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 78^5 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. 



As 30 : 725 : : T— T'' : x 

 X being the answer in grains per square foot per minute, from 

 Avhich 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 hke 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 

 Avinds 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 



