408
THE NEW ZEALAND GAZETTE.
at each place. The objection to this course is the
irregularity of instruments and of the personal error
of observers; and the Board therefore recommend a
modification of this plan to be adopted, as more
likely to give reliable results. This is, that by
telegraphic communication with the Observatory the
difference in time of the actual transits of the same
stars over the meridian of the two stations should be
observed on the same clock.

By a few preliminary observations of high and low
stars, the two instruments can be readily adjusted in
the meridian, and very few observations will suffice
to establish the meridian difference required.

With the view of carrying out this determination,
which, as the foundation of all accurate surveys, is
one of the most important works that can be under-
taken in the Colony, the Board strongly urge that
one of the "Sectors" recommended by Mr. Ellery
for this purpose, and also for the determination
of latitudes, should be obtained, and placed in the
hands of a competent observer.

Colonial Museum,
10th August, 1870.

JAMES HECTOR,
Chairman.

Melbourne Observatory,
15th September, 1869.

GENTLEMEN,-I have been instructed by the head of
my Department, to furnish whatever information in
my power to give, that may be required by the Board
of Longitude of Wellington, and I have now the
honour to assure you of the gratification it will
afford me to be of any service to the Board in
obtaining the objects they have in view. I had
several conferences with Mr. G. A. Woods, during
his late visit to Victoria, relative to the determination
of the geographical position of Wellington, and I
believe I am fully informed of the requirements of
the Board, and of the appliances they have at com-
mand. I was requested by Mr. Woods to place in
writing the various suggestions I made to him con-
cerning the several points on which he questioned
me, and I do so with much pleasure.

1st. With regard to the determination of the
longitude of Wellington, I suggested the method I
should adopt in obtaining the longitude of a place
situated as Wellington is, with reference to Green-
wich and to our own observatory-namely, that
transits of the moon's bright limb and the moon-
culminating stars should be observed regularly,
taking care to have the transits of the first and
second limbs nearly equal each lunation; that copies
of these should be sent to Greenwich Observatory
every mail, requesting in return that such corre-
sponding transits as were obtained there might be
forwarded to Wellington; and that in the mean time
a very close approximation indeed to the true longi-
tude could be obtained by calculating the moon's
R.A. from the data given in the Nautical Almanac
under "Moon's Culminations;"; for since the
adoption, within the last few years, of Hansen's
Lunar Tables in the computation of these data, the
tabular errors of the moon's place are exceedingly
small, and not above one-tenth of what they were
eight or nine years ago, when Burchardt's Eleme-
were used.

I do not think that comparing moon culminations
with Melbourne would give a better result than
taking the Nautical Almanac data. I am of opinion
that a far closer determination of the difference of
longitude between Melbourne and Wellington could
be obtained by transmission, several times, of at
least three chronometers between the two places,
always provided the local time at Wellington can be
obtained with precision; the comparisons of these
chronometers should be made without removing them
from on board the ship they may be sent in, which
can be accomplished by means of a journeyman
chronometer or a time-ball. Three or four com-
parisons each way will, I believe, give the difference
of longitude more exactly than a long series of moon
culminations.

I pointed out the importance of determining the
latitude as well as the longitude, and would suggest
the method of transits in the prime vertical with
a good transit instrument, as the best where a
transit circle is not available. For this purpose it
becomes necessary to know with great precision the
Polar distance of the stars observed; and as the N.A.
list contains so few that will be available, our
Observatory would be able to render direct assistance
with its transit circle in determining the positions
of the stars not in the N.A. list which may be
observed.

In this method it will be absolutely necessary that
a very delicate level, the value of whose divisions is
accurately known, should be used, as the correctness
of the latitude will depend directly on the precision
of the level.

A very useful instrument for determining latitude,
and especially differences of latitude, is the "Zenith
Telescope," which is now being extensively used in
India for this purpose. Enclosed is a photograph of one
for India made by Messrs. Troughton and Simms: it is
portable, and is very useful in determining latitudes
in surveys; this instrument is described in Loomis'
Astronomy. It appears that the transit instrument
already at the disposal of the Board has no collimation
micrometer. I think it would be advisable to have
one added to the instrument, and a collimation
telescope procured, which should be mounted on a
brick or stone pier a few feet to the North or South;
the collimation error could thus be obtained with
precision at any moment with facility. If it be
possible to get a meridian mark at a distance not less
than a mile and a half on the opposite side to the
collimating telescope, it will be found very useful.

In observing R. Ascensions, it is very necessary to
determine the errors of collimation level and azimuth
with each set of observations, and most especially
with small instruments; the collimation error may
be eliminated by obtaining an equal number of
observations with the pivots in reversed positions;
but in moon culminations this cannot well be done,
and the collimation error should be determined fre-
quently with great precision.

Mr. Woods submitted some questions relative to
instituting observations of terrestrial magnetism in
Wellington. I am of opinion that it would be
highly desirable to establish a system of magnetic
observations at Wellington: the result from such a
locality would be highly interesting, and of great
scientific value. At the Melbourne Observatory,
monthly determinations of the declination, dip, and
horizontal force are made; and we have also a set of
the Kew magnetographs (self-registering by photo-
graphy), which gives a continuous record of the
variations of these three forces. This, of course,
involves a considerable amount of work. The
monthly absolute determinations, however, with the
new Kew pattern instruments, only occupy about-
eight hours every month, and these are the only
magnetic observations taken at most Observatories
where the subject of terrestrial magnetism is enter-
tained at all. For these observations, a magnetic
theodolite and dip circle are required-the new Kew
pattern is best by far. They cost about £90
together. A small wooden building, free from iron,
having two small stone piers free from the floor, is all