176 Probable Influence of Evaporation on the 
tions for the basin of the Clyde, assumed that in the Upper 
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 VI., 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 proportion 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, ceteris 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, ifa 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. Ranger, the well known 
Engineering Inspector under the Board of Health, gave for 
England generally a rate of evaporation rather greater than 
that observed locally by Dalton and Charnock. In Scotland 
