MATHEMATICAL FORMULAE 87 



drain taking the water from a large area, say 100 acres, does 

 not require to have ten times the capacity of one taking the 

 water from only 10 acres. 



" If it is assumed that all the factors are constant excepting 

 the run-off and drainage-area, then the above formulae give the 

 following values : 



Hawksley: Q = const. 



Adams: Q = const. 



Blirkli-Ziegler: Q = const. 



McMath: Q = const. X^' 80 . 



N. Y. diagrams: Q = const. XA 85 . 



" From this it is seen that the coefficients fail to show any 

 great difference in the formulae. 

 " All the formulae have the form 



Q = c-r x A x S I . 



" From what was said above, the only formulae giving any 

 other exponent than unity to r are those of Hawksley and Adams, 

 and it is as well to ignore such variation. Therefore the pre- 

 ferred formulae have the form 



As they are practically derived independently of a knowledge 

 of the exact maximum rainfall, we may substitute for c-r the 

 one value C and write 



Q = CA X S X ." 



In the Biirkli-Ziegler formula we may therefore write, 

 for the greatest storms, values for c-r or for C, modifying the 

 numerical values to correspond with the slope in feet per 

 thousand : 



C = n.6i for built-up areas; 

 C= 9.59 for average city areas; 

 C= 4.79 for rural or suburban areas. 



