114 SOME REMARKS ON SEWERAGE SYSTEMS. 



sufficient ; or if, instead of the present open ventilators, 

 openings were formed in the top of sewers communicating 

 only with the subsoil, an inexpensive vent would be pro- 

 vided, without having to pollute the air we breathe. 



Objection might be raised to this suggestion, that the 

 ground air would be polluted, and the polluted air would 

 be sucked up by the heat of fires, etc., into the houses. 

 But care is of course implied, in forming free openings into 

 the sewers in whatever mode they are constructed, and the 

 same would apply to this. Sewer gas is chiefly composed 

 of carbonic acid gas, sulphuretted hydrogen, and ammonia. 

 The former, which is much heavier than air, already 

 exists in the subsoil to ten times the amount, and the two 

 latter are lighter than air, and will rise ; but in rising 

 through the subsoil they will have ample opportunity of 

 filtration and dilution before they reach the surface. In 

 addition — given a properly constructed sewer — the amount 

 of sewer gas is comparatively small, and much of the gas 

 will remain in the sewer. ♦ 



With these remarks, I would leave my subject. Although 

 so much has been accomplished during the last half-century, 

 it clearly cannot by any means be claimed that we have yet 

 arrived at perfection in our systems of sewerage. 



Our views are continually altering with experience, so 

 that methods that were popular a few years ago are almost 

 discarded now, and we lay them down to grasp improved 

 ones. 



I have therefore endeavoured to describe the practice of 

 the present day, with the steps that have led to it, and to 

 indicate the points from which it appears to me fresh 

 departures may be made toward that perfection which is 

 the ultima thiilc of every branch of engineering. 



