Of the winds, currents, and navigation along the east coast of Terra Australis, both without and within the tropic; also on the north coast.
Directions for sailing from Port Jackson, through Torres' Strait, towards India or the Cape of Good Hope.
Advantages of this passage over that round New Guinea.
[EAST COAST. WINDS AND CURRENTS.]
On completing the first portion of the voyage, I entered into an explanation of the winds and currents which had been found to prevail upon the south coast of Terra Australis; and to obtain greater perspicuity and connection, I there anticipated upon the second portion so far as those subjects required. This plan of assembling at the end of each book such general observations upon the coast immediately before examined as could not enter conveniently into the narrative, seeming liable to no material objection, I shall follow it here; and conclude this second part of the voyage with a statement of the winds and currents which appear to prevail most generally along the East and North Coasts; adding thereto such remarks, more particularly on Torres' Strait, as may tend to the safety of navigation. This statement will include the information gained in a subsequent passage, for the reasons which influenced me in the former account; and the reader must not be surprised, should he remark hereafter that I did not, in that passage, follow very closely the directions here given; for besides that my information was then possessed only in part, the directions are intended, not for vessels seeking dangers, which was partly my object, but for those desirous only of navigating these distant shores with expedition and safety.
The East Coast, with respect to winds and currents, requires a division, the part beyond the tropic of Capricorn being placed under different, and almost opposite circumstances, to that within, or close to it. (Atlas, Plate I.)
From Cape Howe, where the South Coast terminates and the East commences, to Sandy Cape, within a degree of the tropic, the south-east trade most generally prevails in the summer season, from the beginning of October to the end of April; and produces sea and land breezes near the shore, with fine weather. There are however many occasional intermissions, especially in the southern parts, wherein gales from South or S. W., and strong breezes between North and N. E., bring heavy rain, with thunder and lightning; but these are usually of short duration. A sultry land wind from the N. W. in the summer, is almost certainly followed by a sudden gust from between S. E. and S. S. W., against which a ship near the coast should be particularly guarded; I have seen the thermometer descend at Port Jackson, on one of these occasions, from 100° to 64° in less than half an hour.
In the winter season, from May to September, the western winds are most prevalent, and generally accompanied with fine weather; the gales then blow from the eastward, between north-east and south, and bring rain with them; indeed there is no settled weather in the winter, with any winds from the sea, and even between north-west and north there is frequent rain, though the wind be usually light in those quarters. It is however to be understood, that the sea and land breezes in the summer are more regular near the tropic; and that the winter winds partake more of the south-east trade than they do from latitude 30° to Cape Howe.
It is a fact difficult to be reconciled, that whilst the most prevailing winds blow from S. E. in summer, and S. W. in winter, upon this extra-tropical part of the East Coast, the current should almost constantly set to the south; at a rate which sometimes reaches two miles an hour. Its greatest strength is exerted near to the points which project most beyond the general line of the coast; but the usual limits of its force may be reckoned at from four, to twenty leagues from the land. Further out, there seems to be no constancy in the current; and close in with the shore, especially in the bights, there is commonly an eddy setting to the northward, from a quarter, to one mile an hour. It is in the most southern parts that the current runs strongest, and towards Cape Howe it takes a direction to the eastward of south; whereas in other places, it usually follows the line of the coast.
This exposition of the winds and currents beyond the tropic, points out the advantage of keeping at not more than three or four leagues from the land, when sailing northward and intending to touch on the coast; but in the winter season this must be done with caution, because gales then often blow from the eastward. A marine barometer will here be of signal advantage. If the weather be tolerably fine, and the mercury do not stand above 30 inches, there is no probability of danger; but when the mercury much exceeds this elevation and the weather is becoming thick, a gale is to be apprehended; and a ship should immediately steer off, until it is seen how far the wind veers to blow dead on the coast. With respect to a rise and fall in the marine barometer, it may be taken as a general rule upon this East Coast, that a rise denotes either a fresher wind in the quarter where it then may be, or that it will veer more to seaward; and a fall denotes less wind or a breeze more off the land; moreover, the mercury rises highest with a south-east, and falls lowest with a north-west wind; and north-east and south-west are points of mean elevation.