201 HYDRAULICS. 



faction of the air in the barrel on moving the 

 piston : and consequently the pump might fail in 

 its operation. This can only effectually be pre- 

 vented, by placing the pump-work in or near the 

 water ; in which case, should any leak happen 

 upward, it will only occasion the loss of some of 

 the water, without any other inconvenience. And 

 the leather valves, being kept under water, will 

 always be found supple, pliant, and in condition 

 to perform their office. 



Placing the pump-work (that is, the valve and 

 piston) pretty low and near together, will also pre- 

 vent the inconvenience of not being able, in all 

 cases, to fetch up water from the spring by the 

 ordinary pump, when of an equal bore, by reason 

 of the shortness of the stroke ; which, therefore, 

 cannot rarefy the air sufficiently to bring the water 

 up to the piston from the lower valve. For in- 

 stance : take a smooth barrelled-pump, 21 feet 

 long, having its piston fetching, suppose a foot- 

 stroke, placed above, and the clack or fixed valve 

 at the other end below. By the playing of the 

 piston, admit it possible for water to rise 11 feet ; 

 or if you will, let water be poured on the clack, to 

 the height of 11 feet, and refit the piston, there 

 will remain still 9 feet of air between it and the 

 water, which cannot be sufficiently rarefied by a 

 foot-stroke to open the clack, or fetch up more 

 water ; for, in this case, the air can only be rare- 

 fied in the proportion of 9 to 10; whereas, to 

 make a bare equilibrium with the atmosphere, it 

 ought to be as 9 to 13£ : since, as 22 or the com- 

 plement of 11, to 33 feet of water, the weight of 

 the whole atmosphere, so is the interval spoken of, 

 9 to 13i : to complete which, the stroke ought to 

 be at least H feet lomj-. 



