196 



Report of the Commissioners on the 



obtain the sectional area of the winter discharge, equal to 

 forty-one square feet, and having already the fall in two miles 

 equal to 8*1 inches, Ave can obtain by Eytewein's formula as 

 above quoted, the velocity equal to 1*15 feet per second, 

 which multiplied by the sectional area, forty-one square feet, 

 gives 47*15 cubic feet per second as the winter discharge. 



Hence having the sunnuer and winter discharges per second, 

 we obtain the mean discharge by taking the half of their sum, 

 which is 29*51 cubic feet per second, or 34,467,680 cubic 

 yards per annum, and then deducting the loss from the eastern 

 swamp already obtained, equal to 2,224,157 cubic yards, we 

 have 32,243,523 cubic yards as the effective mean discharge 

 of the eastern arm per annum. 



As the western arm has a larger drainage area than the 

 eastern arm, and the loss in its swamp not so much, and as 

 all its conditions relative to impcrvlousness, slope, i^ainfall, 

 &c., are precisely similar, it is evident that It must 

 have at least as great a mean discharge, if not 

 greater ; hence the combined mean discharge of both arms 

 will be at least double that of the eastern, or 64,487,046 

 cubic yards, equal to the effective mean discharge of the 

 Plenty above the reservoir, according to this calculation. 



But the result previously arrived at by the application of 

 the English per centage of evaporation of 57*6 of the rainfall, 

 was 63,663,297 cubic yards, thus leaving only 823,749 cubic 

 yards difference between the two calculations. 



As the calculation of effective discharge for the Plenty, 

 previously made upon the assumed correctness of 57*6 per 

 centage of loss from evaporation is so amply confirmed by this 

 last calculation, Ave adopt it as correct. 



The Avhole effective discharge of the Plenty above reser- 

 voir, or 63,663,297 cubic yards, is therefore the amount 

 available for collection. 



But as it cannot be supposed that the Avhole Avaters of the 

 Plenty can be diverted into the reservoir, and thus be ab- 

 stracted from the settlers along its banks, we assume that 

 at least half of its whole amount Avill be retained for their 

 use, leaving the other half, or 31,831,648 cubic yards, for the 

 supply of the reservoir. ^ 



As therefore this amount has to be conveyed by the aque- 

 duct before mentioned, leading from the Plenty to the 

 reservoir, it is necessary to ascertain the aqueduct's capacity 

 of discharge, especially as some of this supply avIU come in the 

 form of floods, and also the amount likely to be carried down 

 by the Plenty during the greatest ordinary floods. 



