14 
BULLETIN 57, TJ. S. DEPARTMENT OF AGRICULTURE. 
THE GRAVITY SYSTEM. 
Iii the gravity system water is forced into an elevated tank placed 
higher than the highest discharge cock. A storage tank may be placed 
in the attic, on the roof, or on a tower outside. The agricultural ex- 
periment station at Ames, Iowa, has designed a silo with the storage 
tank placed on top. Figure 12 shows a gravity system with the stor- 
age tank in the attic and 
figure 13 (p. 16) shows the 
storage tank placed on the 
windmill tower. 
Since there is consider- 
able frictional resistance to 
the flow of water through 
the distribution pipes, the 
tank should be placed at 
least 10 feet higher than 
the highest discharge cock 
to insure a flow under 
pressure. 
Water weighs 62.5 
pounds per cubic foot, or 
about 8.4 pounds per gal- 
lon, so that in placing a 
tank in the attic or on the 
roof the supports should be 
made sufficiently strong to 
uphold this weight. 
Either wooden or gal- 
vanized metal tanks may 
be used. Wooden tanks 
may be obtained of almost 
any size, either circular or 
rect ang u 1 a r in shape. 
They are generally built of 
cedar or cypress, and are 
slightly conical. They are 
Fig. 12.— Gravity supply system with storage tank in attic. USUally knocked down 
when shipped, and should be set up and filled with water as soon as 
received. The foundation should be good and solid and the weight of 
the tank should rest on the tank bottom and not on that part of the 
stave which projects below. The capacities of circular tanks maybe 
found by the following: Capacity in gallons equals diameter hi feet, 
squared, multiplied by 0.7854, multiplied by the depth hi feet, 
multiplied by 7.48. One cubic foot equals 7.48 gallons. When 
located in buildings wooden tanks are common! v made rectangular. 
