Yan Yean Reservoir. 



181 



tlons 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 was only half aninch. 



I must, however, protest against the greatly exaggerated 

 notions relative to the diminished rate of evaporation in largo 

 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. 



If, 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 cant 

 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. J 



Evaporation for March, April, and November, (de-^ 

 termined by analogical deduction, from a com- 

 parison of English and Australian Meteorological f 

 observations, &c. J 



Evaporation for March, September, and May. ... 14*2 



Evaporation for June, July, and August. ... 8-8 



GG-6 



Durin'^ the period that has ehipsed since the lecture of this paper and my 

 inspection of the proof sheets, the President of the Commission of Sewerage 



