PLANETARIUM. 
evidently goveined by the same law, since 
they descend with an accelerated motion, 
as they approach towards the Sun, and 
ascend asjain with a retarded motion, 
bending their way about the Sun, and de- 
scribing equal areas in eqii'al times, by rays 
drawn from them to his centre. See As- 
tronomy. 
■' PLANETARIUM, an astronomical ma- 
chine, contrived to represent the motions, 
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 Leyden. In this planetarium, 
the five primary planets perform their revo- 
lutions about the Sun, and the Moon per- 
forms her revolution about the Earth, in the 
same time that they are really performed 
in the heavens. Also the orbits of the Moon 
and planets are represented with their true 
proportions, excentricity, position, and de- 
clination from the ecliptic or orbit of the 
Earth. So that, by this machine, the situa- 
tion of the planets, with the conjunctions, 
oupositions, &c. may be known, not only 
for the present time, but for any otlier time, 
either past or yet to come, as in a perpe- 
tual ephemeris. There was exhibited in 
London, viz. in the year 1791, a still much 
more complete planetarium of this sort, 
called “ a planetarium, or astronomical ma- 
chine, which exhibits the most remarkable 
phenomena, motions, and revolutions of the 
universe ; invented, and partly executed, 
by the celebrated M. Hahn, member of the 
academy of sciences at Erfurt ; but finished 
and completed by M. A. de Mylius.” This is a 
moststiipendousand elaborate machine, con- 
sisting of the solar system in general, with all 
the orbits and planets in their due proportions 
and positions; as also the several parti- 
cular planetary systems of such as have sa- 
tellites, as of the Earth, Jupiter, &c. ; the 
whole kept in continual motion by a chro- 
nometer, or grand eight-day clock; by 
which all these systems are made peipe- 
tually to perform all their motions exactly 
as in nature, exhibiting at all times the true 
and real motions, positions, aspects, pheno- 
mena, &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 machine ; and it was purchased 
and sent as one of the presents to the Em- 
peror of China, in the embassy of Lord Bla- 
cartney. 
AVe shall now give a description of one 
of these machines in common use. 
Fig. 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, support- 
ed by a wire screwed to a bridge, 6, 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 spindle to turn it by ; it gives 
motion to a worm-wheel, 60, of sixty teeth, 
the arbor of this wiieel is a tube, and goes 
over the central wire sustaining the Sun, to 
its upper end is fixed the fi ame, E E, con- 
taining the wheel-work, and carrying the 
Earth, 0, and Moon, C- The plan, (fig. 2) 
is this frame of wheels, the upper plate of 
the fiame being removed, d is the first 
wheel of sixty-four teeth, fixed fast to the 
central wire of the sun, and having no mo- 
tion, it works with another of sixty-four, on 
the same arbor, h h, with several others to 
be hereafter described ; it turns another, 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, y which is on the other side, will 
have no motion on its axis, and the axis of 
the Earth, fixed to it, will remain parallel to 
itself, while it describes an orbit round the 
Sun, by the motion of the frame, E E. The 
next wheel, 60, 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, g . The third wheel 
from the bottom, on the arbor, ft, has forty 
teeth, and by the wheel, ,56, communicates 
motion to a small wheel of twenty-four, 
which has the planet, Venus, ? , fixed to its 
tubular arbor. The upper wlieel of the ar- 
bor, ft, has seventy-four teeth, and turns a 
pinion of six, on a tube concentric with g, 
and with it the Moon. There is a small 
wheel of fourteen teeth between the wheel 
and pinion, but it does not alter the velo- 
city : k, (fig. 1), is a thin brass ring seen 
edgeways, which has a wire diametrically 
across it on which it turns as an axis, to set 
it at any given obliquity to the axis, g, sup- 
porting the Earth, the wire is fixed into a 
short tube, which turns stiffly In a hole 
made in the upper plate of the frame, EE, 
and tlius the circle can be turned round, 
while its plane continues oblique to the 
