PROPORTION REACHING THE SEWERS 55 



for the rain to fall; but this relation, depending as it does on 

 the conditions already mentioned, is uncertain, and therefore 

 the method cannot be regarded as reliable. It has, however, 

 been stated that, judging from the limited number of observa- 

 tions accessible, in none of which was the time for discharge 

 from the sewers as short as twice the duration of the storm, 

 but rather exceeding this three, four, and five times, it is always 

 possible to divide the rate of rainfall by at least two to get the 

 rate of discharge. But this must be the result of imperfect 

 observations and inattention to details. Col. Adams reports 

 using a series of gagings made in London by Mr. Wm. Hay- 

 ward, Engineer to the Metropolitan Board of Works, London, 

 and designing the Brooklyn sewers to carry off one-half the 

 rainfall, believing from his study of those gagings that his 

 sewers would have twice as long to discharge the rain as it takes 

 to fall. Therefore, having decided that a rainfall of i inch 

 per hour was to be expected with sufficient frequency to make 

 a provision for it desirable, he made the sewer of such size as 

 to take care of half an inch per hour over all the territory drain- 

 ing to that sewer. Other English experiments, which are given 

 by Baldwin Latham, and on which most of the work done in 

 this country has apparently been based, were made in London 

 in 1857. Here the Savoy Street sewer, draining an entirely 

 built-up part of London, discharged from a rainfall of i 

 inch in one and a quarter hours 0.34 cubic foot per second, 

 or 34 per cent of the rainfall. Later Sir Jos. Bazelgette, in 

 the Savoy Street and Ratcliff Street sewers, determined that 

 from rainfalls of 2.9 inches in thirty-six and twenty-five hours 

 there was discharged an average amount equal to 64.5 and 

 52 per cent respectively. From these gagings and a few 

 others the engineers of the London Main Drainage Works 

 concluded that a rainfall of 0.25 inch would discharge 0.125 

 inch, while one of 0.40 inch might discharge 0.25 inch. In 

 1865 Col. Wm. Hayward published a gaging of another Lon- 

 don sewer, showing that of a rain of 2.75 inches in thirty-six 

 hours 53 per cent was discharged, and in 1858 of a rain of 0.24 



