180 Probable Irifluence 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 in 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 1st, 1855, 

 fifty-one inches. On December 1st, 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 1st I found the 

 water had sunk 16*2 inches. The proportion of the rainfall 

 drainini^ 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 m small 

 vessels, constructed of materials that are good conductors of 

 heat, proceeds with very much greater rapidity than m large 

 vessels or ponds, I should not feel safe in placmg any rehance 

 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 ot 

 evaporation from the surface of the Yan Yean reservon^ ^ 



Durincv the month of February I placed in an exposed site 

 in my Lwden a large butt, which I filled nearly to the brim 

 Y/ith water, and protected the external wood-work ot the 

 butt from the influence of the sun and hot winds by woollen 

 HaviDo- unfortunately lost the record of my observa- 



rugs. 



