144 Dr Pearson, On a new kind of self-acting weir. [Feb. 25, 



above the cill in a way which, will be dangerous to craft passing 

 through. 



The self-acting valves, properly so called, were only designed to 

 be placed on the top of the solid weir adjoining the navigable 

 pass : their principles of construction are identical with those of 

 the valves employed in the pass, but they were left, according to 

 their original design, to act for themselves, rotating, when the 

 water rises above a certain level, and recovering their position when 

 it falls below it. A description of the way in which they would 

 act on this theory will be found in the volume of the A. d. P. 0. for 

 1681 ; but from more recent notices, it seems to have been found 

 better to add a light bridge carrying a windlass, similar to that 

 employed in M. Poirde's system, so that the natural movement of 

 the valves may be regulated by means of a chain ; the whole to 

 be partly removed, partly lowered into the bed of the stream, in 

 case of a serious flood. 



As stated before, if the axis were placed at J of the distance from 

 the lower end of the valve, it would revolve as soon as the water 

 passed over the top : to remedy this, the height of the axis is 

 raised a little, and the lower end of the valve had originally a 

 counterpoise attached, moveable by the sluice-keeper or of itself in 

 a peculiar way described in the volume last referred to : but this 

 contrivance has now been superseded, as has been already ob- 

 served. 



The calculation of the point at which the valves will open of 

 themselves seems difficult, as of course the water rises to some 

 extent simultaneously above and below the weir; but it seems 

 clear that equilibrium will be destroyed : 



(1) by the water rising on the weir from above, 



(2) and also, supposing the water to be more than 3 feet above 

 its normal level ; the axis of rotation being fixed somewhat less 

 than 4 feet above this point. 



It does not come within the scope of this paper to give any 

 longer abstract of the naathematical investigation of the question 

 as stated by M. Chanoine in the Ann. des P. et G. for 1861 ; but 

 a brief description of the several parts of the weir seems desirable, 

 and with the help of the annexed plate, there will be little diffi- 

 culty in comprehending its action. 



Fig. 1. 1. represents M. Poiree's design as described in the 

 Ann. des P. et G. for 1839. A is here the abutment of masonry. B.B". 

 are two bars which support the aiguilles, resting on staunchions or 

 fermettes, G.C'. moving on hinges at (7\ and capable of being 

 lowered by aid of them across the stream. D.D. are the staves or 

 aiguilles. In Fig. 1. 1. the water L is represented as at a nearly 

 uniform level above and below the weir ; the sluice-keeper is en- 

 gaged in replacing the staves, in order to close it. Fig. 1, 2. ex- 



