GENERAL CONSIDERATIONS 5 



dried may float off in the air to escape through the traps into 

 houses or through manholes into the streets. 



3. On account of the larger air-space over the flowing liquid 

 in the combined sewer, the gases of decomposition given off 

 by the sewage are largely diluted, and there is nothing to fear 

 from them; whereas, with the small sewers, the degree of con- 

 centration is greater, and there is consequently more danger 

 of forced traps and greater annoyance from ventilating manholes. 



The reply is that, owing to the greater amount of air to be 

 moved, the ventilation is really less perfect in the large sewer, 

 and that, from the slime which accumulates on the walls after 

 flooding, there is more matter to decompose. The deposits 

 which, it is admitted, occur in the mains of the combined sys- 

 tem add considerably to the offensive gases during decom- 

 position. 



4. The combined system is the more economical; for if 

 the use of the sewer is restricted to house-sewage, then there 

 will be required for the rain-water another system of pipes 

 of equal extent, and the cost of the two systems will be greater 

 than that of one. This follows from the fact that the cost of 

 engineering, superintendence, pumping, sheeting, etc., are 

 practically the same for a large sewer as for a small one, and 

 that the cost of excavation does not increase in a direct ratio 

 with the size of the pipe used; and further, since the flow of 

 sewage is insignificant compared with that of the storm-water, 

 a sewer large enough for the latter will serve for the former pur- 

 pose without additional expense. 



The answer to this is that the system for rain-water need 

 never be co-extensive with that for house-sewage, since the 

 street-gutters will serve for the former purpose so long as the 

 flow in them does not become a nuisance; consequently the 

 length of the large main may be reduced nearly one-half. Fur- 

 ther, that the rain-water drains when built seldom need be 

 laid to the same depth or to the same outfall, as they may be 

 discharged into any convenient watercourse at the nearest 

 point. Also, if rain-water flowing on the streets does accumulate 



