AREA OF WATERWAYS. 



623 



signed to them. It is generally assumed that a maximum rate of 

 rainfall should be a factor ; but whether the hourly rate, a certain 

 percentage of it, or a five minute, ten minute, or twenty-four hours 

 maximum, or other rate has not been universally conceded, although 

 the maximum hourly rate is the more generally favoui^ed. The 

 area must also be taken into consideration. These two elements, 

 rainfall and area, are of cardinal importance, and should be the 

 basis of any such formula. The effect of slope of surface, and ratio 

 of length to breadth of catchment have been incorporated in the 

 formulae of several well-known authors, but they are not by any 

 means generally conceded as essential, or if so the relative values 

 assigned to them are not considered of general applicability. Some 

 even go so far as to assign a constant value to rainfall, but evidently 

 their authors generally intended them for local application only. 



The following is a list of some of the better knowTi formulae : — 

 Table I. 



