axis of the ecliptic; by which means, the different lengths of days and nights, and the caufe of the various feafons, are demonftrated to fight. (671.) There is a broad horizon, to the upper fide of which is fixed a meridian femicircle, in the north and fouth points. From the lower fide of this thin horizontal plate ftand out 4 fmall wires, to which is fixed a twilight-circle, 18 degrees from the graduated fide 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 folar ray, which may be put on, fo as to proceed directly from the fun's centre towards the earth's, but to come no farther, than almoft to touch the horizon. The beginning of twilight time of fun rifing, with his amplitude, meridian altitude, time of fetting, amplitude then, and end of twilight, are fhown for every day of the year, at that place to which the horizon is rectified.

(672) The Moon, (N° 5.) exhibits all the phafes already defcribed. When the horizon is rectified to the latitude of any given place, the times of the Moon's rifing and fetting, together with her amplitude, are shown to that place, as well as the fun's; and all the various phænomena of the harveft moon, are made obvious to fight. The Moon's Orbit, (N° 9.) is inclined to the ecliptic, (N° 11.) one half being above, and the other below it. The nodes, or points at o and o, lie in the plane of the ecliptic, as before defcribed, and shift backward, through all its fines and degrees, in 18 years.

(673.) The degrees of the Moon's latitude, to the higheft at NL, (north latitude,) and loweft at SL, (fouth latitude,) are engraven both ways from her nodes at o and o; and, as the moon rifes and falls in her orbit, according to its inclination, her latitude and distance from her nodes are fhown for every day, having firft rectified her orbit, so as to fet the nodes to their proper places in the ecliptic; and then, as they come about, at different, and almoft oppofite times of the year, and then point towards the fun, all the eclipfes may be shown for hundreds of years, (without any new rectification,) by turning the machinery backward, for time paft, or forward for time to

come.

(674.) At 17° distance from each node, on both fides, is engraved a small fun; and at 12 degrees distance, a small moon, which fhow the limits of folar and lunar eclipfes: and when, at any change, the moon falls between either of thefe funs and the node, the fun will be eclipfed on the day pointed to by the annual index, (N° 10.) And when at any full, the moon falls between either of the little moons and node, the will be eclipfed, and the annual index fhows the day of that eclipfe. There is a circle of 294 equal parts, (N° 8.) on the cover of the machine, on which an index fhows the days of the moon's age. There is a jointed wire, of which, one end being put into a hole in the upright ftem that holds the earth's cap, and the wire laid into a small forked piece, which may be occafionally put upon Venus or Mercury, shows the direct and retrograde motions of these two planets, with their ftationary VOL. II. PART II.

times and places, as feen from the earth. The whole machinery is turned by a winch, (N° 12.) and is fo eafily moved, that a clock might turn it, without any danger of flopping.

(675.) Mr JONES'S PLANLTARIUM, Pl. XXXIL fig: 1. reprefents in a general manner, by various parts of its machinery, all the motions and phanomena of the planetary fyftem. This machine confifts of, the Sun in the centre, with the planets, Mercury, Venus, the Earth and Moon, Mars, Jupiter and his four moons, Saturn and his five moons; and to it is occafionally applied an extra long arm for the planet Herfchel and his two moons. To the earth and moon is applied a frame CD, containing only four wheels and two pinions which ferve to preferve the earth's axis in its proper parallelifm in its motion round the fur, and to give the moon her due revolution about the earth at the fame time. Thefe wheels are connected with the wheel-work in the round box below, and the whole is fet in motion by the winch H. The arm M that carries round the moon, points out on the plate B her age and phafes for any fituation in her orbit, and which accordingly are engraved thereon. In the fame manner the arm points out her place in the ecliptic B, in figns and degrees, called her geocentric place. The moon's orbit is reprefented by the flat rim A; the two joints of which, and upon which it turns, denoting her nodes. This orbit is made to incline to any defired angle. The earth of this inftrument is ufually made of a three inch or 14 globe, papered, &c. for the purpofe; and by means of the terminating wire that goes over it, points out the changes of the feafons, and the different lengths of days and nights more confpicuously. This machine is alfo made to represent the Ptolemaic fyftem, or fuch as is vulgarly received; which places the earth in the centre, and the planets and fun revolving about it. This is done by an auxili ary fmall fun and an earth, which change their places in the inftrument.

(676.) The true caufes of the folar and lunar eclipfes are here very clearly feen; for by placing the lamp, fig. 3. upon the centre, in room of the brafs ball denoting the fun, and turning the winch until the moon comes into a right line between the centres of the lamp (or fun) and the earth, the fhadow of the moon will fall upon the earth. On the other fide, the moon paffes (in the aforefaid cafe) through the fhadow of the earth, and is by that means eclipfed. And the orbit A, fig. 1. is fo moveable on the two joints called nodes, that any perfon may eafily reprefent the due pofition of the nodes and intermediate spaces of the moon's orbit; and thence flow when there will or will not be an eclipse of either luminary, and what the quantity of each will be. While the moon is continuing to move round the earth, the lamp on the centre will fo illuminate her, that all her phases, as new, dichotomized, gibbous, full, waining, &c. will be feen juft as they appear in the heavens. All the fame phafes of the earth, as they appear at the moon, will alfo be exhibited. The fatellites of Jupiter and Saturn are moveable only by the hand; yet all their phænomena may be eafily reprefented, excepting the true relative motions and diftances. (6:7.) The

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(677) The IMPROVED CELESTIAL GLOBE, is represented in PLATE XXXII. fig. 4. on the north pole of the axis, above the hour circle, is fixed an arch MKH of 23 degrees; and at the end H is fixed an upright pin HG, which stands directly over the north pole of the ecliptic, and perpendicular to that part of the furface of the globe. On this pin are two moveable collets at D and H, to which are fixed the quadrantile wires N and O, having two little balls on their ends for the fun and moon, as in the figure. The collet Dis fixed to the circular plate F, whereon the 29 days of the moon's age are engraven, beginning juft under the fun's wire N; and as this wire is moved round the globe, the plate F turns round with it. These wires are easily turned, if the fcrew G be flackened: and when they are set to their proper places, the screw ferves to fix them there, fo as in turning the ball of the globe, the wires with the fun and moon go round with it; and these two little balls rife and fet at the fame times, and on the fame points of the horizon, for the day to which they are rectified, as the fun and moon do in the heavens.

(678.) As the moon does not keep her courfe in the ecliptic, but has a declination of 5 degrees en each fide from it in every lunation, her ball may be screwed as many degrees to either fide of the ecliptic as her latitude or declination from the ecliptic amounts to any given time. The horizon is fupported by two, femicircular arches, becaufe pillars would stop the progress of the balls when they go below the horizon in an oblique fphere. To rectify this globe, elevate the pole to the latitude of the place; then bring the fun's place in the ecliptic for the given day to the brazen meridian, and fet the hour index at 12 at noon, that is, to the upper 12 on the hour circle; keeping the globe in that fituation, flacken the fcrew G, and fet the fun directly over his place on the meridian; which done, set the moon's wire under the number that expreffes her age for that day on the Plate F, and the will then ftand over her place in the ecliptic, and fhow what conftellation the is in. Lily, faften the fcrew G, and adjust the moon to her latitude, and the globe will be rectified.

of 29 equal parts, which are the days of the moon's age. The wheel Y has the months and days of the year all around its limb; and in the bar AA is fixed the index 1, which points out the days of the months anfwering the days of the moon's age, fhown by the index, F, in the circie of 294 equal parts at the other end of the bar, On the axis of the wheel is put the piece D, be low the cock C, in which this axis turns round; and in D are put the pencils e and m directly under the earth C and moon M; so that m is carried round e as M is round E.

(680.) Lay the machine on an even floor, pref fing gently on the wheel Y, to cause its spiked feet (of which two appear at P, the third being fuppofed to be hid from the fight by the wheel) enter a little into the floor to fecure the wheel from turning. Then lay a paper about 4 feet long under the pencils e and m, crofswife to the bar; which done, move the bar flowly round the as g of the wheel Y; and as the earth E goes round the Sun S, the moon M will go round the earth with a duly proportioned velocity; and the fric tion wheel W, running on the floor, will keep the bar from bearing too heavily on the pencils and m, which will delineate the paths of the earth and moon. As the index I points out the days of the months, the index F fhows the moon's age an thefe days, in the circle of 29 equal parts. And as this last index points to the different days in its circle, the like numeral figures may be fet to thofe parts of the curves of the earth's path and moon's, where the pencils and m are at these times refpectively, to fhow the places of the earth and moon. If the pencil e be pushed a very little off, as if from the pencil m, to about part of their diftance, and the pencil m pushed as much towards e, to bring them to the fame diftans again, though not to the fame points of space; then, m goes round e, e will go as it were round the centre of gravity between the earth and moon m; but this motion will not fenfibly aler the figure of the earth's path or the moon's.

(681.) If a pin, asp be put through the perc m, with its head towards that of the pin q in the pencil e, its head will always keep thereto as goes round e, or as the fame fide of the moon is till obverted to the earth. But the pin p, which may be confidered as an equatorial diameter d the moon, will turn quite round the port, making all poffible angles with the line of its pr grefs, or line of the moon's path. This is an ocular proof of the moon's turning round be

SECT. II. Of the PRINCIPAL INSTRUMENTS for MAKING ASTRONOMICAL OBSERVATIONS. (682.) In practical aftronomy it is neceffary t have a place conveniently fituated, and futby furnished with proper aftronomical inftrume's It should have an uninterrupted view from L. zenith down to, or even below, the horizon, leaft towards its cardinal points. For this pat pofe that part of the roof which lies in the tion of the meridian in particular, fhould base moveable covers, which may eatily be moved put on again; by which means an inftrument be directed to any point of the heavens between the