Meteorology of Melbourne. 167 
case may be, almost instantly to a certain temperature, an 
infinitesimally thin film of water, and it is from this surface 
film that the evaporation takes place, however cold the 
water below may be. This certain temperature will be 
nearly midway between the actual temperature of the air 
and its dew point. It is true that the air itself may not only 
become cooled, but also partially saturated with moisture on 
passing over a very large surface of water, but in this case, 
the thermometer and hygrometer ought to indicate a notable 
difference on the windward and lee sides of the sheet of 
water. 2nd. The rate of evaporation, as far as it depends 
on temperature, will be affected rather by the range of the 
thermometer than by its mean. Inasmuch as the tension of 
vapour increases almost in a geometrical ratio to the increase 
of temperature, it is easy to see that if a surface of water 
were exposed for twelve hours to a temperature of 50° and 
for another twelve hours to 90° it would evaporate more 
rapidly than if exposed for the whole twenty-four hours to 
the mean of 70°. Hence I suppose that evaporation will be 
more rapid in Melbourne, as compared with London, than 
the mean temperature alone would indicate. 3rd. The 
evaporation from any given area of wet land, will be greater 
in proportion than from a similar area of a sheet of water, 
because it exposes a greater surface. 
By direct experiments upon water in different vessels and 
under different circumstances, exposed to the air during the 
months of January, February, and March, and comparing 
these with the hygrometric observations made also in De- 
cember, I am led to estimate the mean evaporation of the 
three summer months of December, January, and February, 
as 0°55 per day, or four feet one and half inches. I have no 
certain data from which to infer what it may be during the 
remaining nine months of the year; and would not, therefore, 
except in the absence of precise information, presume to offer 
an estimate as a matter of opinion founded upon general 
observation. I cannot suppose that the evaporation during 
the six months of spring and autumn, viz:—March, April, 
May, September, October, and November, is less than half 
of that which occurs in summer; and I assume that of the 
three winter months, viz:—June, July, and August, to be 
one-fifth. We shall thus have 
Summer se ae 4 1h 
Spring and Autumn ... 4 1% 
Winter oe aa 9 
9 O feet. 
