WATERWAYS OF BRIDGES AND CULVERTS. 559 



Colonel Dickens, as quoted in Jackson's " Hydraulic Manual," 

 gives the following : — 



Discharge in cubic feet ) -ook /a i ■ i • „„ ™'i^ Af 



*= , y =,825 I Area drained in square miles I* 



per second. J \ ' 



as working well in Bengal and Bahar, where the rainfall is about 

 double what it is in most parts of South-eastern Australia. 



Lastly, some authorities have endeavoured to take into account 

 the shape as well as the area of the basin. 



Mr. Burge, of the Madras Railway* gives — 



Discharge in cubic feet = 1300 Area in square miles 

 per second. Length in miles. § 



Mr. L. Jackson, author of the aforesaid " Hydraulic Manual,"^ 

 proposes a formula of the subjoined shape — 



Discharge = C ; — (Area)^ 



^ length ^ ' 



and Mr. G. R B. Steane,t advocates the following : — 



Discharge in cubic ( ^ Area in square chains x 181 



feet per second | Length in chains ^'^^ x 1,800 



from which the maximum safe velocity through the opening being- 

 determined, the waterway is at once known. 



It will be seen that these rules group themselves into three 

 classes — 



1. Those of Messrs. Stanley and M. E. Kernot, in which the 

 waterway varies directly as the area drained. Such rules are 

 plainly inapplicable over any very wide range of areas, for there 

 are many reasons why the discharge should not, and universal 

 experience shows that it does not, increase nearly as fast as the 

 area drained. Amongst these reasons may be mentioned the fact 

 that the excessively heavy downpours are very transient and very 

 local, and consequently atfect large areas much less than smaller 

 ones, also that owing to length of channel, bends and obstructions 

 of various sorts, tlie water falling on a large area often takes 

 several times as long to drain off as it takes to fall, a days rainfall 

 of exceptional severity, taking for example a week to run off. 

 Hence, this class of formula is very limited in its application, and 

 Mr. M. E. Kernot very properly limits his rule to areas of not 

 more than three square miles. What limit Mr. Stanley adopts I 

 do not know. 



The marked difference between these two rules, one giving about 

 three times the waterway of the otlier, is probably connected with 



* Jackson's " Hydraulic Manuel," p. 20. 

 t Trans. R. Soc. Victoria, 1S87, p. 152. 



