ORRERY. 
X. This ftem rifes and falls, as has been lai and car- 
alot his al 
every ano 
revolution, by the motion of the crank, reduce an Sibi as 
nearly as may be like the real orbit of the moon, not onl 
with refpe& to fhape and pofition, but to acceleration and re- 
tardation alfo; for if the bend of the crank, meafured from 
the ftem to the moon’s ftem, be made to bear the fame pro- 
portion to the mean diftance of the moon from the earth, 
that tavice the excentricity of the lunar orbit does to its ra- 
dius, then the crank piece, 4, will reprefent the whole equation 
of the centre, and bea be alternately additive and fubtractive, 
as in nature. ing twice the excentricity, it muft be 
granted, will fuer the lunar orbit too excentric for its dif- 
tance from the earth, but php it is confidered that the earth 
and moon are conftruéted on a large {cale, the diflances with 
refpe& to each other, and at with refpeét to the fun, cannot 
poffibly be preferved, without making the annual bar im- 
menfely long, therefore the objeCtion on this {core vanifhes. 
Over the cock, ¢, is fixed a circular face to indicate the 
moon’s diftane es and equations dependent upon them, by the 
help of ae revolving hand fj laps is pte on the upper 
extremit th ich revolves in 
an an nomliftic sevlution. oi rifing aad falling with the 
F the 
and 4, earned by the lunar arm, indicates the 
moon's age, meridian paflage, tides, &c. on the fixed 
plate N. 
From this ea in detail of all the feparate parts of 
the inftrument in of their application, it is 
omed to 
made to er. The 
urpofely avoided, left : 5 confulon 
fhould take place in the defcription ; aad. that may be 
more conveniently referred to, a table of them is er ones to 
the end of this acceunt, for the ufe of the workm man 
It would enlarge our account tuo much, to introduce 
in this place particular direCtions for dividing the circles 
on the various faces of the plates; and would imply, 
befides, a knowledge of the application of aftrono 
mical 
tables in the reader, in which few, comparatively fpeak- 
ing, are converfant. We fhall, therefore, pafs over the 
neceflary diretions in this place, as bein worthy of 
an 
w words refpeGing the method o 
dividing the quadruple fpiral, in ich ay that each 
pe may contain 3654 {paces, in order to take in the 29th 
February every fourth year. To effe& this condition, 
ie {piral muft begin at noon of the 1ft day of ae civil 
time, and end on the laft of February ; aid alfo 
{paces muft be proportioned to the velocity of the aah 
motion in its annual circuit, which, we have feen, is either 
fun, in confequence of the unequal teeth of the large wheel 
. Here the {kilful mathematician and the unlettered 
wigs would probably find themfelves equally at a lofs how 
o fet about a bufinefs apparently fo complex. The fpiral, 
ever, being once drawn, which requires but little 
mathematical knowledge, (and {till lefs pra€tice in me- 
chanics, provided a beam-compafs be at hand,) the bufinefs 
is perfectly finple and eafy ; for the inftrument will divide 
ats own paces in the beft poffible manner: becaufe, as there 
are 365.242 turns of the handle for one revolution of the 
annual bar, all that is age le after 
€ 
firft fpira nother t 
ively, a four _ are ied through ; care pag taken 
to mark, by a long line, each 5, 10, 15, &c. 
month; and that the 
tooth would accelerate or retard the 
mode of dividing will always accommodate the day {paces to 
the fize of the teeth oppofite them, without the ne ance 
or even knowledge, of the perfon su divides ; and as t 
fame wheelwork muft indicate which divided, there i 5 the 
greateft probability of the day {paces completely a aniwering 
their intention, which could not be fo well expected fro 
any other mode of divifion. An inftrument a from this 
plan, well executed in its parts, and properly adjutted, will 
work the various problems, adapted for the i ie globe, 
y the mere turning of the handle, and that 
tural way. The globe will be in reality alae the new 
mounted globe only profefles to be; and, what is of no 
mall advantage to the learner, the reafon will a 
drawing all the imaginary circles, which are ufu 
on the furface of the globe, and which are generally sic ll 
a eae to be piiseiat without a reafon being affigned. 
e inftrument will combine all the ufes a the 
ms 
Like- 
it s they happen 
in a particu zeal when {mall pullies, foe keeping the 
pointing properly to the earth, are duly ap- 
plied carne the es of the crank, 
Table of Dimenfions. 
Feet. Inches. 
Diameter of the ftand or table - - 2 6 
Dittance from the fun to an glee . . 1 8 
i oe § 
From the table to the pee of t die a - Io 
Diameter 
