l 2Q c 2 HYDRAULICS. 



taper; and this part of the trunk must be of suf- 

 iicient width round the beam, to allow free passage 

 into the rising-pipe; or which is better, the rising- 

 pipe must branch off from the bottom of the trunk. 

 A discharge may be made from the cistern F, so 

 that as little water as possible may descend along 

 the trunk, when the piston is raised. 



The requisites of a valve are, that it be tight, 

 and of sufficient strength to resist the great pres- 

 sures to which it is exposed ; that it afford a free 

 passage to the water; and that it do not allow 

 much to go back whilst it is shutting. The clack- 

 valve is of all others the most obvious and common. 

 It consists merely of a leather flap covering the 

 aperture, and having a piece of metal on the 

 upper side, both to strengthen and to make it 

 heavier, that it may shut of itself. Sometimes the 

 hinge is of metal. The hinge being liable to be 

 worn by such incessant motion, and as it is trouble- 

 some, especially in deep mines, and under water, 

 to undo the joint of the pump, in order to put in a 

 new valve, it is frequently annexed to a box like a 

 piston, made a little conical on the outside, and 

 dropped into a conical seat made for it in the pipe, 

 where it sticks fast; and to draw it up again, there 

 is a handle like that of a basket, fixed to it, which 

 can be laid hold of by a long grappling-iron. The 

 only defect of this valve is, that by opening very 

 wide, when pushed up by the stream of water, 

 it allows a good deal to go back during its shutting 

 again. 



The buttery-valve is free from most of these in- 

 conveniences, and seems to be the most perfect of 

 the clack-valves. It consists of two semicircular 

 flaps, revolving round their diameters, which are 

 fixed to a bar placed across the opening through 



