CONVICT LABOR FOB BOAD WOBK. 
79 
water. As the water is drawn off the pressure of the confined 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 water is pumped into a hydropneumatic tank. 
Part of tank filled with water. 
Pressure 
caused by 
compression 
of trapped 
air only. 
Initial 
pressure 
pumped into 
tank. 
Empty 
Pounds per 
square inch. 
0.0 
4.9 
7.4 
14.7 
29.4 
44.1 
Pounds per 
square inch. 
10.0 
18.2 
One-third full of water 
22.4 
One-half full of water 
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 will 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, all 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 
Height. 
Pressure 
in tank. 
Height. 
Pressure 
in tank. 
Height. 
Pressure 
in tank. 
Pounds 
Pounds 
Pounds 
per square 
per square 
per square 
Feet. 
inch. 
Feet. 
inch. 
Feet. 
inch. 
1 
0.43 
35 
15.16 
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 
5 
2.17 
55 
23.82 
150 
64.95 
6 
2.60 
60 
25.98 
160 
69.28 
7 
3.03 
65 
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 
8.66 
90 
38.97 
220 
95.26 
25 
10.83 
95 
41.14 
230 
99.60 
30 
13.00 
100 
43.30 
240 
103. 92 
53577°— Bull. 414—16 6 
