112 SOME REMARKS ON SEWERAGE SYSTEMS. 



constructed, so that sewage is obstructed in its free pas- 

 sage, will have more sewer gas in it than one in which the 

 obstructions are reduced to a minimum. Hence the utility 

 of a perfectly smooth surface and of a small wetted peri- 

 meter. 



Again, if a sewer be laid at so flat a gradient that there 

 cannot be a self-cleansing velocity, an accumulation of foul 

 matter must follow, and consequently an accumulation of 

 sewer gas. 



Flushing with large quantities of sewage or water has 

 been requisitioned, to improve the velocity and to clear out 

 the holes and corners where foul matter has collected, and 

 with very great success ; but it is chiefly because so many 

 existing sj'stems are faulty in these particulars that the 

 accumulation of gas has been so troublesome. As it was 

 impossible, except at great expense, to prevent the cause, 

 the only way appeared to be to reduce the evil by so 

 thoroughly diluting the foul sewer air with fresh air that 

 the gas would be rendered harmless. It has, therefore, been 

 the approved policy to place a free-air opening into the 

 sewer at each manhole and lamphole ; at first by placing 

 gratings at the street surface, and latterly by erecting 

 special shafts in which an artificial draught is induced, 

 such as by a furnace or gas burners. 



If a smell is complained of as coming through the man- 

 hole, the reply is, that the sewer air must be still further 

 diluted by a larger number of openings. In all this the 

 aim has been, not so much to eradicate the cause, as to mini- 

 mize the evil eflects. 



In a paper read last year before the Sanitary Institute by 

 Mr. J. S. Haldane, upon the Air of Sewers and Buildings, 

 a great deal of new light is thrown upon the eflects of 

 ventilation. He had been making a series of careful obser- 



