MATHEMATICAL FORMULAE 83 



mula, to allow its use under other conditions than those of 

 the London districts. In French units his formula was 



where q = volume of storm-water (in liters) reaching the sewer 



per second from each hectare of surface drained; 

 c = constant varying with the character of the surface; 

 r = average rainfall in liters per hectare per second during 



the heaviest fall; 



5 = general fall of the surface per thousand ; 

 A =area drained in hectares. 



Biirkli-Ziegler recommended that for ordinary conditions 

 c be made 0.60 for thickly populated urban districts and 0.25 

 for suburban ones, with an average value of 0.50, and that the 

 maximum rainfall assumed be taken at 125 to 200 liters per 

 hectare per second. 



One liter per hectare per second equals 0.0143 cubic foot per 

 second, so that the rainfall corresponding to 125 to 200 liters 

 per hectare = 1.79 to 2.86 cubic feet per acre per second, or 

 rainfalls of 1.79 and 2.86 inches per hour. 



Transforming the whole formula into English units, reading 

 Q in cubic feet per second per acre, r in inches per hour, A 

 in acres, s for 5, and making, by definition, s = S/iooo, we have, 



= c>j.o$r .0143 4; 





the values of c corresponding to 0.25 and 0.60 will be in English 

 measure 1.76 and 4.23, and for the mean 3.52, so that the formula 

 in English, if Q = the total discharge, is 



