HYDRAULICS. 203 



the piston. Some engineers make their great 

 valves of a pyramidal form, consisting of four 

 clacks, whose hinges are in the circumference of 

 the water-way, and which meet with their points 

 in the middle, and are supported by four ribs, 

 which rise up from the sides, and unite in the 

 middle. This is a most excellent form, affording 

 a more spacious water-way, and shutting very 

 readily. 



There is another form of a valve, called the 

 button, or tail-valve. It consists of a plate of metal 

 turned conical on the edge, so as exactly to fit the 

 conical cavity of its box. A tail projects from the 

 under side, which passes through a cross bar in the 

 bottom of the box, and has a little knob at the 

 end, to hinder the valve from rising too high. 

 This valve, when nicely made, is unexceptionable. 

 It has great strength, and is therefore proper for 

 all severe strains ; and it may be made perfectly 

 tight by grinding. Accordingly, it is used in all 

 cases where tightness is of indispensable conse- 

 quence. It is most durable, and the only kind 

 that will do for passages where steam or hot water 

 is to pass through. 



A pump intended to raise water to any height 

 whatever, will always work as easy, and require 

 no greater power to give motion to the bucket, if 

 both the valves be placed towards the bottom of 

 the pipe, than if they were fixed 33 feet above the 

 surface of the water. 



The playing of the piston thus low in the pipe 

 will, besides, prevent an inconvenience which 

 might happen were it placed above, viz. in case of 

 a leak beneath the bucket, which, in a great length 

 of pipe, may very easily happen ; the outward air 

 getting through, would hinder the necessary rare- 



