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 have an 
atmosphere almost completely saturated. With the tempera- 
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 hygromeiric 
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 65°, while the dew- 
point was 46°. On January 29th of this year, at 113 a.m. the thermometer 
stood at 109°, and the dew-point at 41°, the wind being from the north; and 
in the afternoon of that day, at 4 o’clock, the thermometer indicated a tempe- 
rature of 78°, the dew-point bemg 58°; the wind, meanwhile, having changed. 
to south. I have drawn these facts from the Meteorological Journal of Dr. 
Davey, the Assay Master. 
