PUMPING MACHINERY 259 



them. If operated by hand the paddles are hinged like 

 valves and are rocked back and forth in the waterway. 

 Flash wheels are used extensively in Holland in draining 

 low lands. 



The Chinese devised at a very early time scoop wheels 

 which have buckets on the periphery. These buckets dip 

 into water and are set at such an incline that they carry 

 almost their full capacity to the upper side, and there they 

 pour their contents into a trough. They are sometimes 

 hinged and are made to discharge their contents by strik- 

 ing against a suitable guide. Wheels of this nature may 

 now be used profitably where a large quantity of water 

 is to be elevated for only short distances. 



One of the oldest water-raising devices made famous 

 by history is the Archimedean screw. It consists essen- 

 tially of a tube wound spirally around an inclined shaft 

 and taking part in the rotation of this shaft. The pitch 

 of the screw and the inclination of the shaft are so 

 chosen that a portion of each turn will always slope 

 downward and form a pocket. A certain quantity of 

 water will be carried up the screw in these pockets 

 as it is rotated. At the upper end of the inclined 

 screw the water is discharged from the open end of the 

 tube. 



368. Reciprocating pumps. As advancement came 

 along other lines of machinery, the early devices for 

 raising water gave way to the introduction of more effi- 

 cient machines to which may properly be given the name 

 of pumps, the most common of which is the reciprocating 

 pump. A reciprocating pump consists essentially of a 

 cylinder and a closely fitting piston. 



369. Classes. Reciprocating pumps may be divided 

 into two classes : 



I. Pumps having solid pistons or plunger pumps. 



