GRADES AND SELF-CLEANSING VELOCITIES 163 



An ingenious arrangement was adopted some years ago by Mr. 

 J. H. Fuertes in the case of the Paxton Creek intercepting 

 sewer for Harrisburg, Pa. The permissible grade for this 

 pipe line was so light that the resulting velocity was altogether 

 inadequate, so the diameter was increased from about 18 

 inches to 5 feet and a full cross-section was insured by auto- 

 matically admitting water from 'the creek so that this water 

 added to the sewage would secure the needed velocity. The 

 increased cost of the sewer was here justified by the elimination 

 of pumping which would otherwise have been necessary. 



For sewers flowing constantly, either full or half full (the 

 velocity is the same at both points, increasing at a point A full 

 to a maximum of 112 per cent), at a velocity of 2 feet per second, 

 there are required, according to Kutter's formula, grades as 

 follows ^ = . 



6" 8" 9" 10" 12" 15" 18" 20" 24" size 

 .7 .5 .4 32 .22 .16 .11 .10 .09 grade 



Latham's tables, based on Weisbach's formula, give the 

 following : 



6" 8" 9" 10" 12" 15" 18" 20" 24" size 

 .34 .26 .23 .20 .18 .14 .11 .10 .09 grade 



showing a large difference for the smaller sizes as explained 

 in the discussion of the two formulae (see p. 167). 



Staley and Pierson say that a 6-inch lateral laid on a r \ per 

 cent grade works in a fairly satisfactory way, but Hering 

 advises T S TT per cent is possible. At Ithaca, where all the sewage 

 has to be pumped and where all the sewers in the valley are 

 laid on the minimum grade, the grades adopted were: 



6" 8" 10" 12" 15" 18" 20" 24" size 

 .5 .5 .45 .40 .35 .25 .20 .20 grade 



Examination of the plans of engineers throughout the coun- 

 try discloses that fact that there is considerable latitude in the 



