170 Meteorology of Melbourne. 
and resistance to the wind which occurs at the earth’s surface 
than in the upper and free regions of the atmosphere. From 
the influence of this cause it is easy tounderstand that the upper 
current of air would retain its velocity or momentum in its 
progress to the poles, so as to be enabled at a certain distance 
from the tropics to overcome the more sluggish lower current, 
and force it into its own direction. It is evident that the 
current of air towards the poles can only be equal to that 
which is flowing towards the equator; I suppose, therefore, 
that in the belt in which the westerly winds prevail, the upper 
current has overcome the lower, only by communicating its 
own westerly and not its southerly direction. 
With respect to the winds of Australia it is to be observed 
that we have a very large island or continent, of a compact 
form, not deviating greatly from that of a circle, and for the 
most part remote from any other large tract of land. Of this 
immense region a part is situated within the tropics, and the 
remainder very close outside the tropics, so that, having much 
higher temperature than that of the surrounding ocean, the land 
may be considered as a vast heating surface, which, on the prin- 
ciples already explained, will have a tendency to draw currents 
of air in all directions towards its centre of heat. There can 
be no doubt that the hottest part of the island exists to the 
northward, and probably also to the westward of the geogra- 
phical centre. The influence of the trade winds within the 
tropics would probably throw this point towards the west ; 
and, in confirmation of this view, I would observe that the 
hottest part of the ocean in the vicinity is south of the line, 
and to the N. W. of Australia, according to Black’s map of 
Physical Geography :—viz. Java and Timor. For the sake 
of illustration, we will ‘suppose the hottest point to be in 
long. 130° and lat. 22° or a little to the N. W. of Sturt’s 
Desert. Confining our investigations to the southern portions 
of the continent, we have next to consider that the wind, 
blowing outside the coast, has a prevailing direction from 
west to east, and therefore on approaching the coast it would 
not at once assume a direction towards the centre of heat, but 
rather a direction intermediate between that and its original 
eastward motion. 
On proceeding along the line of coast from west to east, we 
shall expect, on this principle, to find the westerly character of 
the wind gradually diminished, and that it willat a certain point 
become southerly, and beyond that again more or less easterly. 
Thus in South Australia the prevailing winds ought to be 
