ENGINEERING CONSTRUCTION AND RAINFALL, LV. 



much. But even when he published the formula he had accom- 

 panied it by a paragraph, which was quoted, to the effect that the 

 evidence of flood marks and the experience of the engineer were 

 to be preferred to any formula. 



Mr. B. 0. Simpson thought that all known formulae were 

 useless in determining the area of waterways for engineering works, 

 since they were all empirical and founded more or less accurately 

 on certain particular local conditions. The enormous discrepancy 

 in the results obtained by different formulae, and the correspond- 

 ence given by the author of the paper from American and other 

 engineers, both indicate the unreliability of formulae. The author's 

 assumption of a velocity of six feet per second was not in accord 

 with facts. Ganguillet and Kutter gave many examples of 

 rivers with a velocity of 13 feet per second. Professor Kernot's 

 formula, founded on the success or failure of certain waterway 

 works, could have no practical value unless the opening necessary 

 for the outlet of a certain area of watershed had been determined 

 by the success or failure of a very large number of such works, 

 and the actual area of opening in each instance had been qualified 

 by the constant which would be applicable to that particular 

 watershed, according to his formula. The curve on which he 

 based his equation was fixed at many points by only one or two 

 examples. Mr. Haycroft's attempt to prove the similarity of 

 Biirkli-Ziegler's formula and Professor Kernot's could only be 

 described as mathematical juggling. Considering the unreliable 

 nature of all known formulae, and that the engineer must after 

 all use his judgment in the selection of his formula, and in the 

 application of his co-efficient, it was better to take the quantity of 

 water falling on the watershed during the period necessary for 

 the discharge from its furthest limits, and modify the total 

 resulting discharge per second by a co-efficient which would be found 

 to vary from -05 to *5, except in the extreme cases of town 

 drainage ; such coefficient to be arrived at by considering the 

 conditions of shape, slope, and porosity of watershed. 



Mr. Chamier said that for different catchments, varying 

 from one square mile up to five hundred, the factor R in his 



