For our Water Supply. 247 
‘and frosty nights, and I was anxious that the reservoir should 
get every possible advantage. 
I must say, therefore, that I was greatly surprised to find 
that Mr. Hodgkinson relied on ten inches of dew for the re- 
servolr. 
I have looked in vain for any authority to bear out this 
extraordinary opinion respecting dew, and I feel assured that 
Mr. Hodgkinson could not have consulted the best authorities 
on the subject. 
In England, from four to five inches of dew are supposed 
to be condensed on the surface of the ground, and its produc- 
tion is easily explained, and well understood. 
But Mr. Hodgkinson obtains ten inches for the reservoir, 
by assuming that this amount of dew is condensed on the 
surface of water in this colony. 
This is a very important assumption, as ten inches in the 
reservoir will supply 22,727, at forty gallons, per head, per 
day, and 100,000 at nine gallons, which some allege is really 
all that is required for ordinary consumption. At this rate 
the dew condensed on the surface of the reservoir would 
suffice to supply Melbourne, with all its suburban towns and 
villages. 
Here, again, it is to be regretted that Mr. Hodgkinson 
does not say upon whose authority he assumes this enormous 
amount of dew. He certainly states that Mr. Thom, the 
eminent practical engineer of the Paisley Water Works, and the 
energetic promoter of the gravitation schemes of water supply 
in Scotland, considers that the evaporation in large reservoirs 
is counterbalanced by the condensation of dew, but this is 
only to be regarded as his individual opinion, and is certainly 
not based on accurate observation, or experiment. 
It is scarcely possible that Mr. Thom could have directed 
much attention to the subject of dew at the time that he 
uttered this opinion, and Mr. Hodgkinson himself shows that 
Mr. Thom’s statement is altogether inconsistant with the 
production of salt by the evaporation of sea water, which has 
been carried on for ages. 
Mr. Thom’s opinions, therefore, on scientific subjects are 
not very remarkable for their minute accuracy. __ 
Mr. Hodgkinson quotes his estimate of the available rain 
on which he relies for the Paisley Water Works, which is 
thirty-nine inches out of an annual rainfall of fifty-four inches. 
I can readily understand how low swampy ground, that is 
thoroughly intersected with catch-water drains, should yield 
a much larger amount of water than the whole rainfall, because 
