9 
Yan Vean Reservoir. 183 
burn, was sufficiently extensive to maintain the permanent 
discharge of the streams in question in the following manner: 
the rain percolates through the very permeable soil on the 
top of the range, and lodges in extensive interior cavities 
and fissures in the granite; and from these subterranean 
reservoirs the water is gradually exuded at lower levels, 
through the spongy masses of decomposed vegetation which 
fill up the external interstices of the granite, and constitute 
the soil from which spring up, in such rank luxuriance, the 
gigantic mountain Hucalyptus, and dense undergrowth of 
tree-ferns and creepers that choke up the ravines of Mount 
Disappointment. 
This opinion of the probable cause of the very permanent 
and regular flow during the summer months of the mountain 
streams, was briefly enunciated in one of my reports in 1852, 
and has been since corroborated by Messrs. Acheson and 
Christie. - 
During the four hottest months of the year, the enormou 
quantity of water lost from the effects of evaporation and 
absorption in the river swamps between Mount Disappoint- 
ment and Whittlesea (equivalent to a discharge per minute 
of about 2500 gallons) would render it necessary that, for 
these months, a quantity of water, equivalent to a discharge 
of at least 3000 gallons per minute, should be deducted from 
the total yield of the mountain streams, in order to maintain 
a sufficiency of water in the Lower Plenty for the adequate 
supply of the setttlers and stock thereon. During the other 
eight months of the year, a quantity equivalent to a dis- 
charge of 2000 gallons per minute would suffice to effect this. 
The guagings of the Plenty made from time to time by 
Mr. Blackburn, Mr. Jackson, your Committee, and myself, 
_are from their want of connexion in a regular series, of little 
value in affording data for determining the total annual dis- 
charge of the Plenty. The information afforded by the 
settlers on that river relative to its winter discharge, floods, 
&c., is also exceedingly contradictory, I therefore considered 
it safer, as explained in some of my foregoing remarks, to 
endeavour to arrive at some estimate of the supply on 
general principles. 
I submit with much diffidence the following very rough 
approximation to the probable amount of population that the 
Yan Yean scheme would prove adequate to furnish with a 
sufficient supply of water. Since my lecture of this paper I 
have however arrived at the conclusion, that sufficient ex- 
periments on dew have not been made in this colony to 
