Yan VYean Water Works. 195 
above the swamp therefore 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, 
will 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 
26°69 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 eubic 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 assumption 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 per centage 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 we 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 amean 
proportional between the hydraulic mean depth and the fall 
in two miles, hence the fall in two miles will be 81 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 
