HYDRAULICS. 185 



At the beginning of the operation, if the leathers 

 be dry, the piston will not exhaust the air suffi- 

 ciently, and the water will not rise ; but if a little 

 water be poured upon the piston, it will swell the 

 leathers, and causing them to fit close, thus make 

 the piston act. This is vulgarly called, fetching 

 the water. 



As it is the pressure of the air or atmosphere 

 which causes the water to rise, and follow the 

 piston or bucket as it is drawn up, and not suc- 

 tion, as is commonly supposed ; and since a column 

 of water S3 feet high is of equal weight with as 

 thick a column of the atmosphere from the earth 

 to the top of the air, therefore the perpendicular 

 height of the piston or bucket from the surface of 

 the water in the well must always be less than 

 33 feet; otherwise the water will never get above 

 the bucket. But when the height is less, the 

 pressure of the atmosphere will be greater than 

 the weight of the water in the pump, and will, 

 therefore, raise it above the bucket ; and when the 

 water has once got above the bucket, it may be 

 lifted to any height, if the piston-rod be made long 

 enough, and a sufficient degree of strength be em- 

 ployed. 



The lifting-pump consists of a body, or barrel 

 A B (Fig. 2.), with narrow apertures at the lower 

 end, to prevent the entrance of dirt, stones, or any 

 thing that would impede its operation. At the 

 lower end is a valve, a, opening upwards, and al- 

 lowing the water to pass through it, but prevent- 

 ing it from returning downwards. In the barrel 

 is a piston, 6, perforated, and having on it a valve, 

 also opening upwards. This piston is moved up 

 and down by a rod, worked by a lever, or other 

 machine. Both the piston and the lower valve, 



