ORRERY. 
contriver, and to make fuch obfervations as have occurred 
in the examination. 
The wheelwork, according to Mr. Prieftley, 1s contained 
b f diameter, 2 
five inches, each orbit having its proper excentr 
clination ; and the periodic times, a8 well as 
of motion in each revolution, are nearly 
by the lateft obfervations. 
The covering plates, which are painted blue, have narrow 
n them, through which afs the fem 
lanets : 
Mm 
tic. 
The large gilt ball, reprefenting the fun, moves round 
ts axis, according to our author, in 2 4d yeh 76%, 
the axis itfelf has the proper inclination and pofition given 
It. 
_ The orbits of Jupiter and Saturn extending beyond the 
circumference of the box, arms are neceffarily ufed for 
thefe two planets. There are two feparate arms for J upiter ; 
one of which has wheelwork, that accurately exhibits his 
and the motions of his fatellites in their 
motions of the fatellites, &c. is 
m the 
is fupported by a 
Th f Saturn, being very long, 
flay, ca bar with two fri€tion-wheels; which roll 
along the bottom moulding of the box. This arm, where 
s 
feparates into two branches, and is 
uds, that rife above the covering plates. 
with i 
that Saturn then 
the orbits of the 
ecliptic circle, drawn on 
of ox: the year 1s pointed out by an index fixed to 
the arm of Saturn, and the day of the month is fhewn on 
VoL. XXV. 
a circle adjoining to the earth’s orbit. The proportional 
magnitudes of the planets, but not of the fun, are nearly 
fhewn in this orrery. 
The Georgian planet and his fyftem of moons compofe 
e put on any convenient ftand, 
and placed at the due diftance from the fun in the orrery. 
un, ina line with 51 22'. At this point a {mall pin 
e {crewed into the covering plates, and a ftring, 96 
beliocentric longitude, will give 
ongitude is fhewn a filvered 
The unequal motion of the planets in the different parts 
of their orbits has been attempted to be fhewn; and the 
velocities in this orrery will be found to agree, nearly, with 
the Keplerian law of equal areas, defcribed in equal times. 
Two methods have been employed for this purpofe ; for the 
which is defcribed as “ Mr. 
of dividin 
already a ee to. 
With refpe€t to the phenomena arifing out of the motions 
of the earth, &c. the machinery admits of four changes ; 
WIZ 
1. In fhewing the general appearances of the folar fyf- 
i he fize of the 
are ufed : 
other planets, 
in this cafe the parallelifm of the earth’s axis and the 
oon’s motion round the earth are fhewn; but not the 
moo 
earth’s diurnal motion. 
2. The ball and bead above-mentioned being removed, an 
apparatus is put on in their place for fhewing the apparent, 
fuch as the direé&t, retrograde, and ftationary, motions of the 
primary planets; and the geocentric place of any planet 
is fhewn by an index on a {mall ecliptic circle under the 
earth. 
fied 
