78 BULLETIN 414, U. S. DEPAETMEISTT OF AGEICULTITEE. 



Either wooden or metal tanks may be used. Wooden tanks may 

 be obtained in abnost 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. AU 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 hable 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 HYDROPNEUMATIC-TAKK 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 dehvery. 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 



