USE OF FLUSH-TANKS 



235 



corded as rapidly as possible (usually every seven seconds) the 

 depth as marked on a thin vertical scale placed in the sewer. 

 Figs. 68 to 71 show the wave-forms and the progressive flatten- 

 ing as the wave gets farther and farther from the flush-tank. 



To test the transporting power of the wave small brickbats 

 and gravel of various sizes, coated with paint so as to be recog- 

 nizable, were placed in the inverts at the manholes. A consid- 

 able growth, apparently of vegetable origin, had become at- 

 tached to the sides and bottom of the pipe, and the value of the 

 flush in removing this growth was also noted. The order of pro- 

 cedure was to examine and note the condition of the line, and, 

 after placing the gravel, etc., in the manholes, to make a number 



TIME SCALE IN SECONDS. 



FIG. 71. 



of flushes, each of 20 cubic feet, and note the results. Then, in- 

 creasing the amount discharged to 30, 40, 50, and 60 cubic feet, 

 the respective results were noted. Then either the whole pipe 

 was scraped by a rubber-edged piston-like cleaner, or merely 

 the manhole inverts and about 6 feet each way into the pipe, 

 and the flushing repeated. Tables XXVII-XXX give the 

 results on the different lines. 



Before commencing the work, the examination of the Green 

 Street pipe showed it to be practically clean, with no ground- 

 water, except between the third and fourth manholes, where 

 there was a stream about one-fourth inch deep. No house- 

 connections had been made, but there was a small depth of silt, 

 and bits of cement left from construction, also a slight vegetable 



