292 Mr Thorn's Description of a New Double Valve Sluice. 



which, when full, descends, raises lever BA, and opens the 

 sluice. Again, suppose the water to rise in the aqueduct, the 

 float I rising with it, shuts valve H, when cylinder E is 

 emptied by the small aperture in its bottom, and the weight 

 of lever AB again shuts the sluice. This sluice also acts as 

 a waster, by having a pipe to communicate between the re- 

 servoir and cylinder E, in the same manner as in Fig. 4. 



A sluice of this description was erected at Rothesay in 

 1821. Sluice A is three feet long, and eighteen inches deep; 

 lever AB, three feet long ; cylinder E, two and a half feet 

 diameter, and the same depth. The depth of water above the 

 centre of the sluice, when the reservoir is full, is twenty feet. 



By this contrivance, of making the sluice turn on its centre 

 of pressure, the weight of the column of water resting on it is 

 neutralised ; it being at the same time equally exerted to open 

 and shut the sluice. The acting power has, therefore, only to 

 overcome the friction, to make it move in any direction ; 

 whereas, in the apparatus Fig. 4, the power must not only 

 overcome the friction, but must also be equal to half the 

 weight of the whole column of water pressing upon the 

 sluice.* 



Thus, in the present case, there is a column of ninety cubic 

 feet of water pressing upon the sluice when the reservoir is 

 full. Were the sluice hinged upon one side, as in Fig 4, it 

 would require the cylinder E to contain forty-five cubic feet 

 of water, besides about one-tenth more for friction ; and the 

 chain, lever, &c. would have to be made strong in proportion. 

 But by this contrivance, the power to act against this forty- 

 five feet of water is wholly saved, and the cylinder requires 

 only to contain water sufficient to overcome the friction. 



The apparatus is also simplified by having only one cylin- 

 der and one valve, instead of two of each, as in Fig. 4. But 

 this plan has also some small disadvantages : — the sluice, 

 when it turns upon pivots at its centre, is more difficult to 



* The other half is home by the pivots on which the sluice turns. 

 When the sluice is hinged at the upper side, the power has rather more 

 than half the weight to sustain, and when hinged at the under side, it has 

 rather less; but where the depth of the sluice bears so small a proportion 

 to the depth of water above it, the difference is not worth noticing in 

 practice. 



