Plate CCCCLXII. Fig. 1. exhibits the wheelwork con tained in the preceding Table, given in section, on a sup position that the concentric tubes are divided longitudi nally, and that one half of each is removed to show the sun's stern in the centre. In our description of this com prehensive machine, we propose to direct the attention of our readers to each separate portion in succession, and to begin with that which would constitute a Planetarium, if the other portions were not included. AABB is a cy lindrical brass box, twelve inches in diameter and three deep, supported by a tripod or claw-pillar of brass, as represented in Plate CCCCLXII. Fig. 5. within which the wheelwork, supported on a strong brass plate CC, is made fast by four small pillars, of which one is dotted, another made black, and the two others concealed from view. The handle that is assumed to revolve in a solar day, enters the side of the box, and taking hold of a ho rizontal arbor II, gives motion to the diurnal arbor F, by means of two small bevel wheels of 24 teeth each. At the lower end of this diurnal arbor (which rests on a small cork below, and carries the daily index on its supe rior pivot) a pinion 15 is fast, which is the first of the annual train for producing a revolution of the arbor E in a solar year. This train lies below the strong plate CC, the wheel 120 and pinion 23 being fast together, and re volving on an arbor descending from the under face of the said plate, and wheel 61 revolving together with pinion 14 on a stud under cock L, (seen in Fig. 3. more clearly,) while the annual wheel 241 is made fast to the annual ar bor E. Thus the communication of motion from the diurnal handle to the annual arbor does not interfere with the planetary wheels, which arc placed above the plate CC. When the wheel 241 is put in motion, it carries round with it the annual arbor, and all the wheels and pi nions attached to it, as being the denominators of all the simple fractions from the earth to Jupiter, both inclusive, viz. 85, 42, 29, 31, 30, and 14. These eight denomina tors, revolving in a solar year, drive each its numerator in the corresponding planetary periods; and as the dri ven, or planetary wheels, have each a tube at their supe rior ends, they also carry their respective planetary arms in their true periods. In the planetarium above recom mended, Venus, Mercury, and Saturn, have each a pair of similar wheels acting in the same manner, so that this part of our description will equally apply to that more simple machine. The first driver, or denominator of Venus's train, 113, is attached to the tubed wheel 85 of the earth, instead of the annual arbor, and drives wheel 63, and under it 58, made fast together round the annual arbor used as a stud, and the 58 in its turn impels the tubed wheel 64 in the period of Venus; and, in like inanner, 78, the first driver of Mercury's train, is made fast to the 64 of Venus, and drives 97 and 108 fast to it, round the annual arbor, as round a stud, loosely, while the 103 is connected with the small wheel 34. This last wheel, therefore, being fast to the innermost tube, re volves in the period of Nlercury, and carries his arm next to the sun. This innermost tube is the longest, and each succeeding tube decreases in length at both ends, for the purpose of making room for the arms above, and wheels below, successively. The motion of Saturn is derived from Jupiter's last wheel thus : Hie first driver 53 is at tached to Jupiter's 166, and, revolving in Jupiter's pe riod, drives 50 and 46 over it together round the annual arbor, while 46 drives the tubed wheel 121 round in Sa turn's period ; and lastly, Herschel's wheel 86 is fast to Saturn's 121, and drives 46, together with 28, round the annual arbor ; and the latter, being connected with 149, carries it round, together with its tube, in Herschel's pe riod. Thus all the primary planets perform their respec tive revolutions, while the annual index, placed on the upper pivot of the revolving annual arbor, indicates the sun's declination and reduction of the ecliptic to the equator ; the day of the month in a quadruple spiral, and the sun's (or earth's) equation of the ccritte, being shown on a circular plate, seen under the sun, in Fig. 5. not yet described.
Hitherto we have considered the revolutions of the planets as derived from the diurnal handle, which is the case only when all the different portions of the orrery are in motion together ; when the planetarium part alone is used, the diurnal handle is disengaged, and a weekly handle is substituted, which actuates those wheels only that are not connected with the diurnal motion of the earth, nor with Jupiter's moons. This handle is more convenient than the diurnal handle when the revolutions alone are exhibited, on account of the increased velocity thus given to the moving bodies. We shall now explain how this change of motion is effected; which will require some consideration to understand, as some of the parts are not visible in the principal figure. The annual train, under the plate CC, is represented as having the calliper, or pivot, in a straight line, that the wheels essential to be shown might all be in view ; but in reality these pivots, or studs, round which the wheels revolve, are situate round the central stem D, which is a piece of strong steel wire nine inches long. The wheel 120, and the pinion 23 fast 120 to it, revolve in 8 days, and the common arbor on 15 which they are fixed carries, above the plate CC, another wheel, 80, not seen in the principal figure, but exhibited in a detailed state in Fig. 2. without the dotted cock M, which is partly seen in Fig. 1. ; then this wheel 80 impels a weekly wheel 70, on a separate arbor, which is also seen in Fig. 2. and the weekly hand V, placed on its up per pivot, is seen on the lid of the box in Fig. 5. which will explain the relative position of this arbor. The pi nion 23, which we have said revolves in 8 days, impels another similar pinion, 23, on a stud, seen only in Fig. 3.
and that impels a third similar one on a vertical arbor pass ing through a brass bar LL, the cranked part of which bar is seen obliquely in Fig. 1. ; this vertical arbor, therefore, turns in eight days also, and a bevel wheel of 32 teeth, fixed on its superior end, drives another bevel wheel of 28 teeth round in seven days, of 8) together with its horizontal arbor, which extends nearly to the edge of the box ; and when the handle is inserted on this last arbor, it consequently gives seven days' mo tion to each of the planets in each turn. The wheel 61, which has been shown to be one of the annual train, be ing supported by a stud on the lower face of bar LL, is capable of being brought into action with the pinion 23 on the arbor of wheel 120, as in Fig. 1. or of being dis engaged from it, while, in both cases, it remains in ac tion with the second pinion 23, carried by the stud under the bar LL; for this bar has a limited motion round a stud, near the annual arbor, which takes the wheel 61, carried by this bar, out of the teeth of pinion 23, attach ed to 120, but suffers the connexion to remain with pinion 23, carried also by the said bar ; hence this wheel 61 can be made to derive its motion from either the diurnal or weekly handle, as may be required, by the mere partial displacement of the bar LL, the exterior end of which is fixed to either of its two positions by a pressing screw under the box ; and in either situation the handle will only take that arbor which corresponds to the given position of the bar. It might be expected that the pinion 14, made fast to wheel 61, would also be disengaged from the large wheel 241 with which it acts, but this effect is not produced by the motion of the bar that bears it ; and that is occasionally displaced, because the centre of the bar's motion is close to the centre of the large wheel, and the pinion 14, instead of being disengaged, only rolls round the circumference of the large wheel, while their teeth continue engaged ; and thus it is always in a state to perform its office, whichever of the two handles may be used.
The next portion of the orrery is the tellurian, on which the changes of day and night, and vicissitudes of the sea sons depend, as well as various other geocentric pheno mena. Near the middle of the vertical diurnal arbor F, is a wheel, 50, which we have not before noticed, and which drives another of 100 teeth, having a long tube at tached to it ; this latter wheel stands next above the an nual wheel 85, and its tube surrounds that of 85, but its motion is much quicker ; its revolution being performed in two solar days. Immediately under the wheels 85 and 113, which revolve together in a solar year, a cock M, seen clotted in Fig. 1. carries another tube, which has no motion, but which ascends above the annual tube, and holds a circular plate, NN, for containing the days of the month, the ecliptic circle, and the sun's equation, cor responding to his mean anomaly ; the earth's motion being equable in this machine. The earth's arm is clamped fast to the annual tube of wheel 35, and a contrate wheel, 96, is made fast by friction on the tube of wheel 100, re volving in two days ; then two equal pinions, of eight leaves each, at the opposite ends of a horizontal arbor, lying under and parallel to the earth's arm, take the mo tion from wheel 96, and give it to another contrate wheel 48 under the earth ; this smaller wheel, therefore, would revolve in an exact solar day if the earth's arm had no mo tion round the sun, and the bevel wheel 36, at the upper end of its vertical arbor, would give the same motion un altered to a similar bevel wheel 36, attached to the earth's inclined axis ; and thus, a turn of the diurnal handle would produce a rotation of the earth. But the earth's arm makes a revolution in the period of the annual wheel 85, or in 365.2423 days, while the pinion 8 is connected with the contrate wheel 96 ; this pillion consequently re 96 wolves =12 times in a year, by virtue of its circuit round the sun, and these twelve revolutions are given to the wheel 48, by the similar pinion of the opposite end of the long horizontal arbor ; hence the wheel 48 receives 2 x 8 in each year 1 — = 2 revolutions, in addition to the 8 365.2423 derived from the diurnal handle ; and therefore makes 367.2423 turns, which it communicates to the earth's axis in the manner above stated ; and the earth would thus make 367.2423 rotations in a year if there ex isted no other modification than what has hitherto ap peared. But we shall see presently, that one rotation in a year is lost by a cause very similar to that which acids two annually. A contrate wheel, 62, is screwed to the fixed plate NN, and the annual arbor carries another long horizontal arbor above it, with a couple of similar pinions at the opposite ends, one of which is made to revolve by means of its connexion with the fixed wheel 62, and the other pinion gives the same motion to a second contrate wheel 62, on a tube that surrounds the arbor of the 48 above described ; and as the teeth of the second 62 stand in an opposite direction to those of the fixed 62, it re volves once in a year in a direction suitable for preserving the parallelism of the earth's axis, which, therefore, is re trograde ; and as the piece P, which supports the earth's axis, is carried backwards once in a year by the second wheel 62, on the tube of which it is fixed, the wheel 36 on the axis is carried round the revolving wheel 36, that is placed on the arbor Of 48, and thus receives one back ward revolution in a year, and deducts a single rotation from the 367.2423 rotations above explained, and, in this way, the resulting number of the earth's actual rotations is 366.2423, viz. a rotation in each sidereal day as in na ture.