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the index on the hour-circle (No. 7.) shewing the times of these phenomena.

There is a jointed wire, of which one end being put into a hole in the upright stem that holds the Earth's cap, and the wire laid into a small forked piece which may be occasionally put upon Venus or Mercury, shews the direct and retrograde motions of these two planets, with their stationary times and places as seen from the Earth.

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.

398. Another ORRERY. In this machine, which Orrery. is the simplest I ever saw, for shewing the diurnal Plate VI. and annual motions of the Earth, together with the Fig. I.

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 g 2.

g 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 k 2, which turns another pinion 6 2, and that turns a third, which being fixed on a 2, the axis of the Earth U, turns it round, and the earth with it : this last axis inclines in an angle of 23} degrees. The supporter X 2, 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 co- .


vered 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 toward the earth's centre.

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 Wis 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 sup

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porter X 2.

The small wheel E turns the large wheels D and F, of equal diameters, by cat-gut strings crossing between them and the axes of these two wheels are cranked at G and H, above the plate B. The up right 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 toward 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 m 2 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 catgut 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, $ 310, 320.

The following parts of this machine must be considered as distinct from those already described.

Toward the right hand, let S be the earth hung on the wire e, which is fixed into the plate B; and let O be the moon fixed on the axis M, and turning round within the cap P, in which, and in the plate C, the crooked wire Q is fixed. On the axis M is also fixed the index K, which goes round a circle h 2, divided into 29, equal parts, which are the days of the Moon's age: but to avoid confusion in the scheme, it is only marked with the numeral figures 1 2 3 4, for the quarters. As the crank II carries this moon round the earth S in the orbit t, she shews all her phases by means of the cap P for the different days of her age, which are shewn by the index K’; this index turning just as the moon O does, demonstrates her turning round her axis, as she still keeps the same side toward the earth S, $262.

At the other end of the plate C, a moon N goes round an earth R in the orbit p. But this moon's axis is stuck fast into the plate C at S 2, so that neither moon nor axis can turn round; and as this moon goes round her carth, she shews herself all round to it; which proves, that if the Moon was seen all round

from the Earth in a lunation, she could not turn round her axis.

N. B. If there were only the two wheels D and F, with a cat-gut string over them, but not crossing between them, the axis of the earth U would keep its parallelism round the Sun T, and shew all the sea sons; as I sometimes make these machines : and the moon would go round the earth S, shewing her

O phases as above; as likewise would the moon Nround the earth R; but then neither could the diurnal motion of the earth U on its axis be shewn, nor the motion of the moon V round the earth.

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399. In the year 1746 I contrived a very simple The Calmachine, and described its performance in a small Treatise/upon the Phenomena of the Harvest Moon, published in the year 1747. I improved it soon after, by adding another wheel, and called it The Calculator. It may be easily made by any gentleman who has a mechanical genius. The great flat ring supported by twelve pillars, and Plate

VIII. on which the twelve signs with their respective degrees are laid down, is the ecliptic; nearly in the

Fig. I. centre of it is the sun S, supported by the strong crooked wire I; and from the sun proceeds a wire IV, called the solar ray, pointing toward the centre of the earth E, which is furnished with a moveable horizon H, together with a brazen meridian, and quadrant of altitude. R is a small ecliptic, whose plane coincides with that of the great one, and has the like signs and degrees marked upon it; and is supported by two wires D and D, which are put into the plane PP, but may be taken off at pleasure. As the earth goes round the sun, the signs of this small circle keep parallel to themselves, and to those of the great ecliptic. When it is taken off, and the solar ray W drawn farther out, so as almost to touch the horizon H, or the quadrant of altitude, the horizon being rec

tified to any given latitude, and the earth turned round its axis by hand, the point of the wire W shews the sun's declination in passing over the graduated brass meridian, and his height at any given time upon the quadrant of altitude, together with his azimuth, or point of bearing upon the horizon at that time; and likewise his amplitude, and time of rising and setting by the hour-index, for any day of the year that the annual-index U points to in the circle of months below the sun.

M is a solar-index or pointer supported by the wire L, which is fixed into the knob K: the use of this index is to shew the Sun's place in the ecliptic every day in the year; for it goes over the signs and degrees as the index U goes over the months and days; or rather, as they pass under the index U, in moving the cover-plate with the earth and its furniture round the sun; for the index U is fixed tight on the immoveable axis in the centre of the machine. K is a knob or handle for moving the earth round the sun, and the moon round the earth.

As the earth is carried round the sun, its axis constantly keeps the same oblique direction, or parallel to itself, g 48, 202, shewing thereby the different lengths of days and nights at different times of the year, with all the various seasons. And, in one anmuzi revolution of the earth, the moon M times round it from change to change, having an occasional provision for shewing her different phases. The lower end of the moon's axis bears by a small friction-wheel

upon the inclined plane T, which causes the moon to rise above and sink below the ecliptic R in every lunation; crossing it in her nodes, which shift backward through all the signs and degrees of the said ecliptic, by the retrograde motion of the inclined plane T, in 18 years and 225 days. On this plane the degrees and parts of the moon's north and south latitude are laid down from both

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