166 Meteorology of Melbourne, 



made these observations on numerous occasions at all hours, 

 from six in the morning till late in the evening, and have not 

 found them to differ materially from those made daily about 

 noon, the dew point having been nearly the same during the 

 continuance of brisk wind in the same direction. Deducting 

 the dew point from the mean temperature, gives the dryness 

 of December 18^; that of January, 20-5^; and that of 

 February, 18-7^. The mean of the three is 19-0^, wlnle 

 that of England is 8*0^. Thus the dryness of summer in 

 Melbourne is to that of London, as more than 2^ to 1 ; as 

 far as the present season only is concerned. 



The total rain fallen during the four summer months, — 

 December, January, February, and March, was 4*67 inches. 

 I may here be permitted to suggest a caution against any 

 deductions from the annual rain-fall of Melbourne being 

 applied to places even at twenty or thirty miles distance. It 

 is well known that in England the annual rain-fall in some 

 places is more than double of that which occurs in London. 



During the four months in question there has been no 

 appreciable deposit of dew in Melbourne. ^ While on this 

 subject, I may refer to the fallacy of supposing that dew is 

 ordinarily deposited on the surface of water as it is upon 

 that of the land. To attract dew, the surface must be cooled 

 down to the dew point. This on the land is effected by 

 radiation, but not so with the water surface. The cooling 

 effect of evaporation will never reduce the temperature more 

 than half way down to the dew point. A careful consider- 

 ation of circumstances will convince us, that a deposition of 

 dew upon the surface of deep unfrozen water must be a very 

 rare, and almost inconceivable event in this colony. 



The rate at which evaporation will take place from the sur- 

 face of water depends essentially upon three circumstances, — 



1st. The actual temperature. 



2nd. The degree of dryness of the air. 



3rd. The velocity of the wind. / 



It will, however, be difficult from these data alone, to 

 calculate 'otherwise than approximatively the true_ rate of 

 evaporation. Nothing short of direct expermient is to be 

 depended upon ; and even direct expermient upon a small 

 scale is liable to a slight degree of fallacy. 



With respect to the temperature there are two observations 

 to be made:— 1st. It is the temperature of the air rather 

 than that of the water which affects the result. The air, on 

 coming in contact with the water, raises or depresses as the 



