182 Probable Influence of Evaporation on the 
In investigating the probable supply of water derivable 
from the mountains, the possibility of any of the copious 
streams in the ravines of Mount Disappointment being 
derived from sources beyond the apparent limits of the water- 
shed of the Plenty must next be considered. 
When I guaged these streams and examined their sources 
in 1852, the fine body of water that I saw gushing out with 
great velocity from a fissure in the granite, and forming the 
source of the Saw Pit Creek, on the western branch of the 
Plenty, led me to investigate this point; as I was aware, 
that in the island of Hong Kong, copious streams gush out 
of similar fissures in the granite formation there, and dis- 
charge a much greater quantity of water than the total rain- 
fall of the island. 
But on examining the sources of the eastern branch of the 
Plenty, I saw that the high table land forming the dividing 
range between the Plenty and the tributaries of the Goul- 
and Water Supply has published a vindication of the Yan Yean Scheme of 
Water Supply. In the course of his observations, he states that I have com- 
mitted a palpable error, in ignoring, in the above analogical deduction, the dif- 
ference in the length of days here and in England, for the months of corre- 
sponding mean temperature; as he is of opinion that the evaporation, on 
account of longer duration of daylight, is greater in England, during a month 
of a certain mean temperature, than would be found to occur here, durmg a 
month possessing the same mean temperature. But Mr. Griffiths, in the caleu- 
lation by which he illustrates his views on this subject, unfortunately falls into 
the error of supposing that evaporation invariably ceases at nightfall. I, there- 
fore, beg to remark, that in this Colony, where dry windy nights are of such 
frequent occurrence, the nocturnal evaporation is often very great; so muchso, 
in fact, as to render the correction, to which Mr. Griffiths attaches so much im- 
portance, of a very trivial and indefinite nature; not, however, ignored by me, 
as he supposes, but found to be more than counterbalanced by the greater dry- 
ness of the atmosphere in this Colony. Mr. Griffiths further states, that I am 
one who would prefer the Yarra, with “its tanneries, fellmongeries, and the 
thousand other daily increasing sources of pollution,” to the Plenty. The 
nearest point to Melbourne, at which I ever proposed to derive any supply, was 
two miles higher up the river than the point where the analytical chemist of 
the Commission found the Yarra water to be pwrer than the Plenty water; and 
as regards the nuisances on the banks of the Lower Yarra, below Dight’s Mills, 
I recollect having once stated the necessity, in my humble opinion, (in a casual 
conversation with Mr. Griffiths, two years ago,) of some legislative enactment 
to check the contamination of the water. 
I have also lately received from T. E. Rawlinson, Esq., Engineer to the Fitz- 
roy Ward Improvements, a letter kindly conveying to me the result of his ob- 
servations on the Plenty. Mr. Rawlinson’s high professional standing and long 
connexion with Yan Yean impart great weight to his opinions on the Water 
Supply. He considers that the minimum flow of the eastern arm of the Plenty 
is 4000 gallons per minute, and is convinced, from long personal observation of 
the Yan Yean Swamp, that the evaporation from the surface of the reservoir 
will be very much less than has been estimated by some of the members of this 
Society. 
