210 



Influence of the Physical Character 



evaporation ; but it happens that north winds are the excep- 

 tional and not the prevailing winds. When we have south 

 winds, or winds west or east of south, the dew point is com- 

 monly much higher ; and though the daily mean temperature 

 may yet be very high, the conditions for rapid evaporation 

 do not exist. Unquestionably, as compared with many other 

 climates, evaporation is very rapid in this country ; but I am 

 justified In stating, that the amount of evaporation during the 

 seven months commencing in April bears no proportion to the 

 rapid evaporation during the other five months; and this is 

 owing to the low prevailing temperature, and the moist con- 

 dition of the atmosphere. During the rainy season the daily 

 mean temperature is very low, the air Is laden with moisture, 

 and, excepting a few chance days, when a dry north wind 

 prevails, evaporation progresses very slowly. With a tem- 

 perature of 57"^ the dew-point is not unfrequently 48°or 

 49*, and even higher; and should the wind come from the 

 west or south-west it sometimes happens that we ha-s^e an 

 atmosphere almost completely saturated. With the tem^Dera- 

 ture and dew point as above stated, the daily evaporation is 

 seldom more than 8-100 of an inch per diem, under a stiff 

 breeze. In the absence of figures, derived from daily obser- 

 vations, extending over a period of twelve months, I cannot 

 even venture to hint at the amount of evaporation in 

 Melbourne ; and how much more should we hesitate to 

 express any opinion as to the amount of evaporation in far 

 distant places, under often changing conditions ? It is 

 impossible, I repeat, to calculate the amount of evaporation 

 from tables of temperatures during the several months of the 

 year, unless we are also informed as to the daily hygromcLric 

 condition of the atmosphere, and the force of the wind ; and 

 even with that information, the result would be the mere 

 expression of an opinion, valueless as a matter of fact. I am 

 the more anxious to enforce these facts, as, unless they are 

 borne in mind, much valuable time may be mispent in calcula- 

 tions and discussions, not merely useless but dangerous to the 

 true interests of science. 



Effects of Different Geological Formations. — I will now 



wind had changed to south, and the thermometer fell to 65o, while the dew- 

 point was 460. On January 29th of this year, at a.m. the thermometer 

 stood at 1090, and the dew-point at 4lo the wind being from the north; and 

 in the afternoon of that day, at 4 o'clock, the thermometer indicated a tempe- 

 rature of 780 the dew-point being 58o; the wind, meanwhile, having changed 

 to south. I have drawn these facts from the Meteorological Journal of Pr. 

 Davey, the Assay Master. 



