439. The _Planetary Globe_. In this Machine, _T_ is a terrestrial Globe
fixed on its Axis standing upright on the Pedestal _CDE_, on which is an
Hour Circle, having its Index fixed on the Axis, which turns somewhat
tightly in the Pedestal, so that the Globe may not be liable to shake;
to prevent which, the Pedestal is about two Inches thick, and the Axis
goes quite through it, bearing on a shoulder. The Globe is hung in a
graduated brasen Meridian, much in the usual way; and the thin Plate
_N_, _NE_, _E_, is a moveable Horizon, graduated round the outer edge,
for shewing the Bearings and Amplitudes of the Sun, Moon, and Planets.
The brasen Meridian is grooved round the outer edge; and in this Groove
is a slender Semi-circle of brass, the ends of which are fixed to the
Horizon in its North and South Points: this Semi-circle slides in the
Groove as the Horizon is moved in rectifying it for different Latitudes.
To the middle of the Semi-circle is fixed a Pin which always keeps in
the Zenith of the Horizon, and on this Pin the Quadrant of Altitude _q_
turns; the lower end of which, in all Positions, touches the Horizon as
it is moved round the same. This Quadrant is divided into 90 Degrees
from the Horizon to the zenithal Pin on which it is turned, at 90. The
great flat Circle or Plate _AB_ is the Ecliptic, on the outer edge of
which, the Signs and Degrees are laid down; and every fifth Degree is
drawn through the rest of the surface of this Plate towards its Center.
On this Plate are seven Grooves, to which seven little Balls are
adjusted by sliding Wires, so that they are easily moved in the Grooves,
without danger of starting out of them. The Ball next the terrestrial
Globe is the Moon, the next without it is Mercury, the next Venus, the
next the Sun, then Mars, then Jupiter, and lastly Saturn; and in order
to know them, they are separately stampt with the following Characters;
☽, ☿, ♀, ☉, ♂, ♃, ♄. This Plate or Ecliptic is supported by four strong
Wires, having their lower ends fixed into the Pedestal, at _C_, _D_, and
_E_, the fourth being hid by the Globe. The Ecliptic is inclined 23-1/2
Degrees to the Pedestal, and is therefore properly inclined to the Axis
of the Globe which stands upright on the Pedestal.
[Sidenote: To rectify it.]
_To rectify this Machine._ Set all the planetary Balls to their
geocentric places in the Ecliptic for any given time by an Ephemeris:
then, set the North Point of the Horizon to the Latitude of your place
on the brasen Meridian, and the Quadrant of Altitude to the South Point
of the Horizon; which done, turn the Globe with its Furniture till the
Quadrant of Altitude comes right against the Sun, _viz._ to his place in
the Ecliptic; and keeping it there, set the Hour Index to the XII next
the letter _C_; and the Machine will be rectified, not only for the
following Problems, but for several others, which the Artist may easily
find out.