Meteorology of Melbourne. 



169 



expected on theoretical grounds. Before however explaining 

 my particular Yie^ys 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 

 m 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, tlie 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 



