168 Meteorology of Melbourne. 
It may be greater; but is not likely to be much less than 
is here estimated; and it may be a question whether this 
present season has or has not been below the average in point 
of dryness. 
The evaporation on the eighteenth of February was more 
than one inch; the dew point at noon being 37° the mean 
temperature being 78~, the range 42°, and the highest 99°. 
I may here state the formula which is given in scientific 
works for calculating the rate of evaporation from the tem- 
perature of the air and its dryness. Water at 212° is 
ascertained by careful experiments to evaporate at the rate 
of 0°725 grain per square foot per minute. By referring to 
Dalton or Ure’s tables, the tension of watery vapour at any 
given temperature can be found. Let T be the tension at 
the temperature of the air, and T” the tension at the dew-point, 
than by a simple rule of three sum, 
HS) SOU ioi Zo le: Be ees pee ase 
zx being the answer in grains per square foot per minute, from 
which the quantity in inches can readily be calculated. 
This rule however affords us no guide for estimating the 
ratio of increase from the action of wind in proportion to its 
velocity. 
The experiments which I have made on the rate of evapora- 
tion can easily be repeated by other persons, and I have no 
doubt that the correctness of the results, considered as approxi- 
mative will eventually be fully confirmed. But of course it 
is not intended to represent that the level of every natural 
water hole will sink at the rate of half an inch per day in the 
summer months, because the contrary is well known. The 
adjacent soil may be porous and become saturated with water 
which is altogether protected from evaporation. This water 
would of course return into the pool, and supply the place of 
that which had been dissipated by evaporation from the ex- 
posed surface. It is probably only in cases where the sides 
and bottom of the pool consists of impervious clay, that the 
level of the water would sink in anything like the proportion 
I have mentioned. 
I have now to speak of the winds of Melbourne; and it 
will be seen on reference to the Journal, that the prevalent 
winds are those from the south, or within a few degrees of 
south. ‘The next in frequency are those from the north and 
north-west. Winds from all other points of the compass 
occasionally blow, but are not of long duration. It will, I 
think, presently appear that this is just what might have been 
