Yan Yean Reservoir. 181 
tions on the water in this butt, I can only state generally 
that the decrease in depth in consequence of evaporation 
was remarkably less than according to Dr. Davey’s obser- 
vations; and that during the prevalence of one hot wind 
the observed decrease in twenty-four hours wasonly half aninch. 
I must, however, protest against the greatly exaggerated 
notions relative to the diminished rate of evaporation in large 
reservoirs entertained by some practical men. For instance, 
Mr. Stirrat, the promoter of the gravitation schemes of water 
supply in Scotland, actually stated in evidence, that in large 
reservoirs the evaporation was counterbalanced by dew 
condensed on the surface of the water—a remark, which, 
if true, would have entirely precluded the possibility of 
carrying on the well-known process of obtaining salt by 
evaporation of sea water in large tanks formed near the 
sea coasts. 
Tf, in the absence of a complete and satisfactory series of 
observations on the evaporation during the summer months 
here, from water contained in a vessel of adequate depth 
and capacity, well protected from the influences of external 
temperature, my experiment, made during December, 
January, and February, on the pond, be considered. to 
afford some criterion of the probable influence of hot winds 
upon the Yan Yean reservoir, then the approximate deteriora- 
tion of the evaporation of the other months of the year can 
be arrived at with tolerable precision. For if we omit 
December, January, and February, the mean temperature 
of all the other months in the year in this colony agree very 
nearly with the mean temperature of some of the months in 
England, for which the rates of evaporation from water 
surfaces have been registered. By basing a calculation on 
this principle, and applying some additional correction for the 
frequent occurrence of dry winds here, &c., I have computed 
the probable evaporation from the surface of the reservoir to 
be as follows :* 
INCHES. 
Evaporation for December, January and February, } 24-6 
as determined from my experiments on the pond. 
Evaporation for March, April, and November, (de- 
termined by analogical deduction, from a com- 19: 
parison of English and Australian Meteorological 
observations, &c. 
Evaporation for March, Seeeabier! and 1] May. Bo 14:2 
Evaporation for June, July, and August. abe 88 
66°6 
* During the period that has clapsed since the lecture of this paper and my 
inspection of the proof sheets, the President of the Commission of Sewerage 
