Yan Yean Water Works, 



195 



above the swamp tlierefore exceeds that below the same, by 

 16,337 cubic yards per day, this amount therefore fairly re- 

 presents the rate of loss from evaporation for the three summer 

 months ; this spread over the area of the swamp, or 

 3^000,000 square yards, gives a depth of evaporation of '1958 

 inches per day, or 17*82 inches for three summer months. 

 Then by Dr. Davey's rule four-thirds of this, or 23'76 inches, 

 mil equal evaporation for six autumnal months, and one-sixth, 

 or 2'97 inches will be evaporation for three winter months, 

 the sum of all these give 44'55 inches for the year, but we 

 Have already allowed 17'856 inches for surface evaporation all 

 over the basin, which must therefore be deducted, leaving 

 2 6 '6 9 inches depth of evaporation for the year in the eastern 

 swamp equal to 2,224,157 cubic yards. 



The total amount of loss by evaporation due to the western 

 and eastern swamps is therefore 4,194,805 cubic yards, and 

 this deducted from the amount received into the Plenty 

 already ascertained, or 67,858,102 cubic yards, leaves 

 63,663,297 cubic yards, equal to the whole amount available 

 for collection from the River Plenty. 



Having now arrived at the total effective discharge of the 

 Plenty, above the reservoir, and as this result has been 

 obtained on the assuinption that 57 '6 per cent of the rainfall, 

 or 17*854 inches truly represents the amount of loss from 

 surface evaporation, we now propose to test the correctness 

 of that assumption through the medium of the results obtained 

 therefrom. If therefore the effective discharge of the Plenty 

 per annum, or 63,663,297 cubic yards, obtained on this as- 

 sumption of the correctness of 57 '6 as the percentage of 

 evaporation, be confirmed by legitimate calculations and deduc- 

 tions, it may hence be inferred that the assumption of 

 evaporation itself, or 57'6 is correct. 



The eastern arm of the Plenty discharges in summer, as 

 before stated, 11*87 cubic feet per second, which w^e were 

 informed was its ordinary least discharge ; the sectional 

 area of this discharge is fourteen square feet, and mean velocity 

 10*18 inches per second as before obtained; having both which, 

 we obtain the fall in two miles by Eytewein's formula, as 

 follows: — The velocity in a second is ten-elevenths of a mean 

 proportional between the hydraulic mean depth and the fall 

 in two miles, hence the fall in two miles will be 8"1 inches'; 

 now the section of the eastern area at this point is rectangular, 

 and the ordinary winter level, irrespective of floods, as pointed 

 out by a resident on the spot, on particular inquiry, is exactly 

 three feet above the summer level at this point, hence we 



