178 Probable Influence of Evajjoration 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 VL 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 less 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- 

 phere of this colony are considered ! . p r 



Although I cannot admit that the configuration ot 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 ^» ^» ^' ^' &c., denote the respective amounts of rainfall m 

 inches for 'each successive month; ^> f &c., the rates of evaporation 

 corresponding to each successive month; then true mean annual rate of evapo- 



E 31-1-E B-J-E E+E E+&C,; 



ration '-^ ^ \ \ ' \ , but according to Dempsey it would be most 



1 2 3 * lire 



erroneously represented by the exj^ression i ^ ^ ■ 



