38 IRRIGATION. 



This cistern, if 20 feet in diameter and 12 feet deep, 

 would hold 30,000 gallons, or enough to water oyer three 

 acres at one time. If the cistern is open the wall could 

 slope outward, making an inverted fru strum of a cone 

 (as seen in fig. 4), 32 feet wide at the surface and 8 feet 

 wide at the bottom. The earth thrown out at the bot- 

 tom will form a support for the upper portion of the 

 wall. But before the wall is built the earth thrown out 

 should be solidly rammed down in layers made hollow or 



Fig. 4. OPEN CISTERN. 



of the form of a basin. The form is shown by the curv- 

 ed lines in that part of the engraving. 



There is a large variety of pumps adapted to the pur- 

 pose of irrigation, but the severe uses to which they are 

 put make it desirable to have only those which are con- 

 structed entirely of metal or wood. Leather valves are 

 soon worn and become useless, causing delays, and serious 

 loss of time in repairs. The double action force pumps, 

 with metal valves, or the rotary pumps of the ordinary 

 kinds with metal pinions which work into each other 

 similarly to cogwheels, or those which work upon the old- 

 fashioned principle of the Archimedean screw, but which 

 nevertheless are protected by a modern patent are all suit- 

 able for this work on account of their durability. A 

 double-acting force pump of the most simple character 

 (fig. 5), made almost entirely of wood, is one of the best 

 for this purpose on account of its cheapness and the ease 

 with which it is kept in working order. It is formed of 

 a block of wood, A, A, in which two parallel holes are 



