Failure of the Van Yean Reservoir. 



121 



gallons per minute^ at the time when the committee measured 

 the eastern arm. The whole discharge above the swamps was 

 therefore 5,790 gallons per minute, and below the swamps 

 2,537 gallons. 



In this manner we ascertain that the loss by evaporation 

 in the swamps amounted to 3,253 gallons per minute, which 

 is at the rate of 4 feet 4 inches in the reservoir ; and as the 

 area of the swamps is one half less than the area of the 

 reservoir, 8 feet 8 inches will represent the rate of evapora- 

 tion in the swamps during the summer months. 



It is thus easy to understand why Mr. Blackburn and Mr. 

 Hodgkinson so strongly urged the necessity of making arti^ 

 ficial watercourses, in order to withdraw the two branches 

 of the river from the influence of the swamps. 



It does not appear that the Commissioners of Sewerage 

 and Water Supply have any present intention of doing this, 

 and I am not sufficiently acquainted with the levels and 

 depths of the swamps to give any opinion as to the best mode, 

 but it is quite clear that some steps must be taken to save 

 the immense loss that is at present occasioned by them. And 

 in estimating the quantity of water that will be available for 

 the reservoir I shall assume that some effectual means will be 

 adopted to accomplish this very desirable object, and that the 

 whole of the 5,000 gallons may be transferred into the reser- 

 voir without loss. 



The measurements would not be complete without men- 

 tioning, that Dr. Mackenna and myself, on our visit to Yan 

 Yean, measured the Plenty where it passes under the bridge, 

 about three miles below the reservoir, and obtained a discharge 

 of 960 gallons per minute, which would give a depth in the 

 reservoir of 1 foot 3 inches. The measurement of the 

 committee at the same place, on their late visit, gave 475 

 gallons per minute, which is equal to 8 inches in 12 months. 



These small results compared with the measurement above 

 Yan Yean arise from the quantity of water that is abstracted 

 by a cut for the purpose of puddling the embankment. 



I have thus assumed Mr. Blackburn's highest estimate of 

 5,000 gallons per minute as the average of the whole discharge 

 of the tributaries of the Plenty. In the drought of the 

 summer of 1851, however, Mr. Blackburn found this amount 

 reduced to 4,040 gallons ; and therefore it is to be presumed 

 that 5,000 gallons arc only to be depended on in ordinary seasons. 



The important question now arises, what proportion of the 

 5^000 gallons can be abstracted from the river without inflict- 



