PLANETARIUM. 



Squares of the distances increase. The 

 comets are evidently governed by the 

 same law, since they descend with an ac- 

 celerated motion, as they approach to- 

 wards the Sun, and ascend again with a 

 retarded motion, bending their way about 

 the Sun, and describing- equal areas in 

 equal times, by rays drawn from them to 

 his centre. See ASTHOKOMY. 



PLANETARIUM, an astronomical ma- 

 chine, contrived to represent the mo- 

 tions, orbits, &c. of the planets, as they 

 really are in nature, or according to the 

 Copernican system. A very remarkable 

 machine of this sort was invented by 

 Huygens, which is still preserved among 

 the curiosities of the University at Ley- 

 iaen. In this planetarium, the five pri- 

 mary planets perform their revolutions 

 about the Sun, and the moon performs 

 her revolution about the Earth, in the 

 same time that they are really perform- 

 ed in the heavens. Also the orbits of the 

 Moon and planets are represented with 

 their true proportions, excentricity, po- 

 sition, and declination from the ecliptic, 

 or orbit of the Earth. So that, by this 

 machine, the situation of the planets, with 

 the conjunctions, oppositions, &c. may 

 be known, not only for the present time, 

 but for any other time, either past or yet 

 to come, as in a perpetual ephemeris. 

 There was exhibited in London, viz. in 

 the year 1791, a still much more com- 

 plete planetarium of this sort, called "a 

 planetarium, or astronomical machine, 

 which exhibits the most remarkable phe- 

 nomena, motions, and revolutions of the 

 universe ; invented, and partly executed, 

 by tJhe celebrated M. Hahn, member of 

 the academy of sciences at Erfurt; but 

 finished and completed by M. A. de My- 

 lius." This is a most stupendous and 

 .elaborate machine, consisting of the so- 

 lar system in general, with all the orbits 

 and planets in their due proportions and 

 positions ; as also the several particular 

 planetary systems of such as have satel- 

 lites, as of the Earth, Jupiter, &c.; the 

 whole kept, in continual motion by a 

 chronometer, or grand eight-day clock ; 

 by which all these systems are made per- 

 petually to perform all their motions ex- 

 actly as" in nature, exhibiting at all times 

 the true and real motions, positions, as- 

 pects, phenomena, &c. of all the celestial 

 bodies, even to the very diurnal rotation 

 of the planets, and the unequal motions 

 in their elliptic orbits. A description 

 was published of this most superb ma- 

 chine, and it was purchased and sent as 

 one of the presents to the Emperor of 



China, in the embassy of Lord Macart- 

 ney. 



We shall now give a description of one 

 cf th^se machines in common use. 



Jb'ig, 1, Plate Planetarium, is an eleva- 

 tion of the mechanism of a planetarium ; 

 and fig. 2, a plan of the same. A, (fig. 

 1,) is a ball of brass representing the 

 sun, supported by a wire screwed to a 

 bridge, b, fixed beneath the board, B B, 

 which supports the whole instrument ; a 

 is the section of an endless screw, which 

 has a small handle on the end of its spin- 

 dle to turn it by ; it gives motion to a 

 worm-wheel, 60, of sixty teeth, the arbor 

 of this wheel is a t^ibe* and goes over the 

 central wire sustaining the Sun, to its 

 upper end is fixed the frame, E E, con- 

 taining the wheel-work, and carrying the 

 Earth, , and Moon, (_ . The plan (fig. 

 2) is this frame of wheels, the upper plate 

 of the frame being removed, d is the first 

 wheel of sixty -four teeth, fixed fast to the 

 central wire "of the sun, and having no 

 motion, it works with another of sixty- 

 four, on the same arbor, h h, with several 

 others to be hereafter described; it turns 

 another, f y of sixty-four, on whose arbor, 

 g, the Earth is fixed ; as d is fixed, and 

 the next wheel, with its frame, E E, rolls 

 round it, and is thereby turned upon its 

 own axis ; the wheel, /, which is on the 

 other side, will have no motion on its axis, 

 and the axis of the Earth, fixed to it, wiH 

 remain parallel to itself, while it des- 

 cribes an orbit round the Sun, by the mo- 

 tion of the frame, E E. The next wheel, 

 6.0, upon the arbor, h, turns a pinion, 14, 

 of fourteen teeth, (not seen in the plan) 

 by the intervention of a wheel, 64, which 

 does not alter its velocity ; the arbor of 

 the pinion is a tube, and fitted upon the 

 central wire ; at its upper end it supports 

 the planet, Mercury, $ . The third wheel 

 from the bottom, on the arbor, h, has for- 

 ty teeth, and by the wheel, 56, communi- 

 cates motion to a small wheel of twenty- 

 four, which has the planet, Venus, ? , fix- 

 ed to its tubular arbor. The upper wheel 

 of the arbor, h, has seventy-four teeth, and 

 turns a pinion of six, on a tube, concen- 

 tric with g, and with it the moon. There 

 is a smaU\vheel of fourteen teeth between 

 the wheel and pinion, but it does not alte.r 

 the velocity: k, (fig. 1), is a thin brass 

 ring seen edgeways, which lias a wire air- 

 metrically across it, on which it turns asan 

 axis, to set it at any given obliquity to the 

 axis, g, supporting the EartV, the wire is 

 fixed into a short tube, which turns stiffly 

 in a hole made in the upper plate of the 

 frame, E E, and thus the circle can be 



