560 PROCEEDINGS OF SECTION J. 



the clifierence of rainfall of the two colonies, the one being tropical 

 and the other temperate. 



The second class includes that of Mr. Cleeman, one of those 

 given by Mr. Jackson, Mr. Steane's first rule, Col. Dickens' rule, 

 and some othei^s. These I'ecoguise the fact that the discharge of 

 large areas is pi'oportionately less than that of small ones, and 

 allow for it, but to a widely differing extent. The conclusion that 

 I have come to from an examination of a number of actual cases 

 is that with the exception of Col. Dickens', they overdo the 

 allow^ance and make too great a reduction on the larger areas. 



The third class includes the more complex formula of Messrs. 

 Burge, Jackson, and Steane. These take account not only of the 

 size, but also of the shape of the basin. This is undoubtedly most 

 reasonable, and has been advocated by Von I'Caven, Blohm, Craig, 

 and other authors, which are quoted with approval in the valuable 

 paper on "Flood Discharges," by Mr. G. Gordon, M. Inst. C.E., 

 published in the "Victorian Engineer" of August and September, 

 1886. But on the other hand, it may as appropriately be urged that 

 the formula are somewhat complex, and in consistency should 

 also take account of the slope, the character of the soil, the cross 

 section of the stream, and other elements, which to do is manifestly 

 impossible. 



My own conclusion is that for basins of an approximately oval 

 shape, in which the length is from twice to three times the greatest 

 width, a very usual case, a modification of Col. Dickens' formula 

 as follows is easy of application, and corresponds as well as one 

 can, under the circumstances reasonably expect, with the best 

 accessible experience. 



Area of watei'way ) _ p / Area of basin in \ 

 in square feet J \ square miles. / 



When C is 40 in average country land with ordinary i-ainfall, 

 but may be reduced to 30 in very flat and absorbent areas and 

 increase to 80 in hard, rocky, or extremely non-absorbent 

 localities, such as occur in towns. 



The practical considerations leading one to this conclusion I 

 have endeavoured to show graphically in the diagrams submitted 

 when horizontal measurements show areas in square miles, and 

 vertical ones waterways in square feet, and where a round dot 

 means a satisfactory case of long standing, and a cross that the 

 opening has proved insufficient. The thick black lines are 

 those corresponding to the suggested rule. 



To detail fully all the particulars of each case would be tedious, 

 suffice it to say that — 



