156 



Failure of the Yan Yean Reservoir, 



might be rendered comparatively pure^ and that its inferiority 

 mighty to a certain extent, be compensated for by its super- 

 abundance. 



It was only lately, in consequence of my visit to Yan Yean 

 in company with Dr. Mackenna, that I was enabled to obtain 

 the information necessary to arrive at more correct conclu- 

 sions, and our astonishment and surprise can be better 

 imagined than described, when we compared the diminutive 

 stream of the Plenty with the wide extent of the reservoir 

 intended for its reception ; and, indeed, while contemplating 

 from one of the heights the grandeur and singular beauty of 

 this vast plain, the conviction forced itself upon our minds 

 that the whole volume of the river would not suffice during 

 the heat of summer, to wet the surface ; and this my subse- 

 quent investigations have proved to be literally true.^ 



The evaporation from the surface of the reservoir is equal 

 to one foot per month for the three summer months. Now, 

 the river, at the entrance of the aqueduct, gave 25537 gallons 

 per minute in January, or three feet four inches in twelve 

 months; and this is above the average for the summer 

 months, as Mr. Blackburn, in the dry summer of 1851, found 

 it reduced in February to 865 gallons per minute, or to 

 nearly one-third. Let us, however, take three feet to repre- 

 sent the discharge ; this would give three inches for each of 

 the summer months. Thus, with twelve inches of evaporation, 

 it would take four rivers equal in size to the Plenty, to keep 

 the reservoir wet. And if we take Mr. Blackburn's lowest i 

 measurement of 865 gallons per minute, it would require; 

 exactly eleven such rivers to give even an appearance of 

 moisture to the surface of the reservoir. 



According to the data which I have submitted to you, there j 

 is no difficulty in predicting the complete failure of the Yanj 

 Yean Waterworks for waut of water ; audit is important to< 

 notice here, that the amount of water in the reservoir, after ^ 

 deducting the evaporation, is far short of the amount that] 

 seems confidently to have been calculated upon. Two feet] 

 six and half inches, as measured for the whole surface, woul* 

 give six feet of depth at the lower end, and this is the very\| 

 lowest point at which it would be practicable to draw off the^ 

 water. Below this point I consider that it would be altogetheii 

 unfit for use, and it has never been contemplated to draw? 

 it ofFat so low a level for the use of the City, as the main pipes 

 are intended to be supplied through two openings m the 

 Tower Well, at ten and seventeen feet from the bottom. 



