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 Gl*^ 



To some persons the doctrine that a hot wind shoxdd 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 

 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 ram. This 

 circumstance at one time appeared to countenance the idea ot 

 an inland sea. It admits however of a very different ex- 

 planation ; bearing in muid, that from a variety of causes, the 

 irregularity of the earth's surface, and the meeting and cross- 



