain or keep any posi- 

 tion in which it may be put ; and, consequently, that any quan- 

 tity of water may be evacuated at pleasure. It may also be 

 closed again with the greatest possible ease ; for the pressure of. 

 the water above and below the axis on which the door turns, 

 being in all positions of the door equal, any effort which is 

 sufficient to overcome the friction of the several parts will be all 

 that is required either to open or shut the door. 



This mode of hanging sluice doors will be extremely useful 

 and convenient in situations where the sluice is liable to be 

 choaked up with sand ; in such situations much labour and 

 expense are often required to open the doors when hung in the 

 common way ; but, according to this method, the door will be 

 easily raised to a small height, which, being effected, the water 

 will soon make its way, and carry away the sand along with it. 



Gates hung in this manner would be the best of any for the 

 head of a mill-course^ and the practical engineer will easily 

 determine other situations in which they may be used with 

 advantage. As these doors or gates must always remain across 

 the river, or sluice way, it is obvious that this mode cannot be 

 applied where large vessels have to pass in and out ; it may do, 

 however, for small craft, and in every situation where navigation 

 is not concerned, this mode of application is manifestly superior 

 to the method now in use. 



Where the upper end of the door is even with the surface of 

 the water, the distance of the Une of pressure from the top of 

 the door is two-thirds of the length of the door ; in every other 

 situation, whether the top of the door be above or below the 

 surface of the water, the centre of pressure coincides with the 

 centre of oscillation, and is not difficult to determine. Tables 

 for the use of mechanics might be easily constructed, if this 

 method should be generally adopted in those situations for 

 which it is best suited. 



