Meteorology of Melbourne. 



167 



case may be, almost instantly to a certain temperatiircj an 

 infinltesimally 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 pi'ecise 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 ... ... 4 



Spring and Autumn ... 4 1^ 



Winter 9 



d 0 feet. 



