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, 1 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 
temperature 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-two 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 higher 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, with a 
great deal of rain, and few hot winds, is not to be regarded 
as an average season. 
