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All the soils, specified in the above table, which have a low absorbing power, and a high radiation, are formed from the disintegration of siliceous rocks, i. e. quartz rock, granite, gneiss, protogene, hyalomicte, sienite, siliceous slate, mica slate, petrosilex, porphyry, sandstone, and conglomerates.
The soils, again, which have a high absorption and low radiation, are those derived from greenstones, basalts, trachytes, and serpentinous rocks.
Now, by recurring to the Geological and Mineralo
gical Section, we shall find, from the proportion which these two kinds of soils bear to each other, in the two colonies, that, on every 100 square miles, New South Wales has 97'56 of siliceous, and 2'43 of pyroxenic rocks; and that Van Diemen's Land, on the contrary, possesses, on every 100 square miles, twenty-five of the first, and seventy-five of the second kind of rock.
From this difference in the rocks composing the crust of New South Wales and Van Diemen's Land, it follows naturally that the calorific effects of the great luminary will be different; and that, considered abstractedly, the reflected heat and high radiating power of the surface in New South Wales must cause an oppressively high temperature during the daytime, and an insufferably cold one during the night; while in Van Diemen's Land, the different mediums must render the nights too warm to be refreshing, after the day's heat.
The wise adjustment of the climatic agencies obviates admirably the inconveniences that would otherwise be felt in such climates.
The influence of vegetation, to which allusion will be made in the next division of this section, and which, owing to the difference of soils, is different in the two colonies, modifies singularly the radiation of their respective surfaces.
Again, the diaphaneity of the atmosphere in Van Diemen's Land, which is superior to that of New South Wales, as greatly checks the emission of heat in one colony, as it favours it in the other.
In some cases, the difference in the relative powers of absorption and emission of heat possessed by a surface, is an influential concomitant of meteorology, not only affecting the mean temperature of a locality (to which reference will be made hereafter), but pointing also to the causes of the extraordinary anomalies which the irregular variation of its barometer 220
presents. Thus, for instance, during the time of my exploring the extreme north-west point of Van Diemen's Land, the barometrical fluctuation at Woolnorth, during twenty-four hours, amounted to 01*600; the maximum occurring during the twelve hours of the day, and the minimum during those of the night.
At Circular Head, on the same parallel, twentyfour miles distant, the daily range of oscillation was = 00* 150; the maximum and minimum occurring indiscriminately in both the one and the other period of the twenty-four hours.
Now upon inquiring into the physical character of the soils belonging to these localities, I found that, while the soils of Circular Head possessed a greater power of absorption of solar heat, those of Woolnorth had a greater power of terrestrial radiation; or, in other words, that the absorption and emission of heat in the one locality stood in an inverse ratio to the absorption and emission of heat in the other; thus producing daily, as regards temperature and currents, a diametrically opposite effect; which may, without detriment to any theory of barometrical oscillation, account for the irregular variations of pressure.
But, independently of these considerations, the observations of the physical character of soils give results somewhat at variance with certain axioms in physics. They are presented here, as furnishing some additional data bearing on the interesting and obscure question, as to whether the absorbing power of bodies, in respect to heat, is in direct ratio to their radiating power; or, whether terrestrial absorption and radiation follow different and independent laws.
DEW, AND MOISTURE OF THE ATMOSPHERE.
The relative amount of dew condensed in New South Wales and Van Diemen's Land was a subject of four years' inquiry, and was ascertained by means of twenty grains of perfectly dry wool, exposed in a clear summer night to terrestrial radiation, and by comparing the increase of weight which such wool gained in a certain time, in one country, with that which it gained within an equal time in the other.
The result of this inquiry proved, that while, on an average, the wool in New South Wales, in the summer months, gained five grains, in Van Diemen's Land, in the corresponding months, it gained nine grains; consequently, that the relative amount of dew, in New South Wales and Van Diemen's Land, during a given number of summer nights, will be as 1 : 1-8, a proportion which the preceding notice of the diaphaneity of the atmosphere fully accounts for.
Further observations, connected with dew, disclosed the fact, that on comparing the number of clear nights during the summer months, when alone a deposition of dew can take place, it was invariably found that the number of such nights in New South Wales was, to that of the nights in the sister colony, as 1 : 2#2.
The register of the psychrometer illustrates still further the condition of the two colonies in respect to dryness and moisture: the instrument was observed three times per day, during a period of two years, both in New South Wales and in Van Diemen's Land.
The tension of vapour at the dew point, the register of which is here offered in a condensed form, was deduced by the formula,
in which t denotes the temperature of the air, as shown by the dry thermometer; t' the temperature indicated by the wet; and /' the tension of aqueous vapour at the temperature t'.
TENSION OF THE AQUEOUS VAPOUK IN NEW SOUTH WALES
The conclusions which were brought forward under the heads of Rain and Evaporation, tend to establish the fact, that the quantity of rain and of evaporation, or, more correctly speaking, the power of the atmosphere to evaporate a certain quantity of water in a given time, is in New South Wales equal to that which exists in Van Diemen's Land.
On comparing, however, the relative amount of dew obtained in the two colonies, and particularly on comparing the means of the above table, it is evident that, notwithstanding the equal share of rain and