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New South Wales and Van Diemen's Land were the effect of the polar, and not of the equatorial wind.
In the two other diagrams (Pl. III. IV.) we see, on the contrary, that the northerly circulation, moving from west to east, extended its segment further to the southward than the S. E.; showing, not only from the effects of the motion which is ascribed to the eddy, but from its temperature, that the prevailing winds of New South Wales and Van Diemen's Land, although coming from different quarters, are originated by the agency of the equatorial wind.
In accordance with the admirable barometrical observations of Flinders and King, and the conclusions to which they have led, viz., that the (S. E. + S.) polar wind raises the barometer, and that the (N. + N. E.) equatorial depresses it; the abridged table of pressures shows that the barometer in winter, on account of the polar wind, gave a higher mean, and that, on the contrary, in summer, when the prevailing wind belonged to the equatorial circulation, it gave a lower mean; thus, not only rendering evident the influence of particular winds on atmospheric pressure, but showing farther, that 'their effects in the southern hemisphere correspond with those of the northern, as deduced by Dove and Kämtz; in other words, that in both, the barometer rises with the polar, and falls with the equatorial wind.
Independently, however, of these influences, the state of the higher region of the atmosphere, in relation to calm or motion, forms of itself a powerful influence upon pressures, and one which must not be lost sight of in the present case.
The Table I. of the preceding division clearly demonstrates, that in summer the number of antagonist currents and surface winds far exceeds those of winter; that is, that in summer the upper regions
are in a state of comparative agitation, and in winter in that of comparative calm.
Now the observation of storms and other meteorological phenomena, tends to establish the fact, that the maximum motion of the air is followed by the minimum, in the range of the barometer; and hence justifies the belief, that the difference in the state of the atmosphere, between the summer and winter, of Table I., formed a concomitant influence in producing a difference in the mean barometrical height for each season of Table IV. V.*
Whether the phenomena associated with barometrical pressure are an effect of the sun's declination, with which, through the medium of winds and currents, they certainly have a connection, or are dependent upon the lunar phases, as stated by Arago, Boussingault, and Rivero, is a question which we must leave to future researches and speculations to answer.
The approach of rain in New South Wales and Van Diemen's Land, as traced through the meteorological concomitants with which it is associated, is indicated by the southerly current, and the N. and N.W. surface wind; and seems to result from the refrigerating action of the upper atmospherical circulation upon the lower one.
Owing, doubtless, to the two distinct kinds of winds which proceed from the northern quarter - one of them being the hot wind, -- the action of the one current upon the other presents two distinct and characteristic phenomena.
* The angle of inclination at which the atmospheric circulation strikes the surface of the earth, must be taken into account when summing up the influences upon the barometrical range. Observations which I have collected on this point are still too imperfect to admit of a conclusion being formed; but I hope that, with the improvement of the instrument, which is to ascertain the angles of such inclination, the above influence will soon be rendered evident.
In the case of the meeting of the south current and the hot wind, the richly illuminated and cloudless sky begins, at the outset of the contest, to grow dark to the southward; masses of nimbi are soon seen rising from that quarter, and, as if checked in their onward movement, they advance first in a lateral direction along the eastern and western horizon, and then increase in height. At an altitude of about 15° the nimbi are observed to disperse in detached cirrocumuli, which also, before reaching the zenith, disappear. The lightning which accompanies the developement of this phenomena is emitted without noise, and in horizontal sheets, of comparatively little brilliance.
But the dispersion of the continually increasing cirro-cumuli, by the hot wind, quickly alters the characters of that wind, and the contact of the two antagonist forces draws to a close: the lower current begins evidently to recede in its upper region; in other words, while maintaining its velocity on the surface of the earth, the upper current causes the clouds to advance in altitude.
The first indication the instruments give of this change, is a decrease in the temperature of the ambient air, a very striking increase in solar radiation, and a fall of the barometer.
The next is the diminution of the tension of vapours, and of solar radiation.
The last is a rise in the barometer and a saturated state of the atmosphere.
This is followed by a gradual commingling of the two currents, out of which proceeds a southerly wind of moderate force, attended with cloudy, warm, and sultry weather, extremely oppressive to the feelings,
and very unfavourable to the condensation of the circumambient vapours.
In most cases these changes are succeeded by a calm, with occasional light airs, which shift the mists and clouds from one quarter to another for several days, until they eventually disappear, leaving a parched and withered mark upon the exotic vegetation, and from which even the indigenous plants are not wholly exempt.
When, however, the wind which proceeds from the northern quarter is not a hot wind, the action of the two currents resolves itself into a second phenomenon, characterised by an abundance of rain, and a vertical discharge of electricity, accompanied with thunder.
In Van Diemen's Land this action is gradual: the precipitation of rain is effected by the northerly wind, and is both steady and equal.
The southerly current becomes a surface wind, when all the floating vapours are condensed ; and this is succeeded, after a short space, by fine and clear weather. In New South Wales, on the contrary, the action is violent, and the northerly wind being suddenly acted upon by the southerly refrigerating wind, condensation takes place, and the rain descends in floods, accompanied generally by southerly gales.
The absolute quantity of rain, which the combined action of winds and currents produces in the two colonies, is expressed in inches, in the following table, which includes 8730 days of observation, brought to the last term of averages for every season, at each station :
The progress of decrease in the annual quantities of rain from the equator to the pole, and which was made by Arago the subject of a most interesting paper, contained in the Annales de Chimie, 1824 and 1825, finds itself well illustrated in the case of Port Macquarie, Port Philip, and Port Arthur, and may serve as an additional evidence of the existence of some laws regulating such decrease. The discrepancies, nevertheless, which are to be met with in the amount of rain of the three stations in Van Diemen's Land must be accounted for by the influence which the winds were shown to exercise upon its condensation : the prevailing winds being different at each station, they must naturally produce a difference in the quantities of rain which those stations receive.
The influence, however, of winds, great as it may be, is not the only one which increases or diminishes the fall of rain: that of vegetation is nearly equal to it. The refrigerating powers of plants, acquired through the nocturnal radiations of heat, and the feeble absorption of the same daring the day, is exemplified in the most striking manner by a comparison of the quantities of rain condensed by two mountainous districts, - the one richly wooded, the other but scantily covered with vegetation. Thus, on Middlesex Plains,