Failure of the Yan Yean Reservoir. 



131 



I come next to determine the amount of loss from evapo- 

 ration and absorption. 



In large reservoirs the annual evaporation from the surface 

 is a very important element to be considered^ and, as it 

 increases nearly in a geometrical ratio, with an arithmetical 

 increase of temperature, a comparatively small difference in 

 the mean temperature might give double the amount of 

 evaporation. It is therefore especially important in warm 

 climates that its actual amount should be ascertained by a 

 careful series of experiments, before any work of magnitude 

 is undertaken, whose success or failure might entirely depend 

 on the result. 



In my first paper, which I only regarded as a preliminary 

 inquiry, I computed the evaporation for this colony from the 

 tables of Dr. Dalton, who gives forty-four inches as the 

 evaporation for England. I took the mean temperature of 

 the different months in Melbourne, and assumed for each 

 month the amount of evaporation corresponding to the month 

 of the same mean temperature in England. I also estimated 

 the increased evaporation proportionate to the increased mean 

 tempei'ature and to hot winds, to which I allowed a mean 

 temperature of 87^, and a duration of fifteen days, and I 

 thus determined the evaporation to be seventy-tAVO inches or 

 six feet. I stated, however, that I felt satisfied that a careful 

 series of experiments would show a still higher result, as, 

 independent altogether of the temperature, the much drier 

 condition of the atmosphere in Australia exercises a powerful 

 influence in promoting evaporation. 



Since our last meeting I have ascertained that Mr. Glaisher, 

 who is the highest authority on meteorological subjects, has 

 estimated the evaporation at Greenwich at sixty inches, or 

 five feet annually. Proceeding upon this Ingher estimate, 

 the evaporation, calculated in the same way, would be equal 

 to eight feet two inches ; and, making due allowance for the 

 dry condition of the atmosphere, nine feet may be safely 

 assumed as the mean evaporation for this colony. 



I have also learned that Dr. Davey, a member of this 

 Society, has devoted a great deal of attention to this subject, 

 and he has furnished me with the result of his experiments. 

 He is quite confident that the mean evaporation is not under 

 nine feet, but he is inclined to believe that it is more probably 

 ten feet. This summer having been remarkably cool, wir.h a 

 great deal of rain, and few hot winds, is not to be regarded 

 as an average season. 



