FORCING PUMP. 



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straight-lined aqueducts, wliich were supported by immense 

 arches or columns. 



The common pump consists of a large tube or pipe, called 

 the barrel, whose lower end is immersed in the water which 

 it is designed to raise. A kind of stopper, called a piston, 

 is fitted to this tube, and is made to slide up and down by 

 means of a metallic or wooden rod. In the piston, there is 

 a valve, or little door, which opening upwards, admits the 

 water to rise through it, but prevents its returning. A simi- 

 lar valve is fixed in the body of the pump. When the pump 

 is in a state of inaction, the two valves are closed by their 

 own weight ; but when the piston is made to ascend, it 

 raises a column of air which rested upon it, and produces a 

 vacuum between itself and the lower valve; the air beneath 

 this valve expands and forces its way through it ; and the 

 water, relieved from the pressure of air, ascends into the 

 pump, being forced up by the weight of the surrounding 

 atmosphere. When the piston now descends it is forced 

 into the water, which, as it cannot repass through the lower 

 valve, must rise through the valve of the moveable piston, by 

 the ascent of which, it is lifted up and runs off at the spout. 

 There must never be so great a distance as thirty-three feet 

 from the level of the water in the well, to the valve in the 

 piston, for in that case, the water would not rise through the 

 valve, because the pressure of the atmosphere will not sus- 

 tain a column of water above that height. But when the 

 water has passed the valve in the moveable piston, it is not the 

 pressure of the air on the reservoir which makes it ascend ; 

 it is raised by lifting it up, as you would raise it in a bucket, 

 of which the piston formed the bottom. 



The forcing pump is not only used to raise water from a 

 well to the surface of the earth, but likewise to force it into 

 reservoirs on the tops of buildings, from which pipes are laid 

 to convey it to different parts as conveniency requires. It 

 differs from the common pump by having the upper piston 

 solid, and a pipe joined to the barrel just above the lower 

 piston, through which the water passes into what is termed 

 the air vessel. In the pipe which leads to the air vessel 

 there is a fixed valve, which opens upwards and prevents the 

 return of the water. Through the upper part of the air 

 vessel a tube is inserted, which reaches nearly to its bottom. 

 Now the air which is above the water in the vessel being 

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