Yan VYean Reservoir. 179 
is so far favorable for lessening evaporation, as to render, in 
my humble opinion, Dr. Wilkie’s estimated rate of evaporation 
too great. 
For the steep slopes of the ordinary forest ranges com- 
prised within the watershed of the Plenty above Yan Yean, 
promote the rapid conduction of rain-water to the channels 
of drainage, and therefore tend to diminish evaporation ; this 
obvious diminution is, however, to a considerable extent, 
counterbalanced by the increased evaporation that occurs on 
some extensive tracts of wet undrained land, from whose ~ 
surface the evaporation is, according to Charnock, nearly as 
great as from a sheet of water. In this condition are the 
swamps that extend from the base of Mount Disappointment 
to the village of Whittlesea,—a swampy tract north of Sher- 
win’s range, the sides of several of the mountain ravines, and 
even part of the table land on the main range. 
From the want of extended meteorological observations 
taken in connexion with the Upper Plenty districts, or what 
would have been much more satisfactory, a complete series of 
stream-guagings to determine the annual discharge, the avyail- 
able rainfall of the district can be only analogically eliminated 
from the general data afforded by the most trustworthy 
English observations on evaporation, corrected for the average 
differences of temperature for the various months of the year, 
in London and Melbourne, as given in the Statistical Register 
for Victoria. Moreover, as wind, and the hygrometrical 
state of the atmosphere exercise a marked influence on 
evaporation, independently of temperature; and as their 
action is more intense here than in England, some additional 
corrections should be applied to the English data for this 
increased action. Due consideration must also be given to 
the favourable nature of part of the surface of the Upper 
Plenty district, especially that portion draining direct into 
the reservoir. Having made allowance for all these eon- 
tingencies I have arrived at the conclusion that the total 
annual rainfall at the Upper Plenty may be taken as equiva- 
lent to thirty-six inches, and that the amount thereof 
evaporated may be assumed to be 30°8 inches, leaving the 
amount available for supply 5:2 inches, over 44,000 acres. 
But if cuts were made on the catch water principle the amount 
available might be greatly augmented. 
The annual evaporation from the surface of water in 
England amounts to from thirty-four to forty-four inches. 
Both Dr. Wilkie and Messrs. Acheson and Christie, have 
assumed nine feet as the annual loss from evaporation on the 
