172 Meteorology of Melbourne. 
clouds. This character of dryness is not peculiar to the hot 
winds of Australia, In the Deccan the wind has been seen 
at 90°, while the dew-point has been as low as 29°, or 3° 
below the freezing point, making the degree of dryness 61° 
To some persons the doctrine that a hot wind should come 
from a cold quarter may appear paradoxical. This is not the 
occasion on which it is necessary or proper to discuss the 
elementary principles of chemistry or natural philosophy. 
I may, however, state that air when expanding in conse- 
quence of mere diminution of pressure, becomes cold indepen- 
dently of any heat being absolutely abstracted from it: its heat 
merely becoming latent; and it acquires its original tempe- 
rature by simply restoring the pressure which previously 
existed. The experiment of igniting tinder by pressing cold 
air in a syringe is sufficiently familiar to every one; and the 
freezing temperature produced by the sudden expansion of 
air let out of a vessel in which it has been condensed is 
equally well known. In fact, the cold which exists on the 
tops of high mountains is attributable solely to the rarefac- 
tion of the air in those situations. 
If the rationale of the hot winds, as above recited, be the 
true one, it becomes easy to understand why it is that hot 
winds occasionally occur in Van Diemen’s Land and in the 
Island of Sicily, raising the thermometer above 100°, not- 
withstanding the great width of the intervening sea, which 
. ° é 
might be supposed to have exerted such a cooling influence 
as to have rendered such a temperature impossible. 
Barometric observation appears to show that hot winds 
originate, not so much from an increase above the average in 
the pressure of the currents from the north, as from the di- 
minution of pressure from the south and west. To account 
for this, I must refer to a previous remark as to the compa- 
rative friction and resistance in varied directions which exists 
at the earth’s surface. From the absence of this influence in 
the upper currents of air, and for other reasons, it is legiti- 
mate to infer that they should be of a much more uniform 
character, and preserve more nearly a mean pressure; 
and hence we find that they do not occur when the barome- 
ter is either at its maximum or its minimum. 
A northerly wind is frequently, and except during the 
middle of summer, almost invariably followed by rain. This 
circumstance at one time appeared to countenance the idea of 
an inland sea. It admits however of a very different ex- 
planation; bearing in mind, that from a variety of causes, the 
irecularity of the earth’s surface, and the meeting and cross- 
