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From these numerical elements it follows, —
1. That in Van Diemen's Land the equatorial wind prevails during both summer and winter.
2. That at Port Philip it is the equatorial in winter, and the polar in summer, that prevail.
3. That at Port Jackson and Port Macquarie the winter season is marked by the prevalence of polar winds, and the summer by that of equatorial.
These discrepancies in the prevailing winds, on a littoral of but 13 degrees of latitude, are striking and extraordinary.
The influence of the sun's declination cannot account for them; neither can they be explained by any local cause: but it is highly probable that they may be successfully traced to the influence of monsoons and winds which are found to exist within a certain distance of Terra Australis.
Consulting the valuable Appendix to Captain P. P. King's Minute Surveys of New Holland, or Horsburgh's Indian Directory, we find, as constituting the atmospheric circulation of the vicinity of New Holland, —
1. In winter, the easterly monsoon to the northward of that continent, and the W. and S. W. winds prevailing to the southward of it.
2. In summer, again, the westerly monsoon and the S. E. and S. winds prevailing, with some exceptions quoted by Horsburgh, both as regards S. W. and S. E. winds, and those also which are on record as having been observed during the summers of 1841 and 1842, and in which the W. and S. W. prevailed.
Now, by projecting the direction of these monsoons and winds, according to the limits and causes which Flinders, King, and Horsburgh assign to them (PI. I. fig. 1.), We see that the littoral of New Holland is surrounded by an exterior belt or circuit of atmospheric circulation, varying with the seasons, as regards its direction, but constant in motion and intensity, and which must necessarily impart to the remaining central atmospheric fluid, of different densities, certain regular eddies, similar to those observed in the sea or in large rivers.
Indeed, were we to determine the form and direction of these atmospheric eddies simply from the analogous motion of other fluids, taking into consideration the influences of the vertical and horizontal configuration of the country upon which they strike, we should obtain a diagram, representing such determinations in curves very similar to those which are furnished by elements of actual observation.
Thus, during the three succeeding winters, the eastern monsoon, and the W. and S. prevailing wind imparted to the ambient air of New South Wales an eddy, of which the centre or axis was the great mountainous district west of Sydney; and the direction, as might be expected, was from the right to the left, facing the equator. In the course of its developement, its southerly segment, striking on the westerly coast of Van Diemen's Land, and the high chain of mountains which fronts it, was inevitably deflected into a subordinate eddy on the eastern part of the island, and resolved into a movement which accounts for the prevaibng N. E. wind at Port Arthur, Launceston, and Port Philip; and the S. W. at Port Jackson, and Port Macquarie. (PI. II. fig. 1.)
Thus again, in the summer of 1840, the W. and S. E. "outside" winds involved the interior air in a rotatory motion from left to right, facing the equator; and which air, revolving on the identical axis of the winter circulation, and meeting with no obstacle on the eastern coast of Van Diemen's Land, performed an uninterrupted circuit, and produced a N. E. wind at Port Arthur, a S. at Port Philip, and a northerly at Port Jackson. (PL III. fig. 1.)
In the following summer, 1841, the centre of the wind's rotation was again the same as that shown in the diagram of former seasons; its direction from left to right, facing the equator, being such as the monsoon would naturally impart to it; but owing to the prevalence of the W. instead of the S.E. winds, a segment of the eddy reaching to the latitude of Flinders Island, and meeting the opposite wind in Bass's Straits, which is on all occasions extremely strong, was split, or rather bifurcated, producing a S. W. wind at Port Arthur, a N. E. at Launceston, a S. E. at Port Philip, and a N.E. at Port Jackson and Port Macquarie.
In the summer of 1842, from the prevalence of the westerly "outside" wind, the courses of winds around the littoral of the two colonies was similar to that of the preceding summer, except that the inclination of the lower current, which was deflected on Van Diemen's Land, took a more easterly direction, producing N. W. winds at Port Arthur, N.E. at Launceston, S. E. at Port Philip, and N. E. at Port Jackson and Port Macquarie. (PI. IV. fig. 1.)
In one of these seasons (1842), during which the westerly winds, instead of the easterly, prevailed, I was engaged in exploring the islands of Bass's Straits, and witnessed on several occasions the effects of the conflict of two antagonist currents, the N. E. and W. I shall relate but one instance, extremely favourable to the illustration of the case before the reader.
On the 13th January, 1842, I ascended, from the westward, the highest peak of Flinders Island, which Captain Stokes, of H. M. surveying ship the " Beagle" has done me the honour of naming Strzelecki's Peak (2550 feet). On making the ascent, the wind was westerly; on descending the mountain on the eastern side, the wind was N.E. The "Eliza," a colonial vessel from Hobart Town, bound for Launceston, passed Banks's Straits about noon, with a N.E. wind, yet met in the longitude of Cape Portland a strong westerly breeze. The same day the " William," Captain Tom, returning to Launceston from Sydney, met in Bass's Straits an easterly breeze, and made Port Dalrymple with a westerly one; while the "Mary Ann," from Hobart Town, bound for Melbourne, passed Banks's Straits with a N.E., but entered Port Philip with a S. E. wind.
However interesting the study of these currents may be to science, there is no phenomenon that has stronger claims upon the attention of a meteorologist, none that presents itself as a more legitimate subject of physical inquiry, than that commonly known in New South Wales and Van Diemen's Land under the name of the hot wind.
The mean direction of this wind is N. W.; its course ranges, therefore, within that of winds associated with the maximum of temperature and dryness.
Its velocity exceeds in some instances that of a regular gale; its motion, however, is different. That motion along the surface, as observed by the light bodies floating in the air, appears at times as if produced by a rotation on a set of horizontal axes <QQS>LQQQ-Qs • at times as resulting from a ricochet movement, thus, — '~\^/~\/~\s^-' ;in which case its effect is that of a wind blowing by puffs.
Its thermometrical condition on the westerly side of the dividing range is such, that the mean temperature of a summer day is increased by its influence 40°; on the eastern side, the increase, both in New South Wales and Van Diemen's Land, ranges between 25° and 30°: caloric, therefore, seems to be an inseparable concomitant of this wind.
All the visible moisture of the atmosphere, from the cirrus-stratus clouds to the light vapours floating above the marshes and rivers, disappears at the approach of the hot wind: rain never occurs while it lasts; and, notwithstanding the sky's apparent clearness from vapours and clouds, both the formation of the dew and the sun's radiation are interfered with.
The temperature of the wet thermometer reaching in New South Wales 78°, and in Van Diemen's Land 72°, while the temperature of the dry thermometer in the one country was 117°, and in the other 90°, indicates strongly the extreme dryness of the hot winds, but does not lead to any calculation showing the absolute measure of actual evaporation which takes place from the surface of the country swept by it. By means of an evaporating dish, to which a glass tube and scale, and a vernier, was affixed, and by which one thousandth part of an inch of evaporation could be indicated, a more direct result, though still of only an approximate nature, was obtained. The annexed figure shows the construction of the instrument.
The elements furnished by the daily register of the above gauge may be summed up in the following fact, that while under ordinary circumstances the mean of evaporated water in New South Wales and Van Diemen's Land during three hours is equal to O045 of an inch, under the influence of the hot wind that evaporation in three hours reaches 0" 150. This leads naturally to the conclusion that dryness, or a peculiar power of decomposing or dispersing the atmospheric moisture, is the characteristic property of the hot wind.
When daily observing the sky, it is not uncommon, during the prevalence of this wind, to see the high clouds, cirrus and stratus, at once disappearing, whilst in the lower clouds no chance is remarked.