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In one instance, being obliged, for the completion of observations, to stop for a week on the top of Ben Lomond (Van Diemen's Land), I experienced the hot wind at the altitude of 5002 feet. At my depot, 3000 feet lower to the windward, this wind was not felt; hence the hot wind is not confined to any particular altitude, but exists sometimes as a surface wind, and at other times as a wind moving above the ambient air.
From personal observations, it appears not less evident, that as this wind is felt in one locality, and not in another, within the zone or track of its course, so its movement is not always parallel to the earth's surface, but at different angles of inclination: thus, in the valley of the Tamar, Van Diemen's Land, the hot wind was felt on several occasions at Break-o'-day and in Campbeltown, but not in Launceston, though the locality offers no obstruction to the progress of a wind.
The stratum of air immediately overlaying the surface of the country swept by the hot wind is generally of a highly rarified character, and produces all the known phenomena of the Mirage.
The air of the upper stratum is, on the contrary, of prodigious density and great refracting power. Thus, three thermometers exposed to the action of the vertical rays of the sun, one covered with black wool, another with an equal quantity of white, and a third blackened with lamp-black, indicated the following temperatures: —
Before the Hot Wind.
The two thermometers covered with wool
showed a difference The thermometer blackened for solar radiation The thermometer in the shade The wet thermometer Pressure reduced to zero of temperature
The hot wind, then, according to the numerical elements which observation furnishes,
Impedes the calorific effects of solar rays;
The influence of this wind on vegetation, both indigenous and exotic, is extremely injurious. All the grammar and leguminosce are parched by it, and the fruit of the Ficus australis, as well as that of the vine, is destroyed. The red and blue grape commonly lose their colour and their watery elements; the green leaves turn yellow and wither; the quality of the crops is generally deteriorated, and whole fields of of most promising wheat and potatoes are often laid waste.
Its effects on the human constitution partake sometimes of the character of those produced in Egypt by the sirocco or simoom; a feverish heat and determination of blood to the head; and, in those subject to disorders of the lungs, a restrained action in breathing, at times bordering on suffocation, are symptoms confined to the whites alone: the suppressed perspiration, or rather its rapid evaporation, the relaxation of the muscles and vessels, inflammatory attacks, affections of the glottis, and ophthalmia, are common both to the aborigines and European races. The lastnamed disease, called in Australia, "the blight," presents symptoms identical with those observed in the ophthalmia of Egypt.
From the circumstance of my being unassisted in my observations of all the various meteorological phenomena which are commonly associated with the hot wind, some allowance will I hope be made for the incompleteness of this notice. Such as it is, it will afford sufficient ground for venturing upon an analysis, and for tracing at least the proximate cause, or causes, of the properties which the hot wind of New South Wales and Van Diemen's Land uniformly displays.
Thus the circumstances of its velocity and its motion, whether rotary or ricochet, by which impalpable particles of earthly matter are raised in the air, combined with the mineralogical and physical character of the particles thus raised *, will lead us to the solution of the question respecting the high temperature of the hot wind: for when we consider the power of absorption of solar heat possessed by that dense medium of floating particles, and their power of radiation towards the earth, we have a full explanation of the anomalous.fact, that with the decreased solar intensity, as shown by the blackened thermometer, there was a considerable increase of heat in the ambient air.f
• In tailing from New Zealand to New South Wales, in the "Justine," I was prevented making the harbour of Port Jackson for two successive days by the violence of the hot wind. The distance from the shore, on the parallel of Sydney, was sixty miles, and the heat ex. ceeded 90°. The lee sails and reefs of the "Justine" were covered with a quantity of impalpable dust, which was at first mistaken for ashes; but, on examination, proved to be a sand, containing one-fourth of aluminous and three-fourths of siliceous and metallic matter. Those who shape their course to the East Indies by way of Cape Verd Islands may have seen the same efFect produced by the north-east African hot wind.
\ That keen observer Humboldt, reflecting upon the anomalies of the temperature of Mourzouk in the Fezan, as stated by Ritchie, and
But the fine bodies floating in the air are endowed with other properties besides that of increasing, by absorption and radiation, the solar heat: they are susceptible of acquiring an electric property, by means of friction, pressure, contact, or caloric.
Now, the motion of the wind, and the solar action, combine all these means and necessary conditions to the full developement of that electricity, which, once effected, becomes again a cause of the extreme dryness of the hot wind; for electricity promotes evaporation (Volta, Saussure); and the observed disappearance of clouds and vapours, with the drying up of the watery elements in leaves or tender plants and fruits, are but modified effects of that evaporation.
The zone, then, of this hot wind may be looked upon as a huge electric apparatus, highly charged, and endowed, not with a capacity for moisture, but xcith the power of dissolving moisture, in whatsoever form it is collected.
Under the head of rain, it will be proved that the hot wind begins only to show a capacity for moisture, and an increase in saturation, when the southerly current interposes between it and the sun, and when heat, light, and motion, the necessary conditions of electricity, are interferred with.
Nor is the deleterious effect of the hot wind on human constitution a phenomenon more inaccessible to analysis than are the causes of its heat and dryness: for when we bear in mind the fine particles of dust floating in the air, we can readily explain, by their highly electric state, the causes of ophthalmia, and its inflammatory character; and when we add to what has been said already, that, at the altitude in which the hot wind is at times seen moving, a proportional increase of carbonic acid in the atmosphere is found (Saussure), and that, in the deflection of the wind to the surface which follows, this amount is added to that which is left unconsumed by the crisped vegetation, the farther causes of the deleteriousness suggest themselves in the origin of the new and noxious gaseous compounds which a disturbed economy of nature has necessarily entailed upon the atmosphere.
corroborated by Captain Lyons, said, "Mais l'air de l'oasis de Mourzouk n'est il pas constamment charge' de poussiere, de petits grains terreux qui s'echauffent bien autrement que l'air, et qui par leur rayonnement rlevent la temperature dea basses couches de l'atmosphere ?" — Asie Centrale, vol. iii. p. 177.
But New South Wales and Van Diemen's Land are not the only countries where the hot wind is felt: in other parts of the world that phenomenon is attended with aggravated effects, both on animal and vegetable life.*
No datum, however, is as yet offered, by which one could legitimately indulge in a theory concerning the origin of this remarkable meteorological phenomenon: neither is there a sufficient collection of facts and observations to bear out the tempting and inviting conclusion, that the winds similar to the hot wind of New South AVales and Van Diemen's Land, which have been observed, partly by myself, partly by others, in Egypt, Abyssinia, Syria, Arabia, Bombay, Diabekir, Persia, California, and Atacama, are associated by a common origin, or belong to a common system of atmospheric circulation.
As regards the subject of atmospheric pressure in New South Wales, it is to be regretted that the ba
• Vide, Edin. Cabinet Library, Africa, p. 414. et seq. ; Denham's Travels, Edin. Cabinet Library, Arabia, vol. i. p. 68.; Beeloochistan, by Pottinger; Bruce's Travels, vol. vi. pp. 466. 494. 3d edition; Burckhardt, Malcolm, Morier, and Lamartine (in the MS. of Falih Jalla), Voyage en Orient.