For our Water Supply. 259 



hlUs and on the water of the reservoir. The latter will 

 pres^^rve a very uniform temperature while the former wiU be 

 subiect to great diurnal alternations of heat and cold. 



Thus the vapour that is formed on the surface will at all 

 times be quick y removed, and replaced by currents of drier 

 Sr And it is important to notice, that the amount of water 

 ev;porated, other kings being equal, is e-a«%, ^ f ^?^^^^^^^^^ 

 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 sur&ce will at least be doubled. _ 



But independently of winds and atmospheric current , it 

 aDDeai4 to me that those gentlemen who urge this objection 

 Sfe itogerer overlooke^d the law of diffusion which applies 

 equluy to vapour and all other gaseous bodies. In a st 11 

 atmosphere, it is true that diffusion will operate more slowly 

 ^C'Saided by currents; but as the vapour of water is 

 fXeTthan air at the same temi.erature and pressure in the 

 proportion of 62 to 100, its diffusive power is very great even 

 Fn a perfectly still atmosphere; and it may be confidently 



ond&ed tha\ the hygromV^r^t" ttiaUrSS 

 nnd the tension of its vapour will not be materially aftected 

 wie etlporation from the reservoir, which notwithstanding 

 its great extent, is very limited compared with the ocean. 



And? with our Australian atmosphere, which is so re- 

 matkable for its dryness, and with the rapid diffusion that will 

 reruhtherefrom, it would be very unwise to calculate upon 

 Tgreatly dimini bed rate of evaporation in the reservoir. 



fnhLstimate of nine feet of evaporation th^ ^e^™ 

 Dr Davey has made ample allowance for the retarding effects 

 o^- ;xtent If surface. His observations have only extended over 

 Lr Inth,, and the evaporation for these months is as 



follows : — 



Inches, 

 21*710 



January, by approximate data oQ.fi^a 



February, by daily observations "15.470 



April » » 



With respect to these amounts, as Dr. Davey is absent 

 Wtown, 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 Aprd, he thinks, do 

 not require any corrections. 



Now, in computing nine feet as the evaporation from the 



