of bars of steel and brass, adjusted so as to keep the metallic ball always at the same distance from the point of suspension by the difference of their expansions; and by this difference a hand is made to point out the variation of temperature on a divided circle on the top of the rod, which forms a natural thermometer by the sole actions of metals.

On the front of the clock, over the face, is a planisphere marking the age and phases of the moon; in which are besides the day of the week, the names and days of the month and year; and though some months have twenty-eight, thirty, and thirty-one days each, yet the clock makes February to have twenty-nine days every fourth year for bissextile. The mechanism for showing the days of the year will point them out for 10,000 years by means of four indexes, revolving respectively in ten, 100, 1000, and 10,000 years. In this clock and sphere are three contrivances for disengagement: the first for the escapement, the second for the sphere to be detached from the clock work to move by a handle; and the third for detaching the diurnal motion of the earth; and the planets may have a quick motion by means of a corresponding handle. Thus the different portions may be disengaged from each other to make the necessary rectifications. The number of wheels and pinions employed in this mechanism amounts to sixty, some of which are in the interior part of it; the diameter of the sphere is one foot, and is surrounded by a cover of glass; the case of the clock is gilt with four faces, having glass covers neatly designed, well finished, and so exposed to view that all the mechanism may be seen. The whole height from the top of the sphere is seven feet. According to the report of Antide Janvier, who was employed to repair this instrument previously to its being placed in the gallery of the thuilleries at Paris, the following are the numbers of the wheels and pinions, which are put down according to his method of noting them, and also their respective values in time.

The periodic revolution of the moon from a motion of forty-eight hours.

Pinions, 72 25 20 41: 20-27d. 7h. 43′4′′.58"
Wheels, 75 54. 44:31. 735
Revolution of Mercury from a motion of twenty-
seven days seven hours, &c.

Pinions 31 85 Wheels 84 101

}

87d. 23h. 14' 15" 56"

It may be asked,' says Janvier, in a note to his report, where is the revolution of twenty-nine days twelve hours forty-four minutes three seconds which gives motion to this last movement, it being not contained in the preceding statement; but we have seen the periodic revolution, which consists of the time which the moon goes round the heavens; that interval, we know, is shorter than a synodic revolution; but this (synodic) revolution has no real existence, but by means of the earth's change of situation, whose orbit carries a wheel immoveably fixed at the centre of the moon's orbit, round which the moon_revolves really in twenty-nine days twelve hours forty-four minutes three seconds; it is this wheel, then, which gives motion to the wheel-work which represent the eclipses with great accuracy. The last wheel of this movement carries a small dial plate, on which the moon shows her position with respect to her node; this dial carries besides an eccentric piece, which marks the moon's place below and above the plane of the ecliptic within the limits of her greatest latitude.'

The artificial globe serves as a useful instrument for determining, in a rough manner, without calculation, the arrangement of the heavenly bodies at particular times; their places being first ascertained from tables, or, in the case of the sun, merely from a scale of the globe's horizon, or on its surface. We have only to adjust the elevation of the pole of the globe in such a manner that its axis may form the same angle with its horizon, as the axis of the earth does with the real horizon of the place; then, finding a point on its surface corresponding to the place of the sun or planet, we may represent its apparent motion by the motion of this point, and the time occupied by that motion will be shown the length of the day and night, and the time by the index of the globe: thus we may find and place of rising and setting, and, by means of a graduated circle perpendicular to the horizon, we may measure the altitude of the sun or planet at any other time, and also its azimuth or the distance of this circle from the north or south point of the horizon. If we have a ring of any kind parallel to the horizon, and 33′ below it, we may consider this ring as the apparent horizon, allowing for the effects of refraction; if it be still 15' or 16' lower, it will represent the rising or setting of the extreme margin of the sun or moon. We might also have a circle about 1° above either of these which might represent the sensible apparent horizon with regard to the moon, including the correction for her parallax; and a similar ring, placed still lower, would show the duration of twilight, or any supposition that might be formed respecting the depression of the sun required for producing total darkness. By means of the celestial globe the apparent motions of the fixed stars may be represented in a manner nearly similar, proper

attention being paid to the situation of the sun in the ecliptic, as determining the corresponding

time.

Many of these operations may also be performed with equal convenience with a planisphere, which is a stereographical projection of the globe on a plane surface. Professor Bode's planisphere comprehends in one view all the stars that are ever visible at Berlin: he has added to it a moveable circle, representing the horizon of that place, carrying with it the circles of altitude and azimuth, delineated on a transparent paper, which is adjusted, by graduations at the margin of the chart, to the day and hour for which he wished to ascertain the apparent place of the heavenly bodies. Any other chart of the stars, having the pole in its centre, may be applied to a similar use by cutting out a circle, or part of a circle, to represent the horizon of a place of which the latitude is given; and, if the stars are projected as is usual on two equal charts, they must have two equal arcs to represent the respective parts of the horizon belonging to them.

The common orrery or planetarium is represented at fig, 1, plate ORRERY. A brass frame abcd is so contrived as to contain twelve wheels and pinions, actuating one another respectively on such a way as to produce the mean motions of the six primary planets, that have been long discovered, of which our earth is one. A B is a revolving arbor, pivotted into holes made in the upper and lower parts of the frame, and is made to revolve by means of an endless screw, acting with the lowest wheel of 83 teeth, which is made fast to it, as are also the five other wheels

and pinions with the number of teeth specified in the figure. CD is an upright stem of steel wire, screwed fast into the lower plate at C, and ascending through a large hole made in the upper plate, which it does not fill. The six wheels revolving round this stem have each a separate tube, to the inferior end of which they are respectively attached; and the tubes are so contrived that the exterior surface of the innermost forms a stem for the bore of the next largest, till all the six are fitted one within another round the stem of steel, which keeps them in a vertical position while they revolve separately with different velocities. The arbor A B, which receives its motion from the handle by the medium of the endless screw, is assumed as revolving in a year which may be either civil, sidereal, or tropical; but, whichever be the period assumed, the pairs of wheels that act together respectively will be so many fractions of that period. Thus, the lowest wheel of 83 teeth, on the annual arbor, will drive its fellow 20 with its tube, round in of a year, and the little arm that is fixed by friction on its superior end will perform a revolution round the sun in the same time. In like manner, the period of Venus will be performed in of the year; the earth in 8 or one year; Mars in ; Jupiter in; and Saturn in 7; the driving wheels being the denominators of the fraction, and the driven ones the numerators, agreeably to our directions given under the article PLANETARY NUMBERS, for computing the value of wheel-work acting in this simple manner.

If we assume an exact solar year as the period of the first moving or driving wheels, then the fraction will produce the period subjoined, viz. d. h.

S.

m.

4330 10738

Jupiter Saturn

'

If these periods be compared with the true periods, given under PLANETARY NUMBERS,it will be seen that the mean motions produced by this simple mechanism are far from being accurate; and the errors, by continual accumulation, will become sensible in a comparatively short space of time but where general representation only is aimed at, and the respective times of the phenomena exhibited by the planetarium are disregarded, this is the cheapest and simplest construction of a planetary machine that has yet been devised; and, where its imperfection can be dispensed with, its simplicity is no small recommendation. Still, however, the true places of the planets, depending on the equator as well as mean motions of these bodies, are not the places indicated by this planetarium, supposing its mean motions ever so accurate; nor are the motions of the five recently discovered planets attempted to be exhibited, which they might be by additional wheel-work, without altering the construction of the machine. For the use of those who are satisfied with the representation of mean motions only, we beg leave to suggest the numbers suitable for wheels that will produce the Vol. XVI,

both

revolutions of these five diminutive bodies also. Let be taken for Vesta, 4 for Juno, and 13 for Ceres and Pallas, with one tube only for these two pairs; and these wheels, acting in the same way that has been describing, will give the mean revolutions with as much accuracy as the present state of our knowledge of the motions of these little bodies will allow. The planet Herschel would require 335 for its long period, which numbers are impracticable in a small machine; but for this planet a train might be substituted, such as will be described as forming a part of the following machine. When a planetarium of this common construction is fitted up, its appendages are usually alunarium and tellurium, separately adapted to the same stand, but in those the annual and lunar trains are never free from considerable errors.

The late Mr. Ferguson's ingenious orrery may be thus described :

This machine shows the motions of the Sun, Mercury, Venus, Earth, and Moon; and occasionally the superior planets, Mars, Jupiter, and Saturn, may be put on; Jupiter's four satellites are moved round him in their proper times by a

2 A

small winch; and Saturn has his five satellites, and the ring which keeps its parallelism round the Sun.

In the centre, fig. 2, No. 1 represents the Sun, supported by its axis inclining almost 8° from the axis of the ecliptic; and turning round in twenty-five days and a quarter on its axis, of which the north-pole inclines towards the 8° of Pisces in the great ecliptic (No. 11), whereon the months and days are engraven over the signs and degrees in which the sun appears, as seen from the earth, on the different days of the year. The nearest planet (No. 2) to the sun is Mercury, which goes round him in eighty-seven days twenty-three hours, or 87 diurnal rotations of the earth; but has no motion round its axis in the machine, because the time of its diurnal motion in the heavens is not known

The next planet in order is Venus (No. 3), which performs her annual course in 224 days seventeen hours; and turns round her axis in twenty-four days eight hours, or in twenty-four and one-third diurnal rotations of the earth. Her axis inclines 75° from the axis of the ecliptic, and her north-pole inclines towards 20° of Aquarius, according to the observations of Bianchini.

Next without the orbit of Venus is the earth (No. 4), which turns round its axis, to any fixed point at a great distance, in twenty-three hours fifty-six minutes four seconds of mean solar time, but from the sun to the sun again in twenty-four hours of the same time. No. 6 is a sidereal dialplate under the earth; and No. 7 a solar dialplate on the cover of the machine. The index of the former shows sidereal, and of the latter solar time; and hence the former index gains one entire revolution on the latter every year, as 365 solar or natural days contain 366 sidereal days, or apparent revolutions of the stars. In the time that the earth makes 3654 diurnal rotations on its axis, it goes once round the sun in the plane of the ecliptic, and always keeps opposite to a moving index (No. 10), which shows the sun's apparent daily change of place, and also the days of the months.

The earth is half covered with a black cap, for dividing the apparently enlightened half next the sun from the other half, which, when turned away from him, is in the dark. The edge of the cap represents the circle bounding light and darkness, and shows at what time the sun rises and sets to all places throughout the year. The earth's axis inclines 234° from the axis of the ecliptic; the north-pole inclines towards the beginning of Cancer, and keeps its parallelism throughout its annual course; so that in summer the northern parts of the earth incline towards the sun, and in winter from him: by which means the different lengths of days and nights, and the cause of the various seasons, are demonstrated to the sight.

There is a broad horizon, to the upper side of which is fixed a meridian semicircle in the north and south points, graduated on both sides from the horizon to 90° in the zenith, or vertical point. The edge of the horizon is graduated from the cast and west to the south and north points, and

within these divisions are the points of the compass. From the lower side of this thin horizonplate stand out four small wires, to which is fixed a twilight circle 18° from the graduated side of the horizon all round. This horizon may be put upon the earth (when the cap is taken away), and rectified to the latitude of any place : and then, by a small wire, called the solar ray, which may be put on so as to proceed directly from the sun's centre towards the earth's, but to come no farther than almost to touch the horizon, the beginning of twilight, time of sun-rising, with its amplitude, meridian altitude, time of setting, amplitude then, and end of twilight, are shown for every day of the year at that place to which the horizon is rectified.

The moon (No. 5) goes round the earth, from between it and any fixed point at a great distance, in twenty-seven days seven hours forty-three minutes, or through all the signs and degrees of her orbit, which is called her periodical revol tion; but she goes round from the sun to the sun again, or from change to change, in twentynine days twelve hours forty-five minutes, which is her synodical revolution; and in that time she exhibits all the phases already described.

When the above-mentioned horizon is rectified to the latitude of any given place, the times of the moon's rising and setting, together with her amplitude, are shown to that place, as well as the sun's; and all the various phenomena of the harvest moon are made obvious to sight.

The moon's orbit (No. 9) is inclined to the ecliptic (No. 11), one-half being above and the other below it. The nodes or points at 0 and 0 lie in the plane of the ecliptic, and shift backward through all its signs and degrees in eighteen years and two-thirds. The degrees of the moon's latitude, to the highest at N L, north latitude, and the lowest at S L, south latitude, are engraven both ways from her nodes at 0 and 0; and, as the moon rises and falls in her orbit according to its inclination, her latitude and distance from her nodes are shown for every day; having first rectified her orbit, so as to set the nodes to their proper places in the ecliptic, and then, as they come about at different, and almost opposite times of the year, and point twice towards the sun, all the eclipses may be shown for hundreds of years (without any new rectification), by turning the machinery backward for time past, or forward for time to come. At 17° distance from each node, on both sides, is engraved a small sun, and at 12° distance a small moon, which show the limits of solar and lunar eclipses; and when, at any change, the moon falls between either of these suns and the node, the sun will be eclipsed on the day pointed to by the annual index, and, as the moon has then north or south latitude, one may easily judge whether that eclipse will be visible in the northern or southern hemisphere, especially as the earth's axis inclines towards the sun, or from him at that time. And when, at any full, the moon falls between either of the little moons and node, she will be eclipsed, and the annual index shows the day of that eclipse. There is a circle of twenty-nine and a half equal parts, on the cover of the machine, on which an index shows the days of the moon's age.