ORRERY. 
in a year, fimilar to the drawback we have noticed in the 
cafe of Jupiter’s moons, and the rotation of Mars ; becaufe 
while the firft g6 is ftationary, and the fecond moveable, by 
ar, it is clear that this laft hasa 
of + from a handle, 
annual circuit round the ninth 
each now 3 X 9 "= 365 5 
therefore, ‘this 
deduction i is a the rate of one em th per Cc 
confequence will be a ibs the dedudion will now be ws 
== 32° revolutions of the earth’s axis, by the effe& _ the 
annual Bae saa on this wheel of g5, and 3 x 95 + 
365; efore; but for every turn of wheel 96, on ie 
disroal handle, wheel 95 m makes an entire rotation, and one 
hb mor 
one tooth ger day, which would n 
the ninth “in for the tranfmiffion, had no 
teeth as da The balance, therefore, is pre and it 
now appears ow the wheel for the tranfmiffion of motion 
from the 96 to the g5 was made a 365 teeth, in pre- 
ference to any other number. if the aed had con- 
fifted of 3654 days, the compenfaton would n 
confequen 
By this compenfation, the diurnal 
authorifed to difplace. 
365 rotations in the 
handle and the earth’s axis make eac 
nual index, nor an 
the more minutely we amine 
the diftin& portions of this mechanifm, the more clearly we 
ee the reafo of various contrivances, and of 
in more recent con{truGio on of a of 
a day in the year, aus alfo of the daily hand and ecliptic 
index, is an omiffion arifing entirely out of ner in 
which the motion of the handle is tranfmitted to the earth’s 
Venus and Me ercury are at 3% ad 2.2 refpeétively. We 
4 
come, laftly, to the moon, which performs her lunation in ad 
4 
HI ° 
xX —> of a day, which period is 29% 12%; and the 
i plate for the moon’s latitude is carried back- 
95. 
ward, for the motion of the nodes by the wheels 
93 
8 
are equally coher — have Era uch mo 
accurate refult A cap of pein eed 
round in each lean <= ie sual {mall wheels on the 
lanar arm, the in cece one being faft to a fixed tube, as we 
fuppofe, exhibits the phafes for each day of the moon’s age, 
which are “mean a on the cane - fome little diftance 
rom the horary ecliptic circle are 
ereGted a brafs bmicircle for half the ance the a 
ier the fumme and arétic circle, wi 
in %, or 19 civil years. The wheels ==, however, which 
— 
Lary 
SF 
° 
S 
me 
Q 
chine ; and a wor 
orfe, 
our aly fees is, ae 
o much money is expended with fo little real effe&. A 
each fide of the handle is a ee knob: when the right-hand 
one is drawn out, the fecondaries of Jupiter and Saturn, as 
well as the rotation of Mars, ceafe to. move; and when the 
left-hand one is drawn in like manner, the fuperior planets 
oO 
numbers was not attempted : 
produced thofe difficulties in the execution, which 
of a perfect engine has now entirely remove 
Fergufon's Orrery.—James Fergufon invented various’ in- 
ftruments from time to time for illuftrating the phenomena 
the ufe 
n 
comprehends the motions of the » Mercury, Venus, 
Earth, and Moon, and is particularly defcribed both in the 
author s * Select Mechanical Exercifes,”” and in his * Tables 
nd Traéts,”” to which the reader is referred for the plates 
oad defcription of the mechanifm, whic 
m greater accuracy t than they really 
pale we hall fubjoin the wheelwork, 
attributed to t y their author. It 
for our prefent pees to give the fuaibed of the wheels 
“ pin nions of each train, with the correfponding periods, 
"TABLE 
