Meteorology of Melbourne. 171 
S. W., 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 wind 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 havin 
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- 
ponding 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, where its moisture has separated in the form of 
