The whole Machinery is turned by a winch or handle (No. 12,) and is so easily moved that a clock might turn it without any danger of stopping.
To give a Plate of the wheel-work of this Machine, would answer no purpose, because many of the wheels lie so behind others as to hide them from sight in any view whatsoever.
Another Orrery.
[PLATE VI]. Fig. I.
435. Another Orrery. In this Machine, which is the simplest I ever saw, for shewing the diurnal and annual motions of the Earth, together with the motion of the Moon and her Nodes; A and B are two oblong square Plates held together by four upright pillars; of which three appear at f, g, and g2. Under the Plate A is an endless screw on the Axis of the handle b, which works in a wheel fixed on the same Axis with the double grooved wheel E; and on the top of this Axis is fixed the toothed wheel i, which turns the pinion k, on the top of whose Axis is the pinion k2 which turns another pinion b2, and that other turns a third, on the Axis a2 of which is the Earth U turning round; this last Axis inclining 231⁄2 Degrees. The supporter X2, in which the Axis of the Earth turns, is fixed to the moveable Plate C.
In the fixed Plate B, beyond H, is fixed the strong wire d, on which hangs the Sun T so as it may turn round the wire. To this Sun is fixed the wire or solar ray Z, which (as the Earth U turns round its Axis) points to all the places that the Sun passes vertically over, every day of the year. The Earth is half covered with a black cap a, as in the former Orrery, for dividing the day from the night; and, as the different places come out from below the edge of the cap, or go in below it, they shew the times of Sun-rising and setting every day of the year. This cap is fixed on the wire b, which has a forked piece C turning round the wire d: and, as the Earth goes round the Sun, it carries the Cap, Wire, and solar Ray round him; so that the solar Ray constantly points towards the Earth’s Center.
On the Axis of the pinion k is the pinion m, which turns a wheel on the cock or supporter n, and on the Axis of this wheel nearest n is a pinion (hid from view) under the Plate C, which pinion turns a wheel that carries the Moon V round the Earth U; the Moon’s Axis rising and falling in the socket W, which is fixed to the triangular piece above Z; and this piece is fixed to the top of the Axis of the last mentioned wheel. The socket W is slit on the outermost side; and in this slit the two pins near Y, fixed in the Moon’s Axis, move up and down; one of them being above the inclined Plane YX, and the other below it. By this mechanism, the Moon V moves round the Earth T in the inclined Orbit q, parallel to the Plane of the Ring YX; of which the Descending Node is at X, and the Ascending Node opposite to it, but hid by the supporter X2.
The small wheel E turns the large wheels D and F, of equal diameters, by cat-gut strings crossing between them: and the Axis of these two wheels are cranked at G and H, above the Plate B. The upright stems of these cranks going through the Plate C, carry it over and over the fixed Plate B, with a motion which carries the Earth U round the Sun T, keeping the Earth’s Axis always parallel to itself; or still inclining towards the left-hand of the Plate; and shewing the vicissitudes of seasons, as described in the [tenth chapter]. As the Earth goes round the Sun the pinion k goes round the wheel i, for the Axis of k never touches the fixed Plate B; but turns on a wire fixed into the Plate C.
On the top of the crank G is an Index L, which goes round the Circle m2 in the time that the Earth goes round the Sun; and points to the days of the months; which, together with the names of the seasons, are marked in this Circle.
This Index has a small grooved wheel L fixed upon it, round which, and the Plate Z, goes a cat-gut string crossing between them; and by this means the Moon’s inclined Plane YX with its Nodes is turned backward, for shewing the times and returns of Eclipses § [319], [320].
The following parts of this machine must be considered as distinct from those already described.