194 



Report of the Commissioners on the 



as in still water^ to obtain the loss from which, the annual 

 depth evaporated should be multiplied into the superficial ex- 

 tent. We have been kindly favoured by Dr. Davey with the 

 results of his experiments on evaporation in still water, for 

 the three summer months, which give '55 inch per day, equal 

 to 42*5 inches for the three months. He has further furnished 

 us with a proportionate evaporation for the rest of the year, 

 as follows : — 



Evaporation for the six autumnal months, equal | ^^.q jj^^^jj^g 

 to Ibur-tliirds of the three summer months J 



Evaporation for the three winter months, equal j g.r, jj^(>|i(,g 

 to one-sixth of the three summer months, equal) 



And adding evaporation for the three summer \ inches 

 months ... ... ... ... ... J 



We have 123*7 inches 



or 10'3 feet equal depth of water evaporated annually. 



As this amount was ' partly derived from inference and 

 therefore not absolutely proved, however certain; we re- 

 quested Dr. Davey to furnish us with such an amount as in 

 his opinion did not admit of a doubt, and were accordingly 

 informed that we might safely adopt nine feet. 



Hence the amount evaporated in the western swamp can 

 be obtained by multiplying the superficial extent, equal to 

 787,000 square yards by the depth of evaporation, or nine feet, 

 but as 17*856 inches has already been allow^ed for surface 

 evaporation over the whole basin, this amount must be de- 

 ducted from the nine feet, thus leaving 7*512 to be multi- 

 plied into the area of the swamp, thus giving 1,970,648 cubic 

 yards evaporated per annum on the w^estern swamp. 



The loss by evaporation in the sAvamp on the eastern arm, 

 must be determined in a different manner, inasmuch as the 

 water is not spread over its surface, but rather absoi^bed by it- 

 This loss is accurately represented by the difference between 

 the measured discharge above and below the swamp on the 

 same day; the discharge of the eastern arm, near the head of 

 the swamp, was on January 24th, 37,990 cubic yards per day, 

 the sectional area being fourteen square feet, and mean 

 velocity 10.18 inches per second; the discharge below the 

 swamp, at junction of the Plenty with aqueduct, was 21,653 

 cubic yards per day, the sectional area being 13*2 square feet, 

 and mean velocity 6*15 inches per second; the discharge 



