ORRERY. 
pinion has with the fixed wheel, while the annual bar is in 
motion: the horizontal arbor is pivoted into the bridge H 
fe) On 
and horizon circles to be 
latit ude. i 
of the annual bar, axis of the earth is made to point 
always to the fame individual diftant point in every part of 
its annual orbit ; or, in other words, it preferves its parallelifm 
tion on its axis, one night and one day only would be the 
lot of its inhabitants in every year, and the fun would appear. 
to rife in the weft, and fet in the eaft, which phenomenon 
would really take place in nature, if the earth’s motion on 
its axis were flayed, (of which circumftance; we believe, 
aftronomers have taken no notice.) i 
however, it ? oo that an 
the duration of which is only 
a2 56" 3.549 se time, the difference between which and a 
folar day amounts to a day in every folar year ; 
hence in every 365,242 natural days, the earth make 
366,242 rotations hich would be fo man 
’ “Ons ’ 
atural days, if the parallelifm of the earth 
preferved, which parallelifm effeéts a deduction of one day 
‘in a folar year, in the way that has been defcribed. The 
queftion-then arifes, how the additional rotation is given to 
the earth’s axis by the mechanifm before us? e effect is 
' this; viz. while the central wheel of 40, under the end of 
the antarctic circle, revolves 365,242 times, the piece 
carried by the {mall ecliptic plate, carries the next wheel of 
] 
8 by communication 305.242 revo- 
central: wheel, rane | of t 
diminution ; confequently, the earth makes exa@ly 366,242 
haan in 365,242 natural days. It is to a want of atten- 
to this circumftance, that muft be attributed the inac- 
curac r modern tellurians, for either an addition or 
fubtraction, eecseaned by a compound motion cael from 
the annual circuit, has rendered them ufelefs as to all the 
purpofes connected with particular dedutions. By this con- 
ftruGtion of the inftrument, both folar and fidereal time are 
ie aaerin on the fame dial-plate M, placed on the top of the 
e that a in Page -four hours, by means of the 
en hands xt r of whic 
fixed ftem of: feel, round which the tube a ee and the 
latter, P, is carr ried in a retrograde diretion by the ecliptic 
Grete once € round i in every year, fo'as to meet any par rticular 
placed on a’ 
hour on the horary plate 3™ 6. “57 | on ar it did on the 
preceding day; fo if the e put to the 
fame hour on the: on whi ch. le ae enters Aries," the 
pace on - horary oie ene betwee 
nan in the year, will be 
and’ fered time, or, what is the oe thing, oe ail $ mean 
n 
of the earth, which poin _ wher fun is vertical on 
any given day, as alfo the v ration of apnea and a 
variety of other phenomen 
a in a very ing m 
Another effential diab pale “ this inftrument 18, 
that it makes fummer as it is in 
fe) e other, fo that there are pie more 
teeth in one femicircle of the wheel than in the other, and 
contrivance is requifite to aera the 
diurnal from the annual motion of the earth, for otherwife 
the flownefs of the annual motion would be Gaeourabe to 
of wheel 269, is capable of being detached at pleafure from 
the large wheel, by eeu the tapped and milled nut S 
piece of the cock, into a fquare focket faftened t n= 
nual bar, and along with it the aes = the aieet fufficiently 
high for the pinion to clear the wheel; ic elevation 
all the motions oe depend ne the eee wheel of 2 
are allowed to 
Having now detailed ug si of = Fla Ppa 
tion of the inftr Junarian_porti ema . 
attention. 
a perfect coinenene, the motions Por all the different parts of 
the mechanifm, however accurately calculated, ingenioufly 
arranged, and exadtly proportioned, with a view both to 
comparative dimenfions and ftrength, muit commence as 
peated as poflible at the fame time ; otherwife the’ een 
nthe different graduated faces and circles will not ly 
seta with one another: for inftance, we have feen the 
moon, in fome inftruments of Sheol pe or rather we™ 
fhould fay of great price, not commencing i i ; 
the earth, on turning the handle, unt the earth has made 
iisaxis. Such effect was 
therefore to be guarded againft, by taking’ the ea 
tion of motion for the moon, not from a flowly mo ing’ ane © 
nual 
