USE OF FLUSH-TANKS 229 



cent is required for a 6-inch pipe half full for a velocity of 2.5 

 feet per second, and that if the amount of flow constantly de- 

 creases, the depth of flow decreases also, and the grade, in order 

 to maintain the same velocity, must be increased according to 

 the diagram. The diagram is given for two reasons: first, 

 to show that by the accepted laws governing the transportation 

 of material in flowing water, lateral sewers could be laid, theo- 

 retically, on such grades that no flushing would be necessary, 

 since, with grades which continually increase toward the upper 

 end, the corresponding velocities will always be equal to that 

 required to transport matter in suspension; second, to show 

 that as the grade of the sewer increases, the distance from the 

 upper end to the point where the stream reaches the velocity 

 required to carry matter in suspension decreases, and so the aid 

 required from flush-tanks is less. No value can be placed on 

 the grades given, as the diagram is based on the assumption 

 of a house with five persons every 66 feet, and this is not always 

 the case; but it is believed that there is a grade at or beyond 

 which flush- tanks are not required, and if the distance to which 

 the flushing power extends is a function of the amount of 

 water discharged, then this amount should be less on steep 

 grades. 



Referring again to Mr. OdelPs paper, it is first noted that 

 at Mt. Vernon, with grades of from 0.5 to 6 per cent, no flush- 

 tanks are used, and a good hand-flushing twice a year answers 

 every purpose. 



In the discussion, Mr. Hering says that on light grades 

 flushes of 200 to 300 gallons generally lose their flushing power 

 after passing a few hundred feet through the pipe, and that some- 

 times after 500 feet he has been unable to detect any differ- 

 ence in the flow due to the discharge of the tank. 



Mr. Kiersted writes that in one system designed by him 

 he recommended flush- tanks only on laterals of less than 0.5 

 per cent grade, and for five years the system has been in opera- 

 tion with but few stoppages. 



Mr. Folwell writes that in his experience he has omitted 



