138 Failure of the Yan Yean Reservoir. 
per head, the difference between these estimates of eva- 
poration would supply 123,000; deduct this from 190,000, 
and we have 67,000 as Mr. Hodekinson’s estimate, accord- 
ing to this higher rate of evaporation. 
My own estimate, deducting the same evaporation, would 
supply 43,000. 
Regarding these estimates as mere approximations, it will 
thus be seen that the difference between Mr. Hodgkinson’s 
and my own is very small, being equal to only ten inches 
in the reservoir. 
Now, while the greatest importance is to be attached to 
Mr. Hodgkinson’s estimate of the watershed of the Plenty, 
as it is founded on a thorough knowledge of the river, and 
much practical experience both here and in England, I feel 
inclined to prefer Dr. Davy’s estimate of the evaporation, 
and from measurements which I recently made on a larger 
sheet of water than Mr. Hodgkinson’s pond, I found that in 
twenty-eight days the evaporation exactly equalled eleven 
inches, or 0°39 inches per day, which would give very nearly 
nine feet in twelve months. 
But, after all, no scientific result can be obtained from 
ponds and waterholes, unless they are water-tight, which it 
4s impossible to ascertain. 
-Thus, with Dr. Davy’s lowest estimate of evaporation, Mr. 
Hodgkinson’s estimate of the watershed would supply 
94,000, so long as there are no droughts; but Dr. Davy 
‘gives us very little hope of escaping from such calamitous 
visitations. 
Mr. Hodgkinson likewise expresses a very unfavourable 
opinion with respect to the quality of the water, when stored 
in the reservoir, and thinks that the Plenty is more likely to 
suffer deterioration from a resident population than the Yarra. 
It is interesting and highly important in this inquiry to 
know that Mr. Hodgkinson’s estimate of the watershed and 
my own are still further corroborated by the investigations 
of Dr. Davy. 
He is not sufficiently acquainted with the geological 
character of the Plenty basin to give any exact estimate of 
the watershed, but, from observations and calculations which 
he has made respecting the force of evaporation from the 
surface of the ground, in this colony, he regards one-eighth 
of the rain, over any large area of surface, as the most that 
can reach the rivers, under any conditions of slope or geo- 
logical formation, and he thinks that in many districts the 
