Meteorology of Melbourne, 



171 



S. W.5 and in the vicinity of Melbourne almost due east, and 

 still further on south-easterly,, and this we find in point of fact 

 to be the case. 



It is to be presumed that in the upper regions of the at- 

 mosphere, far above the level of the clouds, the returning 

 current of air is almost constantly flowing in the opposite 

 direction to that of the Avind which prevails at the surface ; 

 that, in fact, the north wind is always blowing over our 

 heads. So long as the pressure of wind from the south is 

 sufficient, as indicated by the barometer standing high, a hot 

 wind, according to my observation, does not generally occur. 

 But let the barometer sink one or two tenths below its pre- 

 vious elevation, and the partial vacuum, of which this is the 

 symptom, is liable to be immediately filled by the air 

 nearest at hand, which is that overhead ; and it continues 

 to blow until the barometer either again rises, or else 

 sinks still lower. In the former case, there will be first a 

 lull and then a rapid return of the southerly wind ; in the 

 latter case, which, as I suppose, indicates that the hot wind 

 has blown to a certain distance out to sea before it is met by 

 the southerly wind, there will be rain, and the north having 

 counterbalanced the south direction, the wind will be rather 

 westerly. If the barometer be high, it indicates a tendency 

 to efflux of air in all directions, and no immediate recoil, 

 producing storms or rain, is to be expected. 



It is a very common thing to speak of the hot winds as 

 though they had blown to us direct from the interior desert, 

 horizontally over the surface of the land. I have even read 

 of a proposition to dam up rivers, so as to form artificial 

 lakes towards the north, in order to mollify them. Such a 

 view of the_ case is, however, totally irreconcilable with ob- 

 served facts. 



In the first place, a remarkable feature in these hot winds 

 is their extreme dryness. The meteorological journal will 

 show that the dew-point of a north wind, when blowing 

 strongly, is always very low, sometimes as low as 35^. This 

 wind is drier than that which would blow from the sea in 

 any direction, and especially from the north at the corres- 

 pondiDg season of the year. Air may be heated, but can- 

 not be rendered drier, it cannot be deprived of its moisture 

 by contact with hot sand, however dry. Seeing, therefore, 

 that there are no elevated mountains toward the north, we 

 are led to conclude that this air has been dried simply by 

 having risen into the higher and colder regions of the atmo- 

 sphere, Avhere its moisture has separated in the form of 



