Of the winds and currents on the south coast of Terra Australis,
and in Bass' Strait.
Usual progress of the gales.
Proper seasons for sailing eastward,
and for going westward:
best places of shelter in each case,
with some instructions for the Strait.
[SOUTH COAST. WINDS AND CURRENTS]
Before entering upon the second part of the voyage, it seems proper to
give an account of the winds and currents which prevailed upon the South
Coast; and to add thereto such other general information as may be useful
in rendering the navigation more safe and expeditious, both along the
coast and through Bass' Strait.
The rate and direction of the currents here described, are deduced from
the daily positions of the ship by astronomical observation, compared
with those given by a log kept in the common way, but with somewhat more
than common attention. In the observations, however, there may be some
errors, and a log cannot be depended upon nearer than to five miles in
the distance, and half a point in the course for the twenty-four hours;
and consequently this account of the currents must be taken as subject to
the sum, or to the difference of the errors in the observations and log;
though it is probable they may have been diminished by taking the medium
of several days, which has always been done where it was possible.
Besides the difficulty there is in obtaining the exact rate and direction
of a current, it is known that a continuance of the wind in any
particular quarter may so far change its rate of moving, and even its
direction, that at another time it may be found materially different in
both. Of the probability of these changes the commander of a ship must
form his own judgment, from the winds he may have previously experienced;
and he will consider what is here said upon both winds and currents, as
calculated and intended to give him a general notion, and no more, of
what may usually be expected upon the South Coast.
(Atlas Plate I.)
Several days before making Cape Leeuwin, I experienced a current setting
to the northward, at the rate of twenty-seven miles per day; but at the
mean distance of forty leagues, west-south-west from the cape, the
current ran north-east, twenty-two miles; and when the ship got in with
the South Coast, I found it setting N. 70° E., at the average rate of
twenty-seven miles per day: this was in the month of December. On
approaching Cape Leeuwin in May, from the north-westward, the current for
five days was ten miles to the east; but at forty leagues from the cape,
it ran N. 35° E. fifteen miles; and from the meridian of the cape to past
King George's Sound, the current set east, twenty-seven miles per day,
nearly as it had before done in December. Captain Vancouver and admiral
D'Entrecasteaux do not speak very explicitly as to the currents; but it
may be gathered from both, that they also experienced a set to the
eastward along this part of the South Coast.
The winds seem to blow pretty generally from the westward at Cape
Leeuwin. In the summer time, they vary from north-west in the night, to
south-west in the latter part of the day, though not regularly; and in
the winter season this variation does not seem to take place. A long
swell of the sea, called _ground swell_ to distinguish it from the
lesser, variable one of the surface, appears to come at all times from
the south-westward, which indicates that the strongest and most durable
winds blow from that quarter; and this was partly confirmed by our
experience, for whenever it blew hard, the wind was at, or near to
south-west.
It is from the superior strength and apparent prevalence of this wind,
that the currents in the neighbourhood of Cape Leeuwin may be explained.
The sea being driven in from the south-west, and meeting with the cape,
will necessarily be divided by it, and form two currents, which will
follow the directions of the land; one branch will run northward, along
the west coast of Terra Australis, and the other eastward along the South
Coast: our present business is to follow the latter current.
If a line be drawn from the south-western extremity of New Holland, to
King's Island in Bass' Strait, it will show where the current may be
expected to run strongest; though it will not be equally strong at those
parts of the line which are distant from the land, as at those in its
immediate vicinity. In drawing another line, from the north-eastern isles
of the Archipelago of the Recherche to Cape Northumberland, we shall have
what will commonly be the northern boundary of the current; for within
this line the water does not seem to run in any constant direction, but
is moved according as the wind may happen to blow. This was found by
admiral D'Entrecasteaux; and is conformable to my experience, as I shall
now explain.
It has been said, that the eastwardly current was found in May and
December to run twenty-seven miles per day, from Cape Leeuwin past King
George's Sound. From thence to a little beyond the Archipelago of the
Recherche, keeping in with the shore, I found it to set north-east
thirteen miles; and at a distance from the coast, it ran
north-east-by-east sixteen miles per day, the wind being more from the
south than from the northward in both cases.
In coasting from the Archipelago, all round the Great Bight and as far
south-eastward as to Cape Northumberland, I had no determinate current;
it generally followed the impulsion given to it by the winds, and was
inconsiderable. From the middle of January to the middle of April, the
winds were most prevalent from south-south-east to east-north-east;
coming more from the land at night, and from the sea in the day time.
They seldom had any strength; whereas the winds which occasionally blew
from the westward were fresh, and sometimes became gales, veering in that
case, invariably to the south-west.
On reaching Cape Northumberland I again found the eastwardly current; and
from thence into Bass' Strait it ran N. 80° E., at the rate of twelve
miles a day, the wind blowing strong from the south-westward in the
latter part of the time.
In a subsequent run across the Great Bight in May, from the Archipelago
nearly direct for Bass' Strait, the current set upon the average, N. 39°
E. fourteen miles a day; appearing to be much influenced in its northern
direction by the winds blowing strong from the southward. Mr. Dalrymple,
in reasoning from the analogy of southern Africa, expected that the winds
upon this coast would be found to blow from the northward, or off the
shore, _in the winter time_, and this might possibly be the case if close
in with the land; but at a distance from it, as just observed, the winds
were from the southward.
Such an accumulation of water forcing itself through Bass' Strait, would
naturally lead to the expectation of finding a strong current there,
setting to the east; but on the contrary, the set in common cases was
found to be rather in the opposite direction, the current appearing to be
predominated by the tides, whose superior strength forced it below the
surface. The flood comes from the eastward; and after making high water
at Furneaux's Isles, passes on to Hunter's and King's Islands, where it
meets another flood from the southward; and the high water then made
seems to be nearly at the time that it is low water at Furneaux's Isles.
Another flood is then coming from the east, and so on; whence a ship
going eastward through the Strait, will have more tide meeting than
setting after her, and be commonly astern of her reckoning. This applies
more especially to the middle of the strait, and is what I there found
with winds blowing across it; but the bight on the north side, between
Cape Otway and Wilson's Promontory, seems to be an exception, and in
fact, it lies out of the direct set of the tides. In running from Port
Phillip to the Promontory I was set S. 73° E., thirty-five miles in the
day; but it then blew a gale from the west and south-westward.
Although the eastwardly current be not commonly found at the surface in
Bass' Strait, it is not lost. Navigators find it running with
considerable strength, when passing the strait two or three degrees to
the east of Furneaux's Islands; and it was this current so found, which
led admiral Hunter to the first opinion of the existence of an opening
between New South Wales and Van Diemen's Land.
Every thing in Bass' Strait bespeaks the strongest winds to come from the
south-west; and there is reason to believe that during nine months of the
year, it generally blows from some point in the western quarter. In
January, February, and March, eastern winds with fine weather seem to be
not uncommon; but there is no dependence to be had on them at any other
season. At the eastern side of the strait and of Van Diemen's Land, it is
not unusual to meet a north-east or north wind, though it seldom blows
strong. The gales usually come from between south-west and south-east,
and most frequently from the latter direction; which renders it hazardous
to approach the coast between Cape Howe and Wilson's Promontory.
Thus, speaking generally of the south coast of Terra Australis, it may be
considered that during the six or eight winter months, the winds blow
almost constantly from some western point; and that gales of wind at
south-west are frequent. The progress of the gales is usually this: the
barometer falls to 29½ inches, or lower, and the wind rises from the
north-westward with thick weather, and commonly with rain; it veers
gradually to the west, increasing in strength, and the weather begins to
clear up so soon as it has got to the southward of that point; at
south-west the gale blows hardest, and the barometer rises; and by the
time the wind gets to south or south-south-east, it becomes moderate, the
weather is fine, and the barometer above 30 inches. Sometimes the wind
may return back to west, or something northward, with a fall in the
mercury, and diminish in strength, or die away; but the gale is not over,
although a cessation of a day or two may take place. In some cases, the
wind flies round suddenly from north-west to south-west; and the rainy,
thick weather then continues a longer time.
Such is the usual course of the gales along the South Coast and in Bass'
Strait; but on the east side of the strait the winds partake of the
nature of those on the East Coast, where the gale often blows hardest
between south and south-east. and is accompanied with thick weather, and
frequently with heavy rain.
In the four or five summer months, the south-east and east winds appear
to be most prevalent all round the Great Bight; but even there, the
western winds sometimes blow at that time, and usually with considerable
strength. Thus I had a strong south-west wind in the middle of February,
near the Investigator's Group, and a gale from the same quarter in March,
at the entrance of Spencer's Gulph; which last was felt still more
severely in Bass' Strait by captain Baudin. At the two extremities of the
coast, that is, in the strait and near King George's Sound, the winds
blow sometimes from the west and sometimes from the eastward, in the
summer; but the strongest winds are from the south-west.
It will hence appear, that the summer is alone the proper time for a ship
to come upon, and still more so for exploring the south coast of Terra
Australis; whether she proceed along it from west to east, as I did in
the Investigator, or from east to west, as captain Baudin, seems to be
almost a matter of indifference. From Cape Leeuwin to the end of the
Archipelago of the Recherche, and from Cape Northumberland to Bass'
Strait, it is perhaps most advantageous to proceed eastward, on account
of the current; but in the intermediate and more considerable part of the
coast, a western route is certainly preferable. It has also this general
advantage, that the winds which are fair for running along the coast are
those that blow moderately, and are accompanied with fine weather, most
proper for making a survey; whereas those favourable to the opposite
route frequently blow strong, and render it dangerous to keep in with the
land. As to making a survey of the South Coast in the winter season,
which had been judged from theoretical analogy to be the most proper
time, it appears to be not only a dangerous experiment, but also one from
which very little accuracy of investigation could be expected; and with
as much ardour as most men for such pursuits, I should very unwillingly
undertake the task.
[SOUTH COAST. SAILING DIRECTIONS.]
These observations upon the danger of sailing along the South Coast in
the winter season, are not meant to apply to the commander of a ship
desirous of going eastward through Bass' Strait, and of seeing no more of
the land than is necessary to assure his situation. The strait may be
passed without more than very common danger, at any time of the year,
provided that the navigator be certain of his latitude before approaching
the longitude of 143½°; he should not, however, enter the strait in the
night, unless he have previously seen the land, or be certain both of
latitude and longitude. The parallel of 39°, or 39° 20', according as the
wind may incline, is the best for taking a ship between King's Island and
Cape Otway; and a sight of either, or preferably of both, will point out
his position on the chart. The sole danger to be apprehended here, is the
Harbinger's Reefs, two patches lying nearly two leagues out from the
north end of King's Island; but are so far separated from it, and from
each other, as to leave practicable passages between them, where the
shoalest water found by the Cumberland schooner was 9 fathoms.
(Atlas Plate VI.)
When the position of the ship at the entrance of the strait is
ascertained, a course should be shaped for Curtis' Island, which will be
visible ten or eleven leagues from the deck in fine weather; and as the
distance is between forty and fifty leagues, and nothing lies in the way,
a part of it may be run in the night, with a good look-out. I would
afterwards pass on the south side of Kent's Groups, at not a greater
distance from the largest than two leagues; and then steer
east-north-east by compass, if nearly before the wind, or on either side
of that course as the wind may incline; but taking care not to approach
the northern Long Beach.
In case of meeting with a continuance of foul winds, the most convenient
places in the strait for anchorage, when going eastward, are these:
1st. Under the north-west end of King's Island, near the New Year's
Isles. Of this anchorage I know only, besides what is given in the chart,
that the brig Harrington there rode out a gale from south-west, the heavy
sea being broken off by the New Year's Isles; and the shelter from
eastern winds must certainly be much more complete.
2nd. Port Phillip; anchoring just within the entrance, on the south side.
When a fair wind comes, a ship can get out of the port by means of the
strong tides.
3rd. Hunter's Isles, between Three-hummock and Barren Islands; taking
care not to anchor too close to the weather shore, lest the wind change
suddenly.
4th. The bight between Wilson's Promontory and Cape Liptrap, _in case of
necessity_; but I would not recommend this place, it being very dangerous
should the wind shift to south-west.
5th. Kent's large Group for brigs and lesser vessels; in one of the small
sandy coves under the eastern island.
6th. Furneaux's Isles, between Clarke's and Preservation Islands. If the
ship be not able to weather Clarke's Island, and pass out to the
south-eastward when the fair wind comes, she may run through Armstrong's
Channel, with a boat ahead and a good look-out.
This is all that it seems necessary to say for the information of a
commander desirous of going eastward through Bass' Strait; and with the
chart in the Atlas, (Plate VI.), it is all that a man of moderate
experience and judgment will desire. I have not mentioned the entrance to
the strait between King's Island and Hunter's Isles, thinking it not to
be recommendable; both on account of _Reid's Rocks_, which lie in the
passage, and whose position is not well ascertained, and also because I
am not satisfied that Hunter's Isles are placed in the chart at their
true distance from King's Island: the difference of longitude is from an
approximation only; but the error, if any, cannot exceed eight or ten
miles, and is in excess. However, with daylight and a good look-out, the
strait may be safely entered by this pass, at any time that a ship can
carry sail upon a wind. I entered this way in the Investigator, during
the night; but what a ship on discovery may do is not to be given as an
example to others, whose sole objects are expedition and safety. The
outlet by the pass called Banks' Strait, between Furneaux's Islands and
Cape Portland, is perfectly safe; but is out of the way for a ship bound
to Port Jackson.
It has been observed that the winds are commonly favourable for making a
passage to the _westward_, through Bass' Strait and along the South
Coast, in the months of January, February, and March. I have no personal
experience of such a passage, further than through the strait, though it
has lately been made several times; but to those who may be desirous of
doing the same, and are strangers to these parts, the following
observations may be acceptable.
The first remark is, that the three months when this passage is most easy
to be made, are precisely those in which it is unsafe, if not
impracticable to go through Torres' Strait; and the second, that it will
generally be of no avail for a ship to be in Bass' Strait before the
middle of December, and if it be the middle of January it will be
preferable.
Ships coming from Port Jackson, or anywhere from the north-eastward, may
take a departure from Cape Howe in 37° 30½' south and 150° 5' east; but
from thence, they should not steer a course more westward than
south-south-west by compass, until in latitude 39° 30'; on account of the
danger to be apprehended from south-east winds upon the Long Beach.
Having reached 39° 30' they should steer a true west course, or
west-by-south by compass, leaving the Sisters, the craggy islet, and a
rock, on the larbord hand. The eastern island of Kent's large Group,
which lies in 39° 30' south, 147° 19' east, and may be seen ten, or
perhaps twelve leagues from the deck in fine weather, will come in sight
ahead; and in passing three or four miles on the south side, the small
western group will be seen, and is to be passed in the same way; as are
Curtis' peaked Isles, which will then be in sight. From Curtis' Isles to
the north end of King's Island, the course is nearly true west, and
distance about forty-two leagues, with nothing in the way; but it is
better to steer five or six leagues to the north of King's Island, if the
winds permit. Should they hang to the westward of north, the course may
be safely directed for Three-hummock Island; passing afterwards to the
north or south of King's Island, as the winds may be most favourable.
In the case of foul winds, which, if the weather be thick or rainy, may
be expected to fix at south-west and blow strong, there are many places
where a ship may anchor, to wait a change; but the following appear to be
the most convenient.
1st. Hamilton's Road, at the east end of Preservation Island.
2nd. On the south side of the largest Swan Isle, for small vessels, or
under Isle Waterhouse.
3rd. Port Dalrymple.
4th. Various places amongst Hunter's Isles.
5th. Sea-elephant Bay, on the east side of King's Island, where there is
fresh water; or under the north-east end of that island, if the wind be
from south-west.
6th. Western Port, under Phillip Island; anchoring so soon as the ship is
sheltered. A fair wind for going onward through the strait, will take a
ship out of this port.
7th. Port Phillip.
(Atlas Plate I.)
After clearing Bass' Strait, I think it most advisable to keep at not
more than ten or twenty leagues off the coast, from Cape Otway to
Kangaroo Island; as the wind may there be expected more favourable, and
the contrary current less strong than in steering a straight course
toward Cape Leeuwin. But should the wind rise from the north-westward,
with thick weather and a descent more than usually rapid in the marine
barometer, a stretch off shore should immediately be made, to prepare for
a south-west gale. A look-out must be kept for an island lying to the
west-south-west of Cape Northumberland; it was seen by Mr. Turnbull,
commander of the Britannia, south whaler, but the weather being thick,
its situation was not well ascertained. According to the best information
I could procure, this island lies in 38¼° south, and about 139½° or ¾
east longitude.
From Kangaroo Island, a straight course may be made for the southernmost
part of the Archipelago of the Recherche; but should the winds come from
the westward and not blow a gale, or be light and unsteady, I would steer
more northward, nearer to the land, in the hope of having them more
favourable. From the Archipelago to Cape Leeuwin it seems best to keep at
a distance from the land, unless under the necessity of stopping in that
neighbourhood; for the current runs strong near the shore, and with the
advantage of an offing of twenty or thirty leagues, a ship may lie clear
of the cape with a wind which might otherwise keep her beating for many
days.
There appears to be no place of shelter against western gales, between
Bass' Strait and Kangaroo Island; but there are then, besides various
anchorages under that island, the bays and coves at the entrance of
Spencer's Gulph; and further westward, Coffin's Bay, Petrel Bay in the
island St. Francis, and Fowler's Bay near the head of the Great Bight.
Afterwards come Goose-Island Bay, Thistle's Cove, and the lee of
Observatory Island, all in the Archipelago of the Recherche; the cove
cannot be entered in a gale, but when once secured in the south-west
corner, a ship will be safe; the other two places afford very indifferent
shelter from strong winds, and are indeed fit only for a temporary
anchorage in moderate weather. Doubtful-Island Bay and King George's
Sound afford complete shelter against western gales; but some little time
would be lost in getting out of them, if a ship waited until an eastern
wind set in. Some account of all these places will be found in the
preceding pages of this volume; with the exception of Observatory Island,
for which D'Entrecasteaux's voyage may be consulted.
APPENDIX.
Account of the observations by which the Longitudes of places on the
south coast of Terra Australis have been settled.
The lunar distances and other observations taken in the Investigator's
voyage having been ordered by the Commissioners of the Board of Longitude
to be recalculated by a professed astronomer, with every degree of
correctness which science has hitherto been able to point out as
necessary, this delicate, but laborious task was assigned to Mr. John
Crosley, formerly assistant at the Royal Observatory at Greenwich; a
gentleman who formed part of the expedition as far as the Cape of Good
Hope, but whose ill health had then made it necessary to relinquish the
voyage and return to England. The data and results of all the
observations will probably be made public, by order of the Commissioners;
but in the mean time, for the satisfaction of the geographer, and more
especially for that of the seaman, whose life and property may be
connected with the accuracy of the charts., the _results of the lunar
distances_ observed upon each coast are added in the form of an Appendix
to the volume wherein that coast is described. It is by these results
that the time keepers have been regulated; and the longitudes used in the
construction of the charts are taken from the time keepers.
To appreciate the degree of confidence to which these results may be
entitled, it is necessary to know under what circumstances the
observations were taken; also the method used in the calculations, and
the corrections which have been applied beyond what is usual in the
common practice at sea: of these the following is a general statement.
1st. The instruments used in taking the distances, were a nine-inch
sextant by Ramsden, and three sextants of eight inches radius by
Troughton, the latter being made in 1801, expressly for the voyage. On
board the ship, the sextant was necessarily held in the hand, and the
distances were sometimes so taken on shore; but in most of the latter
cases, it was fixed on a stand admitting of the sextant being turned
easily in any direction. The telescopes were of the largest magnifying
powers which the motion of the ship, or state of the atmosphere could
admit, and each longitude is the result of a set of observations, most
generally consisting of six independent sights. They were taken either by
lieutenant Flinders or by myself; those by him being designated in the
column of _Observers_ by the letter F, the others by C.
2nd. Preparatory to the reduction of the apparent to the true distance,
the four following corrections have been applied.
From the _sun's semi-diameter_, as given in the nautical almanack, 3"
have been subtracted. In the almanacks of the years comprehending our
observations, the semi-diameter was stated from Mayer's tables, which
gave it 3" too great; owing to the imperfection of the telescope with
which Mayer observed.
The semi-diameters of the sun and moon being less in the vertical, than
in the horizontal direction, on account of the differences in the
refraction, they have been reduced proportionally to these differences
and to the angles at the points of contact in measuring the distance.
This correction is called _contraction of the semi-diameter_.
Before using the _moon's horizontal parallax_ in the nautical almanack,
where it is calculated for the equator, it has been corrected (printed as
'diminished', and corrected in the errata) by a number of seconds
depending upon the latitude of the place, and upon this assumed position:
that the earth is a regular spheroid, whose polar axis is to the
equatorial axis, as 320 to 321. This, and the preceding correction are
unnecessary, unless where great exactness may be required.
The _refraction_ of the heavenly bodies given in the tables, being
calculated for a mean height of 50° of Fahrenheit's thermometer, and 29.6
inches of the barometer, it has been corrected for the difference between
these means and what was the state of the atmosphere at the time of
observation.
3rd. In reducing the apparent to the true distance, Mr. Crosley has used
the method of Joseph Mendoza de Rios, Esq., F. R. S., given with his
_Nautical Tables_, second edition, 1809; and the tables from which the
corrections were taken and the computations made, are those of the same
valuable work.
4th. The reduced distance, found as above, has been corrected to the
spheroidal figure of the earth, according to the theory explained in the
_Philosophical Transactions_ of the Royal Society of 1797; and for doing
which, rules are given by Mr. Mendoza with his _Nautical Tables_ of 1801.
This calculation is tedious, and the correction, more especially in low
latitudes, too small to be necessary in common cases.
5th. In the nautical almanack the distances are given to every three
hours, but the irregularities of the moon's motion being such as to cause
some inequality in the different parts of this interval, the distance at
the hour preceding, and at the hour following the time of observation,
was found by interpolation from the two nearest given on each side; and
having the distances at Greenwich for each hour, the observed distance
can never fall more than half an hour from one of them; and the moon's
inequalities do not then produce any sensible error in the corresponding
time, as obtained from common proportion. The correction arising from
this process is seldom so important as to be necessary in sea
observations.
6th. The longitude deduced from a comparison of the true distance at
observation with the hourly distances at Greenwich, is contained in the
following tables under the head of _Longitude from Nautical Almanack_.
But as it frequently happened, that the observation was not taken exactly
in the place which it is intended to fix, this longitude is reduced to
that place by the application of the difference shown by the time keepers
to have existed between the two situations. In ascertaining this
difference, the rates of going allowed to the time keepers are generally
those found at the place which is to be fixed; whether applied to
observations taken before arriving, or after quitting that place. This,
however, could be done only at those stations where rates had been
observed; at the intermediate points, where the result of lunar distances
is given principally as an object of comparison with the time keepers,
the rates allowed in the reduction are those found at the station
previously quitted; but then the difference of longitude is corrected by
the quantity consequent on the following supposition: that the time
keepers altered their rates from those at the previous, to those at the
following station, in a ratio augmenting in arithmetic progression. The
difference of longitude, thus corrected when necessary, is given under
the head of _Reduction by time keepers_; and the longitudes reduced by it
to the place intended to be fixed, are taken to be of equal authority
with those resulting from observations made in the place itself.
7th. But these longitudes, whether reduced to, or observed in, the place
to be fixed, still require a correction which is of more importance than
any of those before mentioned. The theories of the solar and lunar
motions not having reached such a degree of perfection as to accord
perfectly with actual observation at Greenwich, the distances calculated
from those theories and given in the almanack become subject to some
error; and consequently so do the longitudes deduced from them. The
quantities of error in the computed places of the sun and moon, have been
ascertained at Greenwich as often as those luminaries could be observed;
and Mr. Pond, the astronomer royal, having permitted access for this
purpose to the table of errors kept in the Observatory, Mr. Crosley has
calculated the corresponding effects on the longitude, and proportioned
them to the time when our observations were taken. The combined effect of
the two errors forms a correction to the longitudes obtained from the sun
and moon; but when the moon was observed with a star, then the moon's
error alone gives the correction. But it has sometimes happened, that
there were many days interval between the observations of the moon at
Greenwich, and that the errors preceding and following are so extremely
irregular, that no accuracy could be expected in reducing them by
proportion; in these unfortunate cases, that part of the error belonging
to the moon has been taken absolute, such as it was found on the day
nearest to the time of observation; but the sun's error is always from
proportion. These corrections, with the interval in the Greenwich
observations of the moon, are given under their proper heads.
8th. The longitudes thus computed, reduced to the intended point, and
corrected, are placed under each other; and the mean of the whole is
taken to be the true longitude of that point, unless in certain cases
where it is otherwise expressed. The mean is also given of the longitudes
_uncorrected_ for the errors of the sun and moon's places, that the
reader may have an opportunity of comparing them; and some sea officers
who boast of their having never been out more than 5', or at most 10',
may deduce from the column of corrections in the different tables, that
their lunar observations could not be entitled to so much confidence as
they wish to suppose; since, allowing every degree of perfection to
themselves and their instruments, they would probably be 12', and might
be more than 30' wrong.
In the nautical almanacks for 1811 and 1815, the distances are computed
from the new tables of _Burg_ for the moon, and of _Delambre_ for the
sun; and it is to be hoped that the necessity of correcting for errors in
the distances at Greenwich will have ceased, or be at least greatly
diminished. Should the computed places of the sun and moon be happily
found to agree with actual observation, and supposing that our results
may be taken as the average of what practised observers with good
instruments will usually obtain when circumstances are favourable, then
lunar observations taken in 1814 and afterwards, may be entitled to
confidence within the following limits:
From one set of distances, consisting of six independent sights, the
error in longitude may be 30' on either side; but will probably not
exceed 12'.
From six sets on one side of the moon, each set consisting as above, the
error may be 20'; but not probably more than 8'.
Twelve sets of distances, of which six on each side of the moon, are not
likely to err more than 10' from the truth; and may be expected to come
within 5'.
The error in sixty sets, taken during three or four lunations, and one
half on each side of the moon, will not, I think, be wrong more than 5';
and will most probably give the longitude exact to 1' or 2', This degree
of accuracy is far beyond what the hopes of the first proposers of the
lunar method ever extended, and even beyond what astronomers accustomed
only to fixed observatories will be disposed to credit at this time; but
in thinking it _probable_ that sixty sets of lunar distances will come
within 1' or 2' of the truth, when compared with correct tables, I
conceive myself borne out by the following facts.
In Port Lincoln, I observed an eclipse of the sun with a refracting
telescope of forty-six inches focus, and a power of about two hundred. It
was recalculated by Mr. Crosley from Delambre's and Burckhardt's tables,
the one made four and the other ten years afterwards. The longitude
deduced from the beginning differed only 1' 31.5" from that at the end,
and the mean of both only 1' 17" from _thirty_ sets of lunar distances
corrected for the errors of the tables.
The Spanish admiral D'Espinosa observed emersions of the first and second
satellites of Jupiter in 1793, at Port Jackson, and also an eclipse of
the sun which he recalculated by the tables of Burg. He deduces from
thence the longitude of Sydney Cove to be 151° 12' 45"; and from
forty-four sets of lunar distances by lieutenant Flinders, it would be
151° 11' 49" east.
At Port Louis in the Isle Mauritius, the Abbé de la Caille observed an
eclipse of the sun, the transit of mercury over the sun's disk, and
various occultations of Jupiter's satellites; M. d'Aprés also observed
several occultations; and this place should therefore be well determined.
Its longitude in the Requisite Tables is 57° 29' 15" east; and from
_twenty-seven_ sets of distances taken whilst a prisoner there, I made
it, when corrected for the errors of the tables, 57° 29' 57" east.
In appreciating the degrees of accuracy to which a small or larger number
of lunar distances may be expected to give the longitude, I suppose the
observer to be moderately well practised, his sextant or circle, and time
keeper to be good, and his calculations to be carefully made; and it is
also supposed, that the distances in the nautical almanack are perfectly
correct. As, however, there may still be some errors, notwithstanding the
science and the labour employed to obviate them, it cannot be too much
recommended to sea officers to preserve all the data of their
observations; more especially of such as may be used in fixing the
longitudes of places but little, or imperfectly known. The observations
may then be recalculated, if requisite; the corrections found to be
necessary may be applied; and the observer may have the satisfaction of
forwarding the progress of geography and navigation, after having
contributed to the safety of the ship, and benefit of the particular
service in which he may happen to have been engaged.
The following tables, set out in the book, are not reproduced in this
text version of _A Voyage to Terra Australis_--refer to the _html_
version, available from https://gutenberg.org.
Table I. (Reference from Chapter III., "_Longitude_ from thirty-one sets
of distances of the sun east and west of the moon, of which the
particulars are given in Table I. of the Appendix to this volume")
LONGITUDE OF THE OBSERVATORY IN PRINCESS ROYAL HARBOUR, KING GEORGE'S
SOUND
Table II. (Reference from Chapter III, "The _longitude_ from sixteen sets
of distances of the sun east and west of the moon, of which the
individual results are given in Table II. of the Appendix to this
volume")
LONGITUDE OF THE SOUTH POINT OF LUCKY BAY, ARCHIPELAGO OF THE RECHERCHE
Table III. (Reference from Chapter V., "_Longitude_ of the point, deduced
from twenty-two sets of distances (see Table III of the Appendix to this
volume)")
LONGITUDE OF THE ANCHORAGE IN FOWLER'S BAY
Table IV. (Reference from Chapter VI., "The _longitude_, from thirty sets
of distances of the sun and stars from the moon (see Table IV. of the
Appendix to this volume)")
LONGITUDE OF THE TENTS, AT THE HEAD OF PORT LINCOLN
Table V. (Reference from Chapter IX., "Point Nepean is in _latitude_ 38°
18' south. The _longitude_ from twelve sets of distances taken by
lieutenant Flinders in the port, and six others by me ten days before
arriving, the particulars of which are given in Table V. of the Appendix
to this volume")
LONGITUDE OF POINT NEPEAN AT THE ENTRANCE OF PORT PHILLIP
Table VI. (Reference from Chapter X., "_Longitude_ from forty-four sets
of distances of the sun and moon, of which the individual results are
given in Table VI of the Appendix to this volume")
LONGITUDE OF CATTLE POINT, PORT JACKSON
Table VII. Variations in rate and errors in longitude, made by Earnshaw's
time keepers No. 543 and No. 520, between the Cape of Good Hope and Port
Jackson.
END OF VOLUME I
End of Project Gutenberg's A Voyage to Terra Australis, by Matthew Flinders