Failure of the Yan Yean Reservoir, 



133 



Total 11 ft. 6i In. 



Deduct evaporation - - - - 9 0 



Balance - - . - - 3 6-|- 



Kequired for present wants - - - 3 8 



Balance deficiency - - - - 1 Ij 



This is indeed an unfortunate result of the gigantic opera- 

 tions and large expenditure already incurred at Yan Yean. 

 And it seems not a little extraordinary that such unlimited 

 confidence should have been placed in the abundant supply 

 of water ; and it is no less extraordinary that Mr. Blackburn, 

 with a knovv'ledge of the immense loss sustained from evapor- 

 ation in the marshes, should have urged the necessity of 

 rescuing the river from this slough of despond only to plunge 

 it into an abyss of greater magnitude, where it would be 

 scattered, contaiuinated and rapidly dissipated. 



But it will probably be said, that as I have only shown a 

 deficiency of one foot and one and a half inches, which is 

 rather less than one third of the amount required, I may be 

 in error in my estimate, and perhaps the winter rains may 

 furnish the amount. Now, I shall admit that in some seasons 

 even double this amount may possibly be added to the reser- 

 voir from this source, but I do not think that this need be 

 regarded as a subject of congratulation. With a stream of 

 pure water, encircling this city like a horse shoe, the inhabi- 

 tants will not willingly pay £650,000 to be subjected to the 

 chances of the seasons, to be dependent on the casualities of 

 rain for the first necessary of existence ; and, if we must 

 speculate on chances, how oftSn do we have summers remark- 

 able for droughts, and the prevalence of hot winds ? And, 

 what we gain by casualties of rain, w^e shall certainly lose by 

 the casualties of evaporation. An inch a day, for hot winds, 

 which as we have seen is a small allowance for a temperature 

 of 96^, would make short work with two feet in the reser- 

 voir, and this is the greatest addition which could reasonably 

 be expected from the ordinary rainfall of the winter months. 



It will be readily admitted that in estimating the available 

 discharge of a river for the supply of a large city, it is 

 necessary to take a low average, instead of the mean for a 

 number of years, because it is essential to know what amount 

 can really be depended upon for each year, as the water supply 

 of a city should be placed beyond the reaeh of casualties. 



The late Mr, Blackburn was fully impressed with the 

 importance of this principle, hence he assumed his measure- 



