For our Water Supply. 259 
hills, and on the water of the reservoir. The latter will 
preserve a very uniform temperature, while the former will be 
subject to great diurnal alternations of heat and cold. 
Thus the vapour that is formed on the surface will at ali 
times be quickly removed, and replaced by currents of drier 
air. And it is important to notice, that the amount of water 
evaporated, other things being equal, is exactly in proportion 
to the surface exposed; and it it is not difficult to see” that 
when the water is agitated with winds and currents, the extent 
of evaporating surface will at least be doubled. 
’ But, independently of winds and atmospheric currents, it 
appears to me that those gentlemen who urge this objection 
have altogether overlooked the law of diffusion, which applies 
equally to vapour and all other gaseous bodies. In a still 
atmosphere, it is true that diffusion will operate more slowly 
than when aided by currents; but asthe vapour of water is 
lighter than air at the same temperature and pressure, in the 
proportion of 62 to 100, its diffusive power is very great, even 
in a perfectly still atmosphere; and it may be confidently 
concluded that the hygrometric condition of the atmosphere 
and the tension of its vapour will not be materially affected 
by the evaporation from the reservoir, which, notwithstanding 
its great extent, is very limited compared with the ocean. 
And, with our Australian atmosphere, which is so re- 
markable for its dryness, and with the rapid diffusion that will 
result therefrom, it would be very unwise to calculate upon 
a greatly diminished rate of evaporation in the reservoir. 
In his estimate of nine feet of evaporation for the reservoir, 
Dr. Davey has made ample allowance for the retarding effects 
of extent of surface. His observations have only extended over 
four months, and the evaporation for these months is as 
follows :— 
Inches. 
January, by approximate data noo nop nfo 21°710 
February, by daily observations ... O66 ike ... 23°630 
March o 3 a nae ae ie 15470 
April es S ats bos re --« 10:000 
‘ . 
With respect to these amounts, as Dr. Davey is absent 
from town, and as Mr. Brough Smyth thinks that he intended 
to make some corrections on account of the evaporating vessel 
used in January and February, he advises me to assume at 
present, only eighteen inches for December, January, and 
February; the amounts for March, and April, he thinks, do 
not require any corrections. 
Now, in computing nine feet as the evaporation from the 
