Mr Thorn's Description, of a New Double Valve Sluice. 291 



above the centre of the sluice, twenty feet ; of course, there are 

 forty cubic feet of water pressing upon the sluice ; but one- 

 half of this is borne by the pivots, at its upperside. Were the 

 specific gravity of the cylinder GH the same as that of water, 

 this would leave only twenty feet for its contents; but, to 

 make it float freely, it is one-twentieth less ; therefore, allow- 

 ing one foot more for this, and three feet for friction, twenty- 

 four cubic feet would be the necessary contents of the cylinder 

 GH ; — but its contents are fifty cubic feet : it was made thus 

 powerful, that it might work a larger sluice, if ever it should 

 be found necessary. For an improved construction of this 

 apparatus, see Fig. 5. 



Plate IV. Figure 6. 



This apparatus is applicable to the same purposes as that 

 of Fig. 5 ; but the construction is much simplified. 



A, the sluice, which turns upon pivots at its centre of pres- 

 sure. 



AB, a lever attached to that sluice, which, with the small 

 weight B at its extremity, is heavy enough to overcome the 

 friction of the sluice, and keep it shut. 



AC, the aqueduct that conveys the water from the reser- 

 voir to the works. 



D, a pulley which turns easily round its axis. 



E, a light hollow cylinder of copper, having a very small 

 aperture in its bottom, and open at top. 



BDE, a chain, one end of which is fixed to the lever at B, 

 then, passing over pulley D, has its other end fixed to cylin- 

 der E. 



F, a cistern, always full of water, being supplied by a spring 

 from the rising ground. 



FG, a pipe which communicates between that cistern and 

 cylinder E. 



H, a valve that opens or shuts that communication. 



I, afloat, that rises and falls with the water in the aqueduct. 



The sluice A is here represented shut, and the water in the 

 aqueduct at rest. But suppose a part of the water to be 

 drawn from the aqueduct, then, as its surface falls, so will 

 float I, which then leaving the spindle of valve II, that valve 

 opens, and the water flows from cistern F into cylinder E, 



