* 
Failure of the Yan Yean Reservoir. 143 
velocity, as, according to the usual formule it would be very 
small or difficult to compute. 
It is thus very important to notice that they assume the six 
winter months discharge at eight times the amount of the six 
summer months, while the river, when level with its banks, 
can only contain three times the amount. 
They also assume that the river is really level with its 
banks, although they have never seen it in the winter months; 
but they are quite satisfied on this point from the evidence 
of a resident farmer. 
This, it will be observed, is not very scientific or reliable 
evidence to check their calculations, or to justify the expendi- 
ture of £650,000 of public money, and accordingly the 
farmer’s son, an intelligent lad of nineteen, in his father’s 
absence, told Mr. Wekey and Mr. George Wilkie, that the 
river was only half full during the winter months, except in 
floods, which, with the same velocity, exactly accords with 
my own estimate of the winter discharge. 
The principle upon which Mr. Dempsey’s tables are based 
is precisely the same as that adopted by Dr. Dalton and Dr. 
Thomson, for ascertaining the watershed. He calculated 
that 57-6 per cent. of the rain is evaporated, and 42°4 remains 
to supply springs and rivers. 
Whether Mr. Dempsey’s tables are the result of his own 
experiments, or those of others, he does not say, but he 
- clearly states that the evaporation mainly depends upon the 
temperature, heat promoting it, cold retarding it, and 
therefore I cannot conceive that he would commit such 
an egregious blunder as to apply his tables, without correc- 
tion for the great difference of temperature, to determine 
the proportion of the rain evaporated in this colony. 
In calculating the evaporation from the surface of water 
here from English tables, I assumed for each month the - 
evaporation of a corresponding month in England, with the 
same or a less mean temperature, and I thus obtained eight 
- feet two inches, which is sufficiently near to Dr. Davy’s es- 
timate of nine feet to show the correctness of the method. 
I therefore hold it to be scientifically correct to adopt the 
same method with Mr. Dempsey’s tables in order to deter- 
mine the proportion of rain that is evaporated from the 
ground. 
I have drawn up three tables for the purpose of illustrat- 
ing the contrast between the proportion of rain evaporated 
here and in England, allowing the same proportion to the 
