Van Yean Water Works. 193 
soil, combined with slope, is unfavourable to loss from evapor- 
ation on its surface. The rainfalls also being mucn less 
frequent, and consequently heavier than in England, it is 
evident that under similar circumstances there would be less 
loss from evaporation, than if more diffused, as in England. 
It is reasonable therefore to conclude, that if 57°6 per cent. 
of rainfall is lost by surface evaporation in England, under 
circumstances highly favorable to evaporation, a much smaller 
per centage of the rainfall will be lost when the same circum- 
stances are very unfavorable, as in the basin in question. 
We should hence be justified in estimating a much less per 
centage of loss from evaporation in the basin of the Plenty, 
were it not that another important question must enter intothe 
calculation, namely the difference of temperature, equal to 
ten degrees in favour of evaporation in the Plenty basin, which 
must act to some extent as a counterpoise against its un- 
favourable character, for same in other respectsnotwithstanding 
that the action on evaporation does not last nearly as long as 
the English temperature. 
Viewing therefore the conditions of the surface drainage of 
England relatively with those of the basin of the Plenty, we 
are of opinion that the English per centage of evaporation of 
57°6, if correct, embraces that of the Plenty basin, if it does 
not exceed it. We hence proceed in our calculations on this 
assumption, leaving it further on to be shown how it is borne 
out by the facts. 
_ This per centage of 57°6 will therefore give 17°856 inches 
on 31 inches rainfall, the supply from rainfall of 31 inches and 
loss on same by evaporation will therefore stand thus, 
Rainfall of 31 inches on-basin of 60 square 
miles, superficial extent, or 185,856,000 160,042,666 cub. yards. 
square yards ... ae 200 ae 
Loss due to surface absorption and eva- 
poration over same extent, to depth a 92,184,564 cub. yards. 
17°856 inches ... ae se “C0 
——<—_—_——— 
Balance delivered into the Plenty... 67,858,102 cub. yards. 
Having thusascertained the amount discharged into the Plenty, 
on this assumption, it is next necessary to determine the 
amount of loss entailed, by the passage of its western and 
eastern arms through the swamps. 
The swamp on the western arm as before stated, has a 
superficial extent of 787-000 square yards, over which its 
waters are spread, thus exposing them to evaporation almost 
Y 
