78 BULLETIN 414, U. S. DEPARTMENT OE AGRICULTURE. 
Either wooden or metal tanks may be used. Wooden tanks may 
be obtained in almost any size, and are usually circular in section, 
and built of cedar or cypress staves, though juniper, fir, yellow pine, 
and white pine also are fairly satisfactory. As generally built the 
sides are battered not less than one-fourth nor more than one-half 
inch per foot of height; and the staves are held together by means 
of hoops which should be of wrought iron or mild steel and round in 
section rather than flat. Tanks are usually shipped " knockeddown," 
and should be set up and filled with water as soon as they are received. 
They are usually elevated on wooden towers, and, as set up, should 
rest on the tank bottom and not on the part of the stave that 
projects below it. All outdoor tanks should be covered to keep out 
birds and leaves or other debris. To prevent the covering being 
blown off it should be firmly fastened to the top of the tank by straps 
of iron. The ordinary life of wooden tanks is about 15 years. 
Steel tanks cost from 40 to 100 per cent more than wooden tanks 
of the same capacities; but if kept well painted inside and out they 
will last an indefinite time. They are absolutely tight when once 
erected, whereas a wooden tank will shrink and leak if the water gets 
low. They are not liable to sudden failure, as sometimes happens 
with wooden tanks when the hoops burst. On the other hand, steel 
tanks are not well adapted to the use of convict camps because it 
requires skilled boilermakers to erect them. They are also more 
difficult than wooden tanks to protect against freezing. 
This elevated-tank system was in use in six of the camps investi- 
gated. It may be employed satisfactorily in permanent camps, but 
it can not be used economically or conveniently in temporary camps 
on account of the time required to erect and raze the tank and tower 
when the camp is moved. 
THE HYDROPNEUMATIOTANK METHOD. 
The hydropneumatic-tank method is more convenient for the use of 
temporary camps, as the elevation of the tank is avoided. The equip- 
ment necessary consists of a force pump operated by hand or power, 
an air-tight steel tank and valves, pressure gauges, and fittings. The 
operation of the system depends upon the fact that air is elastic and 
can be ^compressed while water can not be compressed. When water 
is pumped into the empty, air-tight tank, the air already in the tank 
is compressed into a smaller space in the upper part of the tank. This 
compression of the air causes it to exert a pressure which forces the 
water through the service pipe to the points of delivery. But, though 
the air can be compressed almost indefinitely, it always will occupy 
some space in the top of the tank, and hence the tank never can be 
filled to capacity with water. In practice it is customary to fill only 
from two-thirds to three-fourths of the volume of the tank with 
