150 Failure of the- Yan Yean Reservoir. 
stream of the Plenty is not confined within its ordinary banks, 
but for eight months of the year is widely extended over 
their level surface; a condition of things which was never 
witnessed by any of the settlers or residents on the river with 
whom I have conversed. 
How much better would it have been for the Committee of 
a Philosophical Society to have disregarded theory altogether, 
and to have rigidly adhered to their own measurements, as 
well as those of Mr. Blackburn and Mr. Hodgkinson, and to 
have made these the basis of their calculations. 
Theory, based upon experiments conducted in this colony, 
would possess a scientific interest and value, but otherwise it 
is practically valueless. , 
Speculative philosophers, who embark in abstruse scientific 
investigations with incorrect or inapplicable data for their 
guide, will soon find themselves lost in a pathless ocean, with- 
out a compass and without a chart. 
Dr. Prout, one of the ablest writers on Meteorology, thus 
expresses himself with reference to the different estimates 
that have been made of the watershed of England:— 
These statements of the water that is condensed and evaporated in 
Great Britain, can only be viewed as rude approximations; and even 
admitting them to be correct, they could scarcely be applied with any 
advantage to an inquiry into the actual condensation and evaporation in 
other countries or climates, which in all instances must be determined 
by observation and experiment. 
Before taking leave of Mr. Dempsey, I wish to state my 
entire concurrence in the liberal views he expresses with 
reference to the amount of water required for a city of 
100,000 inhabitants. 
Although, he says, twenty gallons per head might be suffi- 
cient for domestic and manufacturing purposes, and for the 
extinction of fires, yet he advocates a constant service of 
thirty gallons per head, and is of opinion that extravagance 
in water should always be permitted; and for the purpose of 
cleansing and watering the streets and thoroughfares, for the 
supply of fountains, public gardens, and pleasure grounds, 
and other miscellaneous and occasional purposes, he considers 
that one-tenth of an inch per day, should be allowed for the 
whole area. Part of this is supplied from rain, so that for a 
city covering one thousand acres, he allows fifteen gallons per 
head additional, making in all forty-five gallons, and if he 
were consulted about the proper supply for Melbourne, he 
