IRRIGATION AND DRAINAGE 



193 



because of the pressure of the compressed air. In this way 

 a continuous flow is secured. 



The height to which the water can be forced in the pipe de- 

 pends upon the size and construction of the pump and upon the 

 force with which the plunger can be 

 moved. The larger the stream desired 

 and the greater the height to be reached, 

 the stronger the force needed and the 

 more powerful the construction neces- 

 sary. 



The force pump gets its name from 

 the fact that the moving piston drives or 

 forces the water through the discharge 

 tube. 



185. Irrigation and Drainage. History 

 shows that the lifting pump has been 

 used by man since the fourth century 

 before Christ ; for many present-day 

 enterprises this ancient form of pump 

 is inconvenient and impracticable, and 

 hence it has been replaced in many 

 cases by more modern types, such as 

 rotary and centrifugal pumps (Fig. 136). In these forms, 

 rapidly rotating wheels lift the water and drive it onward 

 into a discharge pipe, from which it issues with great force. 

 There is neither piston nor valve in these pumps, and the 

 quantity of water raised and the force with which it is driven 

 through the pipes depends solely upon the size of the wheels 

 and the speed with which they rotate. 



Irrigation, or the artificial watering of land, is of the greatest 

 importance in those parts of the world where the land is nat- 

 urally too dry for farming. In the United States, approxi- 

 mately two fifths of the land area is so dry as to be worthless 

 CL. GEN. sci. 13 



FIG. 135. The air cham- 

 ber A insures a continu- 

 ous flow of water. 



