Imagens das páginas
PDF

the thermometer covered with swansdown, and the non-radiating one, averages in winter, in New South Wales, 4*3 of a degree, and in Van Diemen's Land, 2-2.

They further show, that in summer the average difference between the two thermometers is in Van Diemen's Land 5°, while in New South Wales it is but 3°; and hence that the diaphaneity of the atmosphere at night is, in the two colonies, inverse to their respective seasons, being greater in New South Wales during the winter, and in Van Diemen's Land during the summer months.

Now the summer nights in Van Diemen's Land being clearer, the terrestrial radiation and its refrigerating action precipitate to the surface all the floating vapours within their reach. With the rising sun the precipitated moisture is reconverted into its elastic form, and the morning is rendered misty until 9 o'clock, at which hour the vapours being raised higher, and diffused or dissipated, the sky becomes clear and bright.

In New South Wales, on the contrary, the summer nights being veiled by the agency of some imperceptible medium, little or no radiation takes place, and scarcely any dew is deposited: the atmosphere at sunrise is then found clear and transparent. At 9 o'clock, at which hour the sea-breeze usually sets in, the degree of that clearness and transparency lessens, owing to the humidity of the air-current; and this continues until 2 P.m., when, the sea-breeze ceasing, the sky gradually re-assumes the appearance it had in the morning.

This difference in the transparency of the medium which intervenes between the surface of each colony and the sun, naturally renders the effect of the rays transmitted to their respective surfaces different also; for we see that misty mornings in Van Diemen's Land, and clear mornings in New South Wales, are marked by a sensible difference in the degree of the intensity of solar rays; whilst again, the dissimilar transparency of the air at noon in the two countries is, by parity of effect, the cause of the startling numerical fact, that Launceston, which is farther from the equator, possesses at noon a higher degree of solar intensity than Port Stephen, which is nearer to it.

The same equally helps to explain the results obtained by the two thermometers, the one covered with black, the other with white wool; and which show at the two stations the greatest difference at the hours when the air is most transparent.

From what has been said, it follows then —

That the degree of the intensity of solar rays is greater in New South Wales than it is Van Diemen's Land; but that, owing to the more diaphanous atmosphere of the latter colony, the register of a blackened thermometer there yields higher numerical elements than in the sister country.

That the effect which solar rays produce by absorption is greater in New South Wales than it is in Van Diemen's Land.

And, lastly, that the diurnal diaphaneity of the atmosphere is indicated more sensibly by two thermometers, of which the recipients absorb differently the solar rays, than by a thermometer blackened with lamp-black.

TERRESTRIAL ABSORPTION OF SOLAR RAYS, AND RADIATION OR EMISSION OF HEAT.

Amongst the soils belonging to the two colonies that came under my notice in the course of an agricultural inquiry, there were some upon which, according to the statement of the farmers, all the early crops were invariably injured by frost; while on other soils such injury was never observed to take place.

The examination of their relative situation, subsoil, cultivation, hygrometrical state, and chemical character, gave no clue to the causes of this failure or success of crops.

The experiments, however, which were instituted for the purpose of ascertaining the rate or power of absorption and emission of solar heat possessed by these soils, have been more successful, as they have rendered evident the influence of the physical character of soils, not only upon vegetation, but upon climate.

For the present, we shall limit ourselves to the consideration of the latter influence, and shall briefly advert to the method or principle upon which the experiments alluded to were conducted.

The apparatus used was similar to that already mentioned for ascertaining solar radiation, with this difference, that instead of three, six thermometers were employed, and their bulbs crowned with truncated cones of paper, tightly fitted, and spreading upwards, so as to contain the soils.

To these was added a seventh thermometer, which was naked, and, in the observations on the absorption of heat, was placed by the side of those covered with the soils; while in the observations on the emission of heat, it was suspended below them, under a roof which protected it well from radiation.

The element of the power of absorption was deduced from comparing the temperature of a thermometer covered by a soil one-twentieth of an inch in thickness with that of a naked reflecting one ; the degree indicated by the latter being considered as zero, and the difference obtained being recorded with a sign of 4.

The element of terrestrial radiation was arrived at by comparing the temperature of one thermometer, covered by a soil one-twentieth part of an inch in thickness, and exposed to radiation towards space, in a clear night, with that of a thermometer screened from such radiation, the degree of temperature indicated by the latter being considered as 2ero, and the difference obtained * being recorded with a sign of-.

The following table embraces the results of these experiments.

[table]

* Thus, taking for instance, the observations relating to the absorption of solar heat; if, in the apparatus, soil A showed 134°, soil B 138°, and soil C 140°, and if the reflecting thermometer indicated a temperature of 110°, soil A was then registered as having -f 24° of absorbing power, soil B + 28°, and soil C + 30° of such power. Again, as regards the emission of heat, if the thermometers covered with soil A showed 57°, soil B 59°, soil C 60°, and the non-radiating thermometer indicated the temperature of 63°, then the power of terrestrial radiation of soil A was registered 6°; that of soil B, — 4°; and that of 6oil C, — 3°. This investigation into the physical properties of soils was made between 1839 and 1843, in New South Wales and Van Diemen's Land. On my return to Europe, in 1844, I met, in the work of Boussingault, which deserves the greatest attention of agriculturists, with similar investigations, which had been undertaken prior to mine, by Dr. ShQbler, with the exception, however, of terrestrial radiation. The methods by which our respective experiments were conducted differ: the results are the same, which to me, unaware as I was of Dr. Shiibler's labours, is a source of some satisfaction.

LOCALITY.

[ocr errors]

VAN DIEMEN S LAND.

Soil from Malachite, the property of the 1

Hon. Mr. Talbot - - - J

Ditto - - - -

Soil from Quamby, the property of Richard "I

Dry, Esq. - - - -J

Ditto ----

Soil from Bothwell, the property of A. 1

M'Dowell, Esq. - - - -J

Ditto

Soil from Glen Leith, the property

Jamieson, Esq.

Ditto ----

Soil from a farm of Mr. Whittle's, on the 1

North Esk river - - - J

Soil from Franklin Village

Soil from a farm called Point Effingham

Soil from the northern littoral of Van

Diemen's Land, east of the Tamar
Soil from a farm at Cape Portland, te-
nanted by Mr. Bowen -
Soil from a farm on St. George's river,

property of the' Hon. Mr. Talbot Average soil from the upper country of Van Diemen's Land, called Lake county Soil covering the western shores of the river Huyon - - -

Soil from one of the small farms of De la 1
Reyne - - - - - J

fMacquarie Harbour --
Ditto
Port Davy

Ditto
St. George's or St. He-
len's river -
Ditto
Ditto

Absorption

of Solar

Heat

Average soils from «

SOILS BELONGING TO NEW SOUTH WALES.

From Strout, a farm of the Agricultural 1
Company - - - J

Ditto - - -

Briton Coast farm, belonging to the A. A. C

Ditto - - - - .

Ditto - - .

Ditto - - - - .

Ditto ----

[ocr errors][merged small][ocr errors]

Radiation of Terrestrial Heat.

- 16°

- 14

- 04

- 6

- 4

- 8

- 1-5

- 2

- 4-5

- 8

- 4

- 5

- 4

- 5

- 2

- 7

- 5

- 5

- 4-5

- 4

- 42

- 5-5

- 42

- 4

- 2-5

- 2

- 4

- 3

- 3-5

- 2-5

« AnteriorContinuar »