62 SMALL WATER SUPPLIES. 



But provide the pipe with the proper gradient and 

 it will run on for ever ! 



Having regard to the weight of cast-iron pipes, 

 tables for use in estimating are published, but if these 

 were not handy we can find out the factor by means 

 of a simple equation also. Given D = the outside 

 and d = inside diameter in inches. W, the weight in 

 Ibs. per yd. of the pipe, is equal to 



W = 7 '35(D 2 - <O . . (19) 

 while if we have flanges, 2 flanges = i ft. of pipe, or 

 if the joints are spigot and faucet, i spigot and i 

 faucet = i ft. of pipe also. 



In water schemes one of the most essential calcu- 

 lations for mains of any importance is their size. 

 Too small a pipe causes shortness of supply, while 

 pipes too large, besides being useless expense, tend 

 to contaminate the water by long standing. A little 

 discretion is necessary. 



Say we have to supply W cub. ft. of water per 

 minute, the total length of pipe being L ft., its dia- 

 meter = D ins., and the pressure = H ft. 



TU 



Then D=-53i^/-j r . (20) 



The fifth root of the equation will of course be solved 

 by logarithms and by dividing by 5. 



If, however, we have to distribute the quantity by 

 means of a number of small pipes in preference to 

 one large one, and their proposed diameter is d ins., 

 the number to be provided will be equal to 



The flow in gravitational mains, however, is not 

 always as simple as it may appear, and mistakes can 



