324 PUMPS, AND THEIR VALUE TO MAN 



In the common pump, water cannot be raised higher than 

 34 feet. (See Chapter XXXVIII.) In many cases it is 

 desirable to force water considerably higher, as, for instance, in 

 the fire hose. Under such circumstances a type of pump is 

 employed which has received the name of force pump. In the 

 force pump valves are placed in the cylinder, and the piston is 

 solid, but the principle is the same as in the lifting pump. 



FIG. 195. Agriculture made possible by irrigation. 



Irrigation and drainage. For many present-day enterprises 

 force pumps and lift pumps are inconvenient and impracticable, 

 and they have been replaced in many cases by more modern 

 types, such as the rotary and centrifugal pumps. In these 

 pumps, rapidly rotating wheels lift the water and drive it on- 

 ward into a discharge pipe, from which it issues with great 

 force. There is neither piston nor valve, and the quantity 

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

 the pipes depend solely upon the size of the wheels and the 

 speed with which they rotate. 



Irrigation, or the artificial watering of land, is of great impor- 

 tance in those parts of the world where the land is naturally too 

 dry for farming. In the United States, approximately two 

 fifths of the land is so dry that it is worthless for agricultural 

 purposes unless artificially watered. In the West, several large 



