156 ON AUSTRALIAN STORMS, 



at their intersections with the curves indicate the simultaneous 

 height of the barometers. Now a cursory glance at the four 

 curves is enough to satisfy any person of their general similarity, 

 but on a closer inspection it will be seen that the barometric 

 maxima and minima, which are of course the prominent features 

 of the curves, are not simultaneous over the large area embraced 

 between the observatories at Adelaide and Brisbane, but have a 

 gradual progression from west to east, or rather perhaps from 

 south-west to north-east. On the average, they occur at Sydney 

 and Brisbane respectively about one and two days later than at 

 Adelaide. The principal corresponding maxima and minima at 

 the four stations are pointed out in the diagrams by the red lines. 

 If we may take the observations for 1861 as sufficient to de- 

 monstrate the rule as to the propagation of our barometric 

 changes, and I think we may, it is incontestably shown by the 

 diagrams that they are propagated gradually over the colonies 

 from about south-west to north-east. The wind phenomena 

 attending these alterations of tension are made up of the two 

 great polar and tropical currents. In the winter months the 

 gradual shifting of these streams of air over the colonies, is as 

 distinctly marked by the observations as the propagation of their 

 accompanying barometric phenomena. The alternation of the 

 tropical and polar winds is distinctly marked at Sydney through 

 the whole year, the former corresponding to a falling and the 

 latter to a rising barometer. During the summer months at 

 Adelaide the tropical currents appear in a great measure to fail, 

 polar winds being remarkably prevalent. This fact may be simply 

 accounted for, by the position of Adelaide with reference to the 

 great Australian Continent. Daring the summer the atmos- 

 phere over the vast area of land to the north is greatly rarified 

 by radiation, and consequently ascends. The tropical streams of 

 air instead of passing over Adelaide are arrested in their progress 

 by this partial vacuum and are met by the polar winds, which 

 are also hastening to restore equilibrium at the same point. Thus 

 it is that polar winds are so remarkably prevalent at Adelaide 

 during the summer. In order to illustrate the way in which our 

 storms are propagated, I may briefly trace out the progress across 

 the colonies of one of the most conspicuous instances of bad weather 



