154 SMALL WATER SUPPLIES. 



weather deliver into the cast-iron tank for house use.. 

 A 4~in. cast-iron pipe conveys the water from this 

 tank to another at the greenhouses lower down, and 

 the overflow also delivers into this pipe so that no 

 water is wasted. 



For the purpose of small supplies cast iron is 

 practically universally used for pipes, except those 

 inside the house of very small diameter. The metal 

 is not usually as strong as that put into large 

 machines and structural work, the ultimate tensile 

 strength being not more than 6 tons per sq. in. on 

 an average. Pipes are always tested at the works 

 under pressure, and this pressure may be put on till 

 the metal is stressed to half its breaking strength ; 

 but this will not usually do the pipe very much good, 

 and in view of the fact that they should be struck 

 with a hammer while under test, this stress should 

 not exceed one-third the ultimate strength. 



But between the time of proving and laying in the 

 ground the pipes are subject to a variety of accidents 

 which tend to produce defects, and where the pipe 

 line is any way long or important it should be tested 

 again when laid. The operation is somewhat trouble- 

 some and is consequently conspicuous by its absence. 

 There should be a clause in the specification binding 

 the contractor to locate and repair any leaks occurring 

 when the full working pressure comes on, but this 

 pressure is not usually sufficient to show up perhaps 

 a very small leak. Where the ground is very un- 

 dulating the mains are laid in sections, starting from 

 the bottom of the valley on each pipe line and work- 

 ing up, socket end first, to the top of each hill. The 

 pipes are here joined or as it is termed " married ", 



