156 Failure of the Yan Yean Reservoir. 
_ might be rendered comparatively pure, and that its inferiority 
might, 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 he 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 2,537 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 
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 
is no difficulty in predicting the complete failure of the Yan 
Yean Waterworks for want of water; and it 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, would 
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 altogether 
unfit for use, and it has never been contemplated to draw 
it off at so low a level for the use of the City, as the main pipes 
are intended to be supplied through two openings in the 
Tower Well, at ten and seventeen feet from the bottom. 
