Failure of the Yan Yean Reservoir. 139 
proportion of rain that reaches the rivers is much less than 
one-eighth. 
Thus, Mr. Hodgkinson, Dr. Davy, and myself, seem to 
have all arrived, by different and independent modes of inves- 
tigation, at very nearly the same result. For all practical 
purposes, our different estimates will produce the same result. 
I attach very little importance to the determination, on 
theoretical principles, of the watershed of the Plenty. 
The only certain way, as stated above, of finding the 
amount, is to measure the streams at least once a month, in 
order to get the mean discharge for the year. But, if the 
aqueduct could be finished within the next two months, we 
should then have the very best means of practically testing 
how much water can be obtained from the winter rains. 
I have availed myself of all the measurements already 
made, and without deducting the immense loss from evapor- 
ation in the swamps, have allowed an increase of two-thirds 
for the greater watershed of the winter months. 
The members of the Committee have disregarded all these 
measurements, because they were taken in the summer 
months, and have calculated the amount of watershed, in 
accordance with the evaporation tables of Mr. Dempsey, 
which profess to show the evaporation due to the mean tem- 
perature of the different months in England, and I should 
have thought that it required no great amount of scientific 
knowledge to see that if these tables give a correct result for 
the mean temperature of England, they are totally inapph- 
cable to the mean temperature of this colony, and would give 
a very incorrect result. 
The Committee, in their report, admit that the evaporation, 
from the surface of water is nine feet, which is nearly double 
the evaporation in England. It seems, therefore, a singular 
oversight on their part, not to see that the force of evaporation 
from the surface of the ground here must bear at least the 
same increased proportion, and, in point of fact, the evapora- 
tion is far greater than double. 
That portion of the drainage area of the Plenty which is of 
the clay slate formation, and which may be estimated at fifteen 
square miles, or one-fourth of the whole, is so much more 
destitute of vegetation than the cultivated soil in England, 
that the surface of the ground becomes intensely heated 
under the influence of the solar rays and evaporation is 
exceedingly rapid. 
Thus, the reasoning upon which the Committee rely, to, 
