﻿and Fluctuations, 329 



passed the place on his return journey*. Whenever the Hot 

 Wind breaks out over South-eastern Australia, it is invariably and 

 immediately followed by a strong cold gale from the south. It 

 is impossible to avoid comparing this with what takes place on 

 opening an oven door : there is, first, a strong outward rush of 

 heated air, then an inward rush of cold air — an alternation 

 which is repeated two or three times with continually diminishing 

 force before equilibrium is established. The same alternation of 

 air-currents was shown very exactly by the late Professor Daniellf. 

 He connected two receivers by a glass tube in which was a stop- 

 cock. From one (B) he partially exhausted the air; into the 

 other (A) he compressed an additional quantity, — thus making 

 a very marked difference in the elastic force of the two volumes. 

 On opening the stopcock, the air rushed violently from A to B ; 

 this rush was immediately followed by a return from B to A, 

 the alternation in the glass tube being distinctly visible. It is 

 evident, therefore, that air putting itself in motion by the ex- 

 ercise of its elastic force, generates in itself such a momentum 

 that far more will yield to the impetus and follow in the same 

 direction than is called for to restore a balance of pressures : the 

 elastic force becomes least in the chamber where it was greatest, 

 so that there is a return movement, and even, when the first 

 difference has been excessive, several alternations of the move- 

 ment. I see no reason to doubt that the Hot Wind of Australia 

 and the cold southerly gale immediately following it are natural 

 instances of the same thing on a very large scale — that the Hot 

 Wind, bursting out from the desert by reason of an elastic force 

 immensely increased by an extraordinary accession of heat, drags 

 in its volume so much air that, as the impetus dies away, it leaves 

 behind it a comparative vacuum, into which the cold air from the 

 south rushes with great violence. We have absolutely no baro- 

 metric observations from the interior of Australia; even if we 

 had, they would be of little value, from our ignorance of the level ; 

 but it seems in the highest degree probable that, when exact ob- 

 servations can be made, we shall find that a noticeable rise pre- 

 cedes the Hot Wind, and that a fall as marked immediately fol- 

 lows it. 



I conceive that the Scirocco of the southern parts of the Me- 

 diterranean is, in its origin, exactly similar. It is, as the other, 

 a hot wind off a burning desert, and is, like it, invariably fol- 

 lowed by a cool wind from the opposite direction. Still, here 

 again we have no barometric observations ; so that I am unable 

 to speak of the rise or fall of the barometer as ascertained facts. 



On the other hand, however, the fact seems sufficiently estab- 

 lished that in high southern latitudes the barometer stands, on 

 * Journals &c. pp. xiii, 473. -\ Meteorology, vol. i. p. 49. 



