ENGINEERING CONSTRUCTION AND RAINFALL. XXXVII. 



ditions, a fact that is well illustrated by the accompanying dia- 

 gram. In many cases this is not to be wondered at, as some 

 of these formulae have been put forward as applicable only to 

 particular districts. Take, for instance, that of Colonel Dickens, 

 which was designed to deal with rainfall in different parts of 

 India, and where the only quantity to be introduced, independently 

 of judgment, is the area of the catchment ; this item can be 

 ascertained with very fair approximation, but no engineer worthy 

 of the name would think of applying such a formula to a country 

 like Australia, without having first determined the suitable con- 

 stant to use in the particular case under consideration ; these 

 constants have a very wide range, reaching from the minimum 

 0'03, to a maximum of 24. The localities from which each were 

 derived are given in Jackson's Hydraulic Manual, and his state- 

 ment as regards the value of the constant .is as follows : " It can 

 be determined and made use of within local limits only, as it 

 depends on an average maximum local downpour, evaporation, 

 quality, inclination, and disposition of surface of the area under 

 consideration : it has hitherto been determined for very few 

 districts, and not sufficiently satisfactorily for some of them. In 

 some cases unfortunately doubtful flood marks have been used to 

 obtain the flood gradients, and the velocities calculated according 

 to very varied formulae. In others the obstructions caused by 

 bridges and embankments have vitiated all the bases of the 

 calculations of discharge." 



After such an explanation, it is evident that before using this 

 formula in Australia, it is necessary to prove that the conditions 

 are identical. Where flood marks are observable, as in catch- 

 ments discharging by streams with defined channels, no such 

 formulae need be applied, nor, indeed, are they necessary ; if 

 doubt exists as to the reliability of the marks, an experienced 

 engineer can make requisite allowances. 



On consideration of the most approved formulae which include 

 factors for absorption, rainfall, slope and area of catchment, it is 

 at once evident that these can be applied only where these 

 several factors are sensibly constant; this constancy cannot 



