180 Probable Influence of Evaporation on the 
surface of the reservoir at Yan Yean; whilst Mr. Blackburn 
considered three feet would be the maximum loss, and others 
have coincided with his opinion, citing instances of the small 
decrease in depth observable in certain ponds. 
Reference to the observed decrease in depth during a 
specified period in a pond, is of no use im facilitating the 
inquiry into the amount of evaporation from the surface of 
the reservoir, unless the rainfall during that period, the 
area of the pond, and the area of the ground draining into 
the pond, be given. 
In the following case these were attainable :—In the vicinity 
of my residence near the Yarra is a pond, to whose surface I 
occasionally have recourse for testing the adjustment of my 
spirit levels. The area of this pond is about one and a-half 
acres, its greatest depth ten feet, and the area of the ground 
draining into it about nine acres. 
The surface of the area of drainage is either trap rock, or 
a thin coat of soil derived from its disintegration, and resting 
on a substratum of very stiff impervious clay. The height 
of the surface of the pond above the nearest point of the 
Yarra was ascertained by me to be, on March Ist, 1855, 
fifty-one inches. On December Ist, 1854, one of my men 
defined, by a peg driven down flush with the surface of the 
pond, its level on that day, and on March Ist I found the 
water had sunk 16-2 inches. The proportion of the rainfall 
draining into the pond, from the small area comprised by its 
watershed, was assumed to be, for the three months of 
December, January, and February, 0°15 of the small amount 
of rain that fell on that area during that period. The rain- 
fall during the three months was known by reference to 
the rain guage kept in Melbourne. From these data I com- 
puted the evaporation for those three months to be 24°6 inches. 
Having been aware that evaporation from water in small 
vessels, constructed of materials that are good conductors of 
heat, proceeds with very much greater rapidity than in large 
vessels or ponds, I should not feel safe in placing any reliance 
on the results afforded by the small copper vessels employed 
by Dr. Davey, and which have led Dr. Wilkie and your 
Committee to assume nine feet as the annual amount of 
evaporation from the surface of the Yan Yean reservoir. 
During the month of February I placed in an exposed site 
in my garden a large butt, which I filled nearly to the brim 
with water, and protected the external wood-work of the 
butt from the influence of the sun and hot winds by woollen 
rugs. Having unfortunately lost the record of my observa- 
