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equal power of evaporation, New South Wales is drier than Van Diemen's Land.
Whence then does this difference arise? The question is of very easy solution.
As stated in the foregoing division of this section, the geological survey proved that, on each 100 square miles, New South Wales possesses 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, 25 of the first, and 75 of the second kind of rock.
Now the difference which is observed to exist in the proportions of the kinds of rocks that form the crust of each of the colonies, naturally influences the composition of the soils, and renders their power of production different, as also the action upon the climate of such vegetation as corresponds to that power.
For it is observed that, in Van Diemen's Land, soils formed by the disintegration of greenstones, basalt, and trachyte give rise to a vegetation characterised by lofty trees, ferns, close-tufted gramince and mosses, which contribute to screen the rain-water imbibed by the soil from evaporation; while, in New South Wales, the soils, derived from granitic rocks and sandstones, yield a shadeless vegetation of Eucalyptce and thin gramince; which being incapable of producing the same effect, the rain-water is returned to the atmosphere and carried away by winds.
Again, the difference in the vegetation produces a difference in the solar action upon the surface.
In New South Wales, where, from the physical character of the soil, the absorption of solar heat ought to be less than in Van Diemen's Land, it is virtually greater, as the soils of Van Diemen's Land, with all their high power of absorption, are screened by vegetation from exercising that power; while those of 224
New South Wales, though possessing that power in a lower degree, are left open to a free and continuous access of solar rays.
The terrestrial emission of heat in New South Wales was likewise proved to exceed that of Van Diemen's Land. The interference of vegetation, however, modifies this tendency, and produces an effect contrary to that which might be expected.
Indeed, the terrestrial radiation in Van Diemen's Land, owing to the influence of vegetation, lowers the temperature of the ambient air in such a degree as to produce, whenever the atmosphere is clear, a copious condensation of the floating vapours in the form of a shower, (as on high mountains, when the passing clouds are within the altitude of their refrigerative action,) or in the form of dew, if no unfavourable circumstances intervene to prevent its deposition.
Such condensation is never observed in New South Wales; and it was owing to the dry air of the night that, during two years of my wanderings in that country, I dispensed entirely with a tent; while, inured as I was to climatic changes, I could not do without one in Van Diemen's Land.
To not taking into account the influence of vegetation, may be ascribed the conflicting opinions of meteorologists with respect to the moisture of the higher regions of the atmosphere; some pronouncing it to be drier, some again, to have the same degree of moisture as the lower. Questions of this nature can only be decided by aeronauts; as, at certain elevations, barren, naked, and reflecting the solar heat, the hygrometrical observations collected, will corroborate the conclusions of De Luc, Saussure, and Humboldt, that the higher regions are drier; while, at elevations, again, which are clothed with a vegetation corresponding to that of the plain or foot of the mountain, the indication of the hygrometer will be
the same as there, and will confirm the results obtained by Kiimtz, Martin, and Beauvais.
If, however, the elevated locality be that of the Cordillieras at the passo from St. Juan to Acongagua (9000 feet), or of the region of Mouna Roa, Sandwich Isles (8000 feet), or of Mount Kosciuszko, New South Wales (5000 feet), or of the table land in Van Diemen's Land (4000 feet), or of Corcovado in Rio Janeiro, before it was clear of timber,—at which altitudes the vegetation is richer than it is on the sealevel,— the hygrometrical results will not agree with either of the above conclusions. In 1841, when the drought destroyed the herbage in the lower part of Van Diemen's Land, the table land of that island furnished throughout the season a most excellent pasture for sheep and cattle.
But, independently of the action of soils through the medium of their vegetation, it has been further observed, that when denuded even of that vegetation, they still possess a direct power of influencing the moisture of a country.
Under the preceding head it was stated that the classification of soils, by the farmer, into "cold and warm" induced an inquiry which ended in determining their relative power of absorption and radiation of heat.
In the present case, also, it was the farmer's having frequently pointed out two paddocks, situated on the same level, both equally drained and cultivated, and one nevertheless possessing a soil drier than the other, that prompted an investigation which proved that soils are endowed with a property of absorbing moisture from the atmosphere, independently of that of the dew; and that this power, which will be called, in the Agricultural Section, "capacity for moisture," varies ad infinitum.
Thus, 100 grains of one kind of soil, dried at a
temperature of 212°, and 100 grains of another kind, equally dried, and both exposed to a free circulation of night air, when the wet and dry thermometer indicate nearly the same temperature, both also equally screened from radiation, will show, next morning, a difference in weight amounting to 30 or 40 grains.
Again, on being exposed to the sun with the collected moisture, and reweighed after two or three hours of such exposure, they will show that the emission of moisture is in an inverse ratio to the power of absorption; that is, that the soil which absorbed the most will lose the least, and trice versd.
The chemical analysis, as will be shown in the proper place, proves, that in such cases the aluminous earth of both the soils was in equal quantities; but that, whenever the soil was found drier, it was deficient in the base of humic acid; and whenever it was moist and retentive of moisture, it had of that base from 20 to 40 per cent.
Now, the indigenous vegetation of Van Diemen's Land, promoted by the kinds of rocks which form the soil, is more favourable than that of New South Wales to the accumulation and decomposition of vegetable matter, which constituent of the noncultivated soils plays so prominent a part in the effects of solar heat and moisture. And while this, in connection with vegetation, tends to explain the difference in the condition of the atmosphere, as regards moisture, it tends not less to justify my humble observations transmitted to His Excellency Sir George Gipps, and published in the Parliamentary Papers of 26th August, 1841; and in which I took the liberty of pointing out the bad consequences which would accrue to the colony from the doing away with vegetation, by overstocking the pasturage, or by burning it,—a practice which, far from improving the grass, as some have imagined, only subtracts from the soil the most essential conductor of moisture, or medium of condensing it in the form of dew or shower.
Whatever difference may exist between New South Wales and Van Diemen's Land in respect to their hygrometrical condition, certain it is that each of these colonies possesses a greater amount of moisture than that which is commonly allotted to them: thus, taking, for instance, the climate of London, as illustrated by Professor Daniell, in his Meteorology, we see that the mean amount of its moisture, represented by *341 of tension of vapour, is, within a trifle, similar to that of Van Diemen's Land, the moisture there being equal to '374.
When it is considered what powerful adjuncts of calorific and frigorific influences are the winds, currents, rain, evaporation, radiation, absorption, and emission of heat, &c, it cannot be wondered at that temperature — alternately cause and effect, acting and reacted upon by the ever-changing and modifying agencies of the atmosphere and the crust of the globe — should seem to be viewed by many as embracing all that is comprehended in the word "climate."
Nor is it to be wondered at, that, in presence of such interferences, connections, and dependencies, the investigation of temperature in the abstract, or "solar climate," should have eluded the ingenuity of meteorologists. The high importance which Arago has attached to the registering of the thermometer, as being capable of yielding elements for the solution of the question of terrestrial temperature, and the permanency of solar heat, refers to the observations