SOME REMARKS ON SEWERAGE SYSTEMS. 99 



Recently there have been many improvements in the 

 materials of which sewers are constructed. In all brick 

 sewers none but the best bricks should be used, of as dense 

 a character as possible, to prevent absorption of sewage. 

 Glazed fire-bricks are excellent. Glazed hollow fireclay in- 

 vert blocks are often used, and the hollow is utilized as a 

 drain for the subsoil water. In some systems these subsoil 

 drains are specially provided ; but they should be used 

 cautiously, as they may easily become fouled from the 

 sewer, and, if the soil is at all loose, may so dislodge it 

 that a settlement of the sewer may take place. 



Sewers have been sometimes built up in four or more 

 segmental terra-cotta blocks ; and sometimes the bricks have 

 been previously cemented into segments before being placed 

 in position. More recently, concrete sewers have become 

 popular, and concrete and brick or concrete and pipe sewers 

 combined. They certainly have the advantage of being 

 practically water-tight and very strong. Of pipe sewers, 

 either circular or oval sections can be obtained. The 

 former are to be preferred, as being likely to make the 

 best joint. There are a number of different pipe-joints. 

 One of the best is Stanford's or Phillip's, but the plain 

 collar pipe is by far the most commonly used. Great 

 attention should be paid to the cementing material, as 

 some limes (such as chalk lime) cannot resist the chemical 

 action of the sewage. So far as I am aware, London Port- 

 land cement has proved itself to be most suitable for the 

 purpose, either used neat or mixed with a little sand. A 

 few years ago clay joints were largely used for pipes ; but I 

 trust the day for this unsafe method has gone by. There 

 are times when some clay-jointed sewers may be thoroughly 

 sound; but much more often the clay gets washed out, or 

 rats work through it, and thus ruin the sewer as a water- 



