Meteorology of Melbourne. 169 
expected on theoretical grounds. Before however explaining 
my particular views on this subject, it will be necessary to 
introduce a brief reference to the phenomena of winds 
generally, and their causes as far as they are well understood. 
Winds arise from the circumstance that different portions of 
the earth’s surface are unequally heated, and that air expands 
and becomes lighter in proportion as its temperature is 
increased. The air in contact with the overheated surface 
has of course a tendency to ascend, and a partial vacuum being 
thus induced, the air from the cooler regions in the vicinity 
flows towards that point. In order to destroy the equilibrium, 
it is manifest that the heated air, after having ascended to a 
certain height, must flow off again horizontally, in a direction 
opposite to that in which it moved at the surface. 
It is on this principle, that the great comparative heat of 
those portions of the earth within the tropics causes a constant 
flow of air from the poles towards the equator at the earth’s 
surface and a current from the equator towards the poles in 
the upper regions of the atmosphere. But in consequence of 
the motion of the earth in its rotation on its axis being greater 
in proportion as we approach the equator, the north and south 
currents of air at the earth’s surface become converted into 
north-easterly and south-easterly, and, from the converse 
operation of the same cause, the upper currents returning 
from the tropics become north and south-westerly. All up 
to this point is so well understood and appears to be so simple 
that it almost needs an apology for its introduction into 
a paper of this description; but, as we further pursue the 
study of the winds, we shall find that they become modified 
by the utmost complexity of causes. We shall have to deal 
with some facts difficult to explain, and with others which 
defy all our powers of calculation; so that the scriptural 
saying in reference to the wind, “ Thou canst not tell whence 
it cometh or whither it goeth,” is still true in the present state 
of our scientific and geographical knowledge. 
From what has been above stated, it might be supposed 
that, irrespectively of local influences, the general tendency of 
the winds at the earth’s surface in all latitudes should be 
north and south-easterly. The fact however is, that beyond 
a certain limit outside the tropics, the prevailing winds are 
more or less westerly. Explanations of this fact are to be 
found in different treatises upon the subject; but I have not 
seen any that entirely meets the case. It appears to me that the 
fact may be accounted for by the greater amount of friction 
a 
