GENERAL CONSIDERATIONS 7 



halls, etc. General statistics of the health of sewer-laborers 

 show no ill effects from their employment. 



It is hard to see why it should not be possible to keep both 

 systems clean, since the inverts of both may be made to the same 

 radius, and so the velocity with the same grade kept equal. 

 If the water for flushing has to be bought, the same quantity 

 used in frequent flushes of small pipes will probably keep the 

 sewer cleaner than single flushes, larger in amount but applied 

 at such infrequent intervals to a large sewer that the deposits 

 become hard and fast between times. However, modern practice 

 seems to have the tendency to do away with flush-tanks and to 

 keep the sewers clean by hand-flushing in such amounts and at 

 such times and places as may be found necessary. 



The alleged advantage of having the sewer large enough 

 to enter is nothing, since with proper grades and velocities the 

 cost of removing the few stoppages that occur is much less than 

 the interest on the money required to build the larger sewer. 



Only a few years ago, the author heard of a city engineer 

 who still believed that large sewers only should be built and 

 therefore, even for laterals of short blocks, constructed only 

 brick sewers 24 inches in diameter. The excessive cost thus 

 incurred was a heavy burden on the small village (near Pitts- 

 burgh) and no better sewer was provided than if a 6-inch pipe 

 had been used. Indeed, on a low grade the large sewer would 

 be distinctly inferior. 



The question of ventilation is based on conditions which 

 do not, or should not, occur. Both sewers are designed to 

 carry all material to the outfall before decomposition has begun, 

 so that, unless by some accident deposits take place, there is 

 no decomposition in the sewer, and therefore no gases to be dis- 

 pelled. Should deposits occur and gases arise, the ventilation 

 through the manholes for the same sewage-flow should be as 

 complete in the one case as in the other; any slime left on the 

 sides of the large sewer after a rain would in decay be so diluted by 

 the greater amount of air that the offence would probably not 

 be any greater. The sewers which are cited to show the bad 



