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, while 
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,— 
Ist. 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 experiment is to be 
depended upon; and eyen direct experiment upon a small 
scale is liable to a slight degree of fallacy. 
With respect to the temperature there are two observations 
to be made:—Ist. 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 
