(50)

establishment, and rise and fall; nearly similar
to what prevails in Table Bay and Simons Bay.

Magnetic Variation.—This was determined
by means of two magnets on the flat ground
between the light-house and beach. The result
(29 deg. 20 min. W.) has been employed for
the compass bearings given in these Instruc-
tions.

4. Description of the Light Apparatus.
   This light is of the first or most brilliant
   class, and is the work of Mr. Henry le Paute.
   It is constructed upon what is technically
   called the Dioptric principle.

A hollow cylinder with a paraboloid top of
about 4½ feet in circumference, and 10 feet in
height; is composed (omitting a door way) of
28 horizontal tiers of glass and reflectors, firmly
fixed in a metallic frame work of six compart-
ments; so that each tier is divided into six
circular segments.

The centre tier is a large hoop-shaped plano-
convex lens, 11 inches in depth. Next, eight
tiers of circular prisms above and eight below
the centre lens. These several tiers of reflectors
above and four tiers below. Three-fourths of
the circumference of the cylinder is formed by
this combination. The remaining quadrant
which is towards the land, and its centre due
no th, serves for the door way into the cylin-
der; but the door is lined with two large con-
cave metallic reflectors, each two feet in
breadth and three feet in height.

The burner consisting of four concentric
Argand wicks, four inches in diameter, is
placed in the centre of the cylinder in the
focus of the large lens. It is supplied with
oil by a force-pump driven by clock-work; and there is a simple contrivance, which gives
notice by the ringing of a bell when the reser-
voir requires to be replenished.

The framework of the lenticular (the name
given to the whole combination) is firmly at-
tached to a cast-iron circular floor, supported by
a stem of cast iron; the lower end of which is
let into a block of granite resting upon the
crown of the supporting dome, where it (the
stone) is surrounded by masonry.

The theory of the lenticular may be under-
stood as follows:—The burner being in the
focus of the plano-convex lens, the rays of
light that impinge upon the inner plane sur-
face emerge outside horizontal.

The bases of the prisms are perpendicular
inside; their upper planes are horizontal, and
their hypothenusal planes outside slant down-
wards and inwards. The depth of each base
successively diminishes from 3 inches to 1 1-8
inch in depth, in the order from the centre
lens upwards and downwards, which is the
order of the increasing inclination of the im-
pinging rays from the burner. The effect of
the prismatic form, is to turn the rays, and
send them out horizontal, upon the same prin-
ciples by which objects in a horizontal direction
in front of a camera lucida are seen by looking
down into it.

The tiers of reflectors commence at the
height and depression where the still greater
inclination of the impinging rays from the
burner would cause a prism to decompose them
in the horizontal direction into the prismatic
or rainbow colours. The reflectors are con-
cave towards the lens, their curvatures increas-
ing as they recede from the centre lens: They
reflect the rays that fall upon them to the
prisms, whence they are sent out horizontal.

The two large reflectors behind, on the in-
side of the door, return the rays from their
direction to the large lens, through which they escape horizontal.

Thus all the light from the burner, save the
small quantity that escapes up the chimney
glass, is bent into one brilliant horizontal sheet
to overspread the horizon.

This splendid specimen of skill is protected
by a lantern 10½ feet in diameter, or 33 feet
in circumference, glazed with thick plate glass,
and surmounted by a copper dome, from which
a lightning conductor of wire rope descends to
the ground.

The light-house was planned by Lieut.-Col.
Mitchell, the late talented Surveyor-General at
the Cape, who made a journey to Paris for
the express purpose of selecting the lenticular
apparatus.

It may be proper to mention, that the tower
of the light-house is to be painted, and will
show alternate horizontal bands of white and
red, two of each.

Messrs. Rees, Carr, and Skead, of H. M.
Ships “President,” “Nimrod,” and “Dee,”
respectively, particularly Mr. Skead, provided
a large portion of the sailing instructions.
These, with the remainder by myself, I have
arranged and compared carefully with the
chart recently constructed from measurement
and rigorous trigonometrical calculation. The
other matter is by myself, and I hold myself
responsible for the accuracy of the whole.

I may add, that one of the trigonometric
stations in the measurement of the arc of the
meridian is on the highest point of the hill
above the light-house; by means of which, the
latitude and longitude of the light house are
derived from the position of the Observatory.
Thus the light is in an exact geographical
point.

THOMAS MACLEAR.

NOTICE.

THE Wardens of the Hundred of One-
hunga hereby give notice, that all stock
not claimed by persons resident within the
said Hundred, must be removed thence by
the 31st instant.

Onehunga, 17th March, 1849.

NOTICE.

THE Wardens of the Hundred of One-
hunga invite all persons therein resident,
who may be desirous of offering suggestions
relative to the regulations about to be issued
for the Depasturing of Cattle, to do so in writ-
ing, on or before the 31st instant, addressing
the same as follows:

The Wardens of Onehunga,
Post Office,
Onehunga.

Onehunga, 17th March, 1849.