RESULTS OF OBSERVATIONS FOR LONGITUDE BY MERIDIAN TRANSITS OF
THE MOON
TAKEN AT THE HUTT OBSERVATORY IN THE YEAR 1870.

No. 1.
Mr. A. F. HALCOMBE to the Hon. W. GISBORNE.

SIR, —

Provincial Secretary's Office,
Wellington, 1st November, 1860.

I have the honor to forward the enclosed paper by the Chief Surveyor of this Province (Henry
Jackson, Esq.), showing the results of observations taken during the present year at the Hutt, for the
purpose of determining an initial longitude for the surveys of the Province.

The Hon. the Colonial Secretary, Wellington.

I have, &c.,
A. FOLLETT HALCOMBE,
Provincial Secretary.

Enclosure in No. 1.
RESULTS OF OBSERVATIONS for LONGITUDE by MERIDIAN TRANSITS of the Moon taken at the
Hutt Observatory in the Year 1870.

THE instrument used for the observations to obtain the longitude at the Hutt Observatory, as given in
the accompanying Table, is a 24-inch transit, of a new construction, by Troughton and Simms. The
standards are half moon shaped and hollow, two inches radius at the base, and tapering slightly towards
the Y's. They are fixed on a massive brass plate, which rests on three levelling foot-screws. The
telescope has five transit wires and a right ascension micrometer wire, the cylindrical head of which is
divided into 100 parts, each indicating 0".61. The axis level has a scale graduated into divisions of 1"
value. The finding circle is 6 inches in diameter, and fixed to one of the pivots. It is read by two
verniers.

The instrument is mounted upon a brick pier built with Portland cement, and sunk 5 feet below
the surface of the ground. The pier is 5 feet 6 inches square at the base. It rises in a pyramidal
shape to the level of the floor of the building, where it is 2 feet 4 inches square. It is then carried
up square to a height of 4 feet above the floor. It is completely isolated from any contact with the
ground or floor of the building from its base.

The building is octagonal, 7 feet 6 inches across, and has a roof revolving on eight cast-iron wheels
upon a circular ring of angle iron. The shutters, one North and the other South, are 1 foot in breadth.
They open upon hinges, and expose the meridian from horizon to horizon.

The collimation error was determined every month by measuring with the micrometer the distance
of the centre transit wire from the meridian pole in both positions of the axis. Ten readings were
taken, the instrument being reversed between each reading. Half the difference between the mean of
the micrometer readings in each position of the axis, multiplied by 0.0407 s, is the collimation error of
the centre wire, to which was applied the reduction to the mean wire. The result is the collimation
error $c$ of the Table. The positive sign is prefixed when the mean wire was West of the collimation
axis.

The level error was determined by reading each end of the axis level in the two positions. The
difference between the sums of the East and West readings divided by 4, and the remainder, multiplied
by 0"067 s., is the level error $b$ of the Table. The positive sign is given when the East readings of the scale
were greater than the West readings. Level error was observed whenever time permitted immediately
after a transit; but when this could not be done, an interpolated value was given to an invervening
transit from the two determinations of level error preceding and following.

The azimuth error was determined for each night's observations in the following manner:—The
difference between the micrometer readings of the collimation axis and centre of the meridian pole was
observed in the evening, and again in the morning. This difference multiplied by 0".61, and applied to
360° 00′ 07"·9 (the bearing of the centre of the meridian pole from the centre of the instrument), gave
the azimuth of the collimation axis morning and evening. A proportional part of the differences
between the morning and evening determinations for the time corresponding to the middle of the night's
observations, was then applied to one of the determinations, and the result, divided by 15, with the
positive sign prefixed when the collimation axis pointed East of North, was adopted for the azimuth
error $a$ of the Table.