176 Prohahle Influence of Evaporation on the 



tlons for the basin of the Clyde, assumed that in the Upper i 

 Plenty District the evaporation would leave only one-ninth ! 

 of the total rainfall available for the Yan Yean reservoir \ 

 whilst your Committee, Messrs. Acheson and Christie, 

 adopted, on the authority of Tables VL, in Dempsey's treatise 

 on Drainage (which Tables they considered corroborated by 

 their own observations), 0'42 as the amount of the total 

 rainfall available, — a quantity nearly four times greater than 

 that assumed by Dr. Wilkie. 



Before discussing the very numerous measurements and 

 experiments made in various localities and climates, and 

 which, in the existing want of a regular and connected 

 series of guagings of the Plenty, afford data for arriving 

 analogically at an approximate determination of the propor- 

 tionate amount of rainfall in the Upper Plenty District, 

 I wish first to remark that in a tract of country of specified 

 area comprised by a watershed, the propoi-tlon between the 

 total amount of the annual rain falling thereon, and that 

 portion of the rainfall that is carried away from it by the main 

 channel of drainage, depends not only upon the climate, 

 geological structure, and vegetation of such a tract, but also 

 (although in a much less degree of course) upon the 

 greater or less extent of the area ; as, ccBteris paribus, the 

 greater the area the longer would be the aggregate distance 

 that the rain water would have to traverse in order to reach 

 the point of outfall of the tract, and consequently the longer 

 time would such rain water be liable to evaporation before 

 final departure from the tract in question at the lowest level. 

 Hence, if a large tract and a small tract present the same 

 physical configuration as regards surface, with the same 

 climate and rainfall, the rate of evaporation for the large tract 

 would be slightly in excess of that for the small tract. 



The most extensive and minutely accurate observations 

 ever made in Great Britain for the determination of the 

 evaporation from surfaces of land and water under various 

 conditions, were those taken at Ferrybridge, in Yorkshire, 

 by Mr. Charnock, Vice President of the Meteorological 

 Society of London. These observations were extended over 

 the five years terminating 1846; and tended to corroborate 

 the general accuracy of Dalton's observations at Manchester 

 for the years 1795, 96, 97. Howard's Table, referred to in 

 recent computations by Mr. Eanger, the well known 

 Engineering Inspector under the Board of Health, gave for 

 England generally a rate of evaporation rather greater than 

 that observed locally bv Dalton and Cliarnock. In Scotland 



