230 SEWER DESIGN 



flush- tanks on grades from 6 to 12 per cent, and on the 6 per 

 cent grades no stoppages were discovered, nor were there any 

 odors. 



Mr. Le Conte intimates that flush-tanks as built do not 

 answer their purpose, for where grades are light and the flush 

 most needed, they do the poorest work; and the large quantity 

 of water needed to be effective must be obtained by some other 

 means. 



Mr. Odell maintains that flushes of 200 gallons or less fail 

 to flush a sewer properly, especially on flat grades where flush- 

 ing is most needed. 



A table by Mr. Allen shows that on grades greater than 

 0.5 per cent a velocity of more than 2^ feet per second is main- 

 tained over 1000 feet from the flush-tank, but on lesser grades 

 the velocity drops to 2 feet or less within 600 feet. 



In order to obtain an insight into general engineering prac- 

 tice in the matter, and, at the same time, reap the benefit of 

 any experience which was to be had, the author sent out, on 

 January i7th, 150 reply postals, reading as follows: 



" ITHACA, N. Y., January 17, 1898. 



"DEAR SIR: 



" To aid me in deciding as to the necessity for flush-tanks 

 for our sewer system, will you kindly answer the following: 



" I. Do you find flush-tanks a necessity, or is periodic 

 hand-flushing sufficient to keep sewers clean? 



" II. Does the element of grade affect the question, and 

 within what limits of grade are tanks required? 



" III. Does your experience show any relation between 

 the minimum amount of water required for effective flushing 

 and the grade of the sewer? 



" Thanking you in advance for your kind assistance in this 



matter, 



" I am, yours very truly, 



" H. N. OGDEN, 

 " Engineer, Ithaca Sewer Commission." 



