SANITATION 



67 



he pipes on subsequent occasion should this be necessary for 

 effecting repairs or for making new connections. 



It is important that all traps be set in a firm bed of cement 

 concrete so that the water seal may remain at the proper level. 



Defects. From the foregoing it will be seen that defects in 

 drains may be clue to several causes, and these may be summarised 

 as follows : (a) blockage of the pipes or fittings through the 

 entrance of large objects; (&) insufficient gradient, or, conversely, 

 too steep a gradient ; (c) want of ventilation resulting in the syphon- 

 age of traps when a w.c. or sink containing a large quantity of 

 water is let into the drain; (d) " waving " of the water in a trap 

 due to alteration in the air pressure of adjacent pipes, usually 

 set up by wind, and resulting in the reduction of the seal; 

 (e) insufficient flushing of the drains resulting in reduction of the 

 seal in traps by evaporation ; (/) imperfect laying of traps so that 

 these become tilted out of line, so reducing the depth of the seal; 

 (g) bad workmanship in laying drains, such as imperfect alignment 

 and the access of cement into the pipes at the faucet during the 

 joint making; (h) imperfect luting of inspection covers ; (i) using 

 pipes too large, or, conversely, too small ; (/) imperfect ventilation 

 of lengths of piping, so that sewer gas accumulates under pressure 

 and forces the water seal of a trap; (k) imperfect flushing of the 

 system with the result that solids accumulate and gases generate; 

 (/) subsidence of the soil with consequent sagging and rupture 

 of pipes of their joints; (m) use of unsuitable substances, such as 

 clay, for jointing material; (n) too many bends in a system, or 

 these improperly made; (0) junctions not effected at a suitable 

 angle. 



TESTING DRAINS. Drains must be tested to ensure that they are 

 gas and water tight, and that the water seals of the traps are satis- 

 factory. Tests are carried out on the various sections of a drainage 

 system during its construction, and a final test on the whole system 

 when completed. Thereafter periodical tests should be made from 

 time to time to ascertain that the system remains satisfactory, as 

 defects may develop from various causes which, if not rectified, 

 may cause serious danger to health. 



There are various methods of testing drains but those generally 

 in use are : 



(1) The Air or Pneumatic Test. 



(2) The Smoke Test. 



(3) The Water or Hydraulic Test. 



(4) The Scent or Smell Test. 



The Air and Smoke Tests. The air test consists in plugging 



