232 SEWER DESIGN 



six believing in hand-flushing, two came to that opinion after 

 becoming disgusted with the uncertainty of tanks. 



On the second question, only twenty-three of the eighty 

 ventured an opinion. Of these, eight thought that the grade 

 did not affect the question, but that flush-tanks were as neces- 

 sary on steep as on flat grades. One engineer explained his 

 position by saying that while the velocity on the steep grades 

 might be greater, yet as the depth would be less, the trans- 

 porting power would be less, and therefore tanks were equally 

 necessary. Of the fifteen who thought that tanks are not 

 needed above a certain grade, six merely ventured it as an 

 opinion, and nine fixed the limit at from 0.5 to 3 per cent; four 

 of these gave i per cent as the limit; one, 3 per cent; and the 

 other four less than i per cent. 



Only six replies were given to the last question, whether 

 the amount of water in the flush-tank should be varied with 

 the grade of the sewer. Of these six, two engineers thought 

 that no difference should be made; three thought that less 

 water could be used on the steep grades, but had no definite 

 opinion as to the relative amounts; while one well-known 

 engineer, who has thoroughly studied the workings of the 

 sewer system under his care, writes that he finds one flush 

 daily on a 2 per cent grade as effective as two flushes daily on 

 a 0.5 per cent grade, each flush of 300 gallons. 



The general conclusion from the replies is that on low grades, 

 probably below i per cent, occasional flushing is needed at the 

 upper ends of laterals; that this may be accomplished either 

 by hand-flushing or by the use of automatic tanks; that if 

 tanks are used, less care and vigilance are required in inspec- 

 tion and oversight, but, on the other hand, the periodic exam- 

 ination of the system, which should not be omitted, is apt 

 to be irregular, and if a tank fails to work or if an obstruction 

 occurs below the effect of the flush, a serious nuisance may 

 result; that if hand -flushing is used, a constant and regular 

 inspection must be practised, although actual flushing may be 

 required but once a month or even less. The amount of water 



