442. The ECLIPSAREON. This Piece of Mechanism exhibits the Time,
Quantity, Duration, and Progress of solar Eclipses, at all Parts of the
Earth.
The principal parts of this Machine are, 1. A terrestrial Globe _A_
turned round its Axis _B_ by the Handle or Winch _M_; the Axis _B_
inclines 23-1/2 Degrees, and has an Index which goes round the Hour
Circle _D_ in each rotation of the Globe. 2. A circular Plate _E_ on the
Limb of which the Months and Days of the year are inserted. This Plate
supports the Globe, and gives its Axis the same position to the Sun, or
to a candle properly placed, that the Earth’s Axis has to the Sun upon
any day of the year § 338, by turning the Plate till the given Day of
the Month comes to the fixed Pointer or annual Index _G_. 3. A crooked
Wire _F_ which points towards the middle of the Earth’s enlightened Disc
at all times, and shews to what place of the Earth the Sun is vertical
at any given time. 4. A Penumbra, or thin circular Plate of brass _I_
divided into 12 Digits by 12 concentric Circles, which represent a
Section of the Moon’s Penumbra, and is proportioned to the size of the
Globe; so that the shadow of this Plate, formed by the Sun, or a candle
placed at a convenient distance, with it’s Rays transmitted through a
convex Lens to make them fall parallel on the Globe, covers exactly all
those places upon it that the Moon’s Shadow and Penumbra do on the
Earth: so that the Phenomena of any solar Eclipse may be shewn by this
Machine with candle-light, almost as well as by the light of the Sun. 5.
An upright frame _HHHH_, on the sides of which are Scales of the Moon’s
Latitude or Declination from the Ecliptic. To these Scales are fitted
two Sliders _K_ and _K_, with Indexes for adjusting the Penumbra’s
Center to the Moon’s Latitude, as it is North or South Ascending or
Descending. 6. A solar Horizon _C_, dividing the enlightened Hemisphere
of the Globe from that which is in the dark at any given time, and
shewing at what places the general Eclipse begins and ends with the
rising or setting Sun. 7. A Handle _M_, which turns the Globe round it’s
Axis by wheel-work, and at the same time moves the Penumbra across the
frame by threads over the Pullies _L, L, L_, with the velocity duly
proportioned to that of the Moon’s shadow over the Earth, as the Earth
turns on its Axis. And as the Moon’s Motion is quicker or slower,
according to her different distances from the Earth, the penumbral
Motion is easily regulated in the Machine by changing one of the
Pullies.
[Sidenote: To rectify it.]
_To rectify the Machine for use._ The true time of New Moon and her
Latitude being known by the foregoing Precepts § 355, 363, if her
Latitude exceeds the number of minutes or divisions on the Scales (which
are on the side of the frame hid from view in the Figure of the Machine)
there can be no Eclipse of the Sun at that Conjunction; but if it does
not, the Sun will be eclipsed to some places of the Earth; and, to shew
the times and various appearances of the Eclipse at those places,
proceed in order as follows.
_To rectify the Machine for performing by the Light of the Sun._ 1. Move
the Sliders _KK_ till their Indexes point to the Moon’s Latitude on the
Scales, as it is North and South Ascending or Descending, at that time.
2. Turn the Month Plate _E_ till the day of the given New Moon comes to
the annual Index _G_. 3. Unscrew the Collar _N_ a little on the Axis of
the Handle, to loosen the contiguous Socket on which the threads that
move the Penumbra are wound; and set the Penumbra by Hand till its
Center comes to the perpendicular thread in the middle of the frame;
which thread represents the Axis of the Ecliptic § 371. 4. Turn the
Handle till the Meridian of _London_ on the Globe comes just under the
point of the crooked Wire _F_; then stop, and turn the Hour Circle _D_
by Hand till XII at Noon comes to its Index. 5. Turn the Handle till the
Hour Index points to the time of New Moon in the Circle _D_; and holding
it there, screw fast the Collar _N_. Lastly, elevate the Machine till
the Sun shines through the Sight-Holes in the small upright Plates _O_,
_O_, on the Pedestal; and the whole Machine will be rectified.
_To rectify the Machine for shewing the Candle-Light_, proceed in every
respect as above, except in that part of the last paragraph where the
Sun is mentioned; instead of which place a Candle before the Machine,
about four yards from it, so as the shadow of Intersection of the cross
threads in the middle of the frame may fall precisely on that part of
the Globe to which the crooked Wire _F_ points: then, with a pair of
Compasses take the distance between the Penumbra’s Center and
Intersection of the threads; and equal to that distance set the Candle
higher or lower as the Penumbra’s Center is above or below the said
Intersection. Lastly, place a large convex Lens between the Machine and
Candle, so as the Candle may be in the Focus of the Lens, and then the
Rays will fall parallel, and cast a strong light on the Globe.
[Sidenote: It’s use.]
These things done, which may be sooner than expressed, turn the Handle
backward until the Penumbra almost touches the side _HF_ of the frame;
then turning it gradually forward, observe the following Phenomena. 1.
Where the eastern edge of the Shadow of the penumbral Plate _I_ first
touches the Globe at the solar Horizon, those who inhabit the
corresponding part of the Earth see the Eclipse begin on the uppermost
edge of the Sun, just at the time of its rising. 2. In that place where
the Penumbra’s Center first touches the Globe, the inhabitants have the
Sun rising upon them centrally eclipsed. 3. When the whole Penumbra just
falls upon the Globe, its western edge, at the solar Horizon, touches
and leaves the place where the Eclipse ends at Sun-rise on his lowermost
edge. Continue turning, and, 4. the cross lines in the Center of the
Penumbra will go over all those places on the Globe where the Sun is
centrally eclipsed. 5. When the eastern edge of the Shadow touches any
place of the Globe, the Eclipse begins there: when the vertical line in
the Penumbra comes to any place, then is the greatest obscuration at
that place; and when the western edge of the Penumbra leaves the place,
the Eclipse ends there; the times of all which are shewn on the Hour
Circle: and from the beginning to the end, the Shadows of the concentric
penumbral Circles shew the number of Digits eclipsed at all the
intermediate times. 6. When the eastern edge of the Penumbra leaves the
Globe at the solar Horizon _C_, the inhabitants see the Sun beginning to
be eclipsed on his lowermost edge at its setting. 7. Where the
Penumbra’s Center leaves the Globe, the inhabitants see the Sun set
centrally eclipsed. And lastly, where the Penumbra is wholly departing
from the Globe, the inhabitants see the Eclipse ending on the uppermost
part of the Sun’s edge, at the time of its disappearing in the Horizon §
343.
_N.B._ If any given day of the year on the Plate _E_ be set to the
annual Index _G_, and the Handle turned till the Meridian of any place
comes under the point of the crooked Wire, and then the Hour Circle _D_
set by the hand till XII comes to its Index; in turning the Globe round
by the Handle, when the said place touches the eastern edge of the Hoop
or solar Horizon _C_, the Index shews the time of Sun-setting at that
place; and when the place is just coming out from below the other edge
of the Hoop _C_, the Index shews the time that the evening Twilight ends
to it. When the place has gone through the dark part _A_, and comes
about so to touch under the back of the Hoop _C_ on the other side, the
Index shews the time that the Morning Twilight begins; and when the same
place is just coming out from below the edge of the Hoop next the frame,
the Index points out the time of Sun-rising. And thus, the times of
Sun-rising and setting are shewn at all places in one rotation of the
Globe, for any given day of the year: and the point of the crooked Wire
_F_ shews all the places that the Sun passes vertically over on that
day.