PLA 
axis, g, this ring represents the plane of the 
Moon’s orbit, and is engraved witli the dif- 
ferent phases of the Moon. The Moon is 
not fixed to the arm which turns it, but its 
stem slides up and down in a short tube fix- 
ed to the arm, and rests upon the ring, so as 
to describe a parallel plane to it. On the 
end of the frame, EE, a pillar is erected to 
support a small semi-circular piece of brass, 
m, inclosing the Earth, and showing the 
line of light and darkness. IV is a tube 
screwed fast to the board, B B, by a flanch 
af the lower end ; it fits the outside of the 
tube of the wheel, 60, beneath the board, 
and thus steadies the whole frame as it turns 
round ; upon this tube long arms are fitted, 
carrying Mars, Jupiter, and the other supe- 
rior planets ; but as there is no wheel-work 
to turn these, they are omitted in the plate. 
This instrument is defective in not having 
the diurnal motion of the Earth upon its 
axis shown, and the rotation of the Moon’s 
nodes ; there haye been instruments made, 
which show all these motions, and those of 
the superior planets with their satellites ; 
but they are so complicated, that it would 
fer exceed the limits of our plates to de- 
scribe them. 
The numbers of the teeth of the wheels 
of this planetarium are not correctly calcu- 
lated to produce true revolutions of the pla- 
nets introduced in it, as the fixed wheel, 
64, and the wheel, 64, on the axis, h, are 
equal ; the latter, and all the wheels on h h, 
revolve once in a tropical year ; the wlieels 
which turn Mercury are 60, upon h, turn- 
ing 14, that is of a tropical year, or 
85.223185 days ; this period, which is in- 
tended to be Uie tropical revolution of Mer- 
cury, viz. 87d. 23 /t. 14m. 35s. is made the 
synodical revolution by the mechanism, by 
reason of the wheel-work being carried 
round the sun again in a year, by the frame, 
E, representing the Earth’s radius vector ; 
so that the planet Mercury goes from con- 
junction with the Earth to conjunction 
again, -instead of going through the ecliptic 
only in this period, and the imperfection of 
the wheel-work is rendered still more im- 
perfect by its position, which ought to have 
been on a stationary bar, to have produced 
the true calculated effect, this error causes 
it to make just one revolution in a year 
more than intended. The tropical period 
of Venus is also turned into a synodic one, 
by the same fault in the position of the 
wheel- work, besides the period itself being 
erroneous, viz. of a year, according to 
the original intention, which time is only 
PLA 
219 d. 3 ft. 29 m. 19.8 s., instead of 224 d. 
16 A. 41 TO. 30s., which is the true tropical 
period. 
The Moon-wheels, making 12i luna- 
tions, or synodic revolutions, give one luna- 
tion at 29 d. 14 ft. 44 w. 29.8s., which is 
greatly too long; the true period being 
29 d. 12ft. 44m. 3s. ; but making 12| lu- 
nations, or one in 29 d, 12 A. 20m. 54s., 
would be much more accurate, and equally 
well made. Thus the instrument before us 
is so very inaccurate, in all respects, that it 
ought to have its numbers rectified, which 
may be done in this manner. 
For Mercury, instead of i§, (or i|), put 
If, in which case the wheel, 63, will pro- 
duce revolutions ; and the Earth’s arm 
will carry the 20 round oval in a year, mak- 
ing, together, 4.15 revolutions of Mercury 
for one of the Earth’s, which is very near 
the truth, producing one tropical revolution 
in 88 d. 0 /«. 14 m. 38 s. 
For Venus, instead of |J, put |§, and one 
revolution per annum will be produced by 
the motion of the Earth’s arm, and of an- 
other by the wheels, making, together, 
1.625 in each year, or one tropical revolu- 
tion in 224 d. 18ft. 21 m. 27s. 
The true synodic periods are : — of Mer- 
cury, 115.877 d., and of Venus, 583.923 d. ; 
theiefore the said periods, by tire present 
wheel-work, are too short by more than 
thirty days in Mercury, and in Venus, by 
364 d., and upwards. 
PLANIMETRY, that part of geometry 
which considers lines and plain figures, with- 
out considering their height or depth. See 
Surveying, &c. 
PLANISPHERE, signifies a projection 
of the sphere, and its various circles on a 
plane ; in which sense maps, wherein are 
exhibited the meridians, and other circles 
of the splrere, are planispheres. See Map, 
Sphere, &c. 
Planisphere, is more particularly used 
for an astronomical instrument used in ob- 
serving the motions of the heavenly bodies. 
It consists of a projection of the celestial 
splrere upon a plaire, representing the stars, 
constellations, &c. in their proper order; 
some being projected on the meridian, and 
others on the eqirator. 
the use of the planisphere is to represent 
the face of the heavens for any day and 
hour : find, on the lesser moveable plate, 
the month and day proposed, and turn the 
plate till the given day of the nronth stand 
against the hour and minute required ; and 
the plate will then represent the face of the 
