DISPOSAL OF EXGEETA 869 



Bends (Fig. 156) also require looking to ; instead of 

 being made with curved pipes they are not uncommonly 

 made with straight pipes fitted into each other so as to give 

 the needful curvature, and leaking at every joint. 



For the purpose of description all pipes within a building 

 are known as stable pipes, those outside it which conduct 

 the material from the building to the sewer are known as 

 seioer pipes, while the main drain, which in cities is placed 

 in the middle of the street and receives the contents of all 

 the buildings, is known as the sewer. 



Sewers are the channels into which the combined drainage 

 system empties, and these unite and reunite until one or 

 more channels of considerable size are formed towards the 

 outlet. When sewers were first established they were 

 built of brick which was pervious, and it was found that 

 they conveyed away not only the material poured into 

 them, but also water from the soil, which passed in through 

 the imperfect joints. The general effect of this sewer 

 system was to lower the height of the ground water in 

 cities, and raise the general health of the inhabitants. In 

 this respect they did immense good, but it was found that 

 not only did water pass in through the brickwork, but 

 drainage frequently passed out, so that soil pollution 

 became common, and poisoning of the ground air and well 

 water occurred. 



Sewers are now rendered impervious by the use of 

 glazed fire bricks with Portland cement for the joints ; in 

 section they are egg-shaped, the narrowed end being below, 

 this is found to be the form which gives rise to the least 

 friction with the contents. 



Inspection chambers are arranged wherever a change of 

 direction occurs, and at other intervals for the purpose of 

 cleaning. These manholes are also used for the purpose 

 of ventilation, the ventilation of sewers being an important 

 point to attend to in sanitary legislation. 



The outlet of a sewer is generally a river ; in days gone 

 by the crude material was poured direct into it, and 

 extreme pollution occurred. Now the sewage has to be 

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