178 Probable Influence of Evaporation on the 
been done), a certain proportion to exist between the rainfall 
and the available supply. 
Mr. Dickenson’s Tabulated Quantities, which indicated an 
annual amount of evaporation so greatly differing from the 
results of all other authorities, were, I believe, made for Mr. 
Parkes, the well known agricultural surveyor, and were in- 
serted without acknowledgement of their author’s name, in 
Dempsey’s Treatise on Drainage. But in Table VI. the 
very great error was committed of obtaining the mean-annual 
evaporation for the total rainfall of the year by adding up 
the mean monthly evaporations and dividing the sum by 
twelve; which result would only be correct if the rainfalls 
for each month were precisely equal.* Hence the annual 
evaporation adopted, on Dempsey’s authority, by your Com- 
mittee, was not only based on observations of an exception- 
able nature, as just shown, but was also greatly vitiated by 
a gross mathematical absurdity, which, whether due to 
Dickenson or Dempsey, was very inexcusable in a pro- 
fessedly scientific series of Tables. 
The inadequate rate of evaporation from the ground, as 
thus assumed by your Committee, is still further shown by 
its actual application. 
Thus, if thirty-six inches represent the rainfall at the 
Upper Plenty, the evaporation, according to the Committees 
assumed rate, 0.58, would be only 20.88 inches ;—that is, a 
quantity Jess than has been observed on the average of 
gathering grounds, whose discharges have been guaged in 
Scotland, and very much less than has been found to prevail 
in the South of England. Yet how obviously must the 
evaporation from the surface of the ground be greater here, 
even on the most favorable surfaces for lessening evaporation, 
when the high temperature, hot winds, and clear dry atmos- 
here of this colony are considered ! 
Although I cannot admit that the configuration of the 
surfaces of the Upper Plenty District is so unusually 
peculiar as to warrant the excessively low rate of evaporation 
assumed by your Committee, yet a portion of these surfaces 
* Let B B, BB &e., denote the respective amounts of rainfall in 
inches for each successive month; %; Es re E, &e., the rates of evaporation 
corresponding to each successive month; then true mean annual rate of evapo- 
ER--E R-+LE R-+E R-+-&e.; 
raion — but according to Dempsey it would be most 
R+R+R+R+ ke; =) 
1 2 3 4 
FEPEPE+ &. 
E 
erroneously represented by the expression 1 
