CONVICT LABOR FOR ROAD WORK. 



79 



water. As the water is drawn oflF the pressure of the conSned air 

 diminishes rapidly and a point is soon reached where, though there 

 still is some water left in the tank, the pressure of the air may not be 

 sufficient to force it to the faucets. The following table shows the 

 increase in the pressure of the air as water is pumped into a hydro- 

 pneumatic tank. 



Table 8. — Increase in pressure as tvater is pumped into a hydropneumatic tank. 



Part of tank filled with water. 



Empty 



One-fourth full of water. . . 



One-third full of water 



One-half full of water 



Two-thirds full of water. . . 

 Three-fourths full of water 



Pressure 



caused by 



compression 



of trapped 



air only. 



Pounds per 

 square inch. 

 0.0 

 4.9 

 7.4 

 14.7 

 29.4 

 44.1 



Initial 



pressure 



pumped into 



tank. 



Pounds per 

 square inch. 

 10.0 

 18.2 

 22.4 

 34.7 

 59.4 

 84.1 



A pressure of 6 or 7 pounds per square inch is necessary to overcome 

 the friction in the piping and force the water to the height of faucets 

 under average conditions, and it wiU therefore appear by reference to 

 the above table that the volume of water which can be delivered at 

 one charging of the tank, when only the air trapped in the tank fur- 

 nishes the pressure, is not greater than one-half the volume of the 

 tank. If, before the water is forced into the tank, a pressure of 10 

 pounds of air be pumped into it, aU the water the tank will hold, which 

 is not more than three-fourths of its volume, can be forced out. In 

 practice, it is always necessary to pump a certain amount of air into 

 the tank at intervals to overcome the loss caused by the gradual 

 absorption and removal of the air by the water. 



The following table gives the pressures in the tank theoretically 

 necessary to force the water to certain elevations above the tank: 



Table 9. — Pressures theoretically necessary to force water to given heights. 



Height. 



Pressure 

 in tank. 



Height. 



Pressure 

 in tank. 



Height. 



Pressure 

 in tank. 





Pounds 





Pounds 





Pounds 





per square 





per square 





per square 



Feel. 



inch. 



Feet. 



inch. 



Feet. 



inch. 



1 



0.43 



35 



1.5. 16 



110 



47.63 



2 



.87 



40 



17.32 



120 



51.96 



3 



1.30 



45 



19.49 



130 



56. 30 



4 



1.73 



50 



21. 65 



140 



60. 62 



6 



2.17 



55 



23. 82 



1.50 



64.95 



6 



2.m 



60 



25. 98 



160 



69.28 



7 



3.03 



G5 



28.15 



170 



73. 61 



8 



3.46 



70 



30.31 



180 



77.94 



9 



3.90 



75 



32. 48 



190 



82.27 



10 



4. .33 



80 



34.64 



200 



86. 60 



15 



6. .50 



85 



36.80 



210 



90. 93 



20 



H.m 



90 



38. 97 



220 



95. 26 



25 



10. 83 



95 



41.14 



230 



99. 60 



30 



13.00 



100 



4.3.30 



240 



103. 92 



53577"— Bull. 414—10- 



