ORRERY. 
the time when, and the —— on the globe where mid st 
will be vifible ; and alfo as to the folution of all th 
fraGtional portion of a day, w 
take into the calculation, ook sding a fapplementary 
ovided for in every lea 
and other inaccuracies, both in te lunar motions, 
rf 
and in ie ae annual courfe, (by the latter of which 
sa corre&t the errors, and fupply the defe&ts of the very 
imperfect ones, which had heretofore been ufed in feminaries 
is, we underftand, the refult of much peel day and o 
had been attempted and 
profeffed to be done by different men on the. prefent fub- 
ject; yet no real improvement, fince the days of Huygens, 
had adtually been effeéted in inftruments for explaining 
celeftial phenomena ; unlefs, indeed, the fcenic effet pro- 
rovement. 
machine for uniting the two requifite 
qualifications may be well founded, will beit be proved by 
an examination of the inftrument itfelf, 
V. of Planetary Machines reprefents a perfpective 
view of the firft sale of the orrery on a reduced fcale, 
as feen when put toget 
is a circular aa or ftand of mahogany two and a 
half feet in diameter, to which is faftened puna | the 
D isa bar of brafs, 
E upon a fhoulder, and refts alfo near the extremity of the 
table-on the brafs fri@ion-wheel above A, and is called the 
annual bar, by reafon of its being carried once igor ina 
year; at the remote end of this bar, beyond fixed 
the annual index, that points out in a ied {piral, 
drawn or a brafs rim on the furface of the table, - days 
of the nth in the firft, fecond, third, or four 
line, accordingly as the year may be leap year, or the firft, fe- 
cond, &c. after; and alfo indicates the fun’s place in the 
plete and his pate in two interior circles, as well as 
his mean anomaly and correfponding equation of the centre ; 
all which graduations are engraved along with the figns of the 
ecliptic. The {mall contrate wheel of 62, which appears in 
E, is alfo fixed faft to the central 
by the annual bar, either above it, or belo ; 
the wares tone by it are put into motion, as it goes round, 
partly by means of the pinion of 10, hidden with the long 
arbor under the annual bar, being connected with the fixed 
wheel of 269, and partly by the pinion.of 8 on the long 
arbor above it, being aCted upon by the immoveable wheel 
° 
~ 
The train of wheels, which carries the annual bar round, 
and alfo gives the daily motion to the earth, is compofed of 
the exact ratio between a day anda iG year, and confifts 
18 1800 
94 657436 
10 
— xX 
26 
which ratio inverted is equal to 3654 5? 48™ 
of the following numbers, viz. Bi x 
269 
This train is agreeable to Lalande’s determination of 
the folar year exaé#ly, there being not an error of fo much 
as even the fraClional portion of a fecond in a year: hence, 
when the machine has been made to revolve for the fpace of 
a million years, it will require no retification of this train, 
tab es themfelves, whence the 
zontal fteel arbor placed under, ae val to the annual a 
whilft this bar moves forwards ; then the contrate wheel of 26 
eing faft upon the fame a bor, which is lowered from 7 
place above A to be feen, revolves in the fame time, and a 
tuates the fecond pinion of 10, placed on the firft se 
lar arbor, and fupported by a cock F beneath the annual bar, 
on which perpendicular bs is alfo fixed the wheel of 94 
above the annual bar and under the bridge G, into which the 
upper end of this arbor is pivoted. Laftly, the wheel of 
94. drives the {mall wheel of 18 (not feen) round i in an exa&t 
day ; which laft wheel is foldered to the lower extremity of 
a long tube almoft fix inches long, that bears alfo the fmall: 
wheel of 40 on its fuperior end, for the purpofe of com. 
municating motion to a fimilar ert placed on the earth’s 
axis, by the help of an i wheel; which three 
w and dimenfions; the 
long tube is f{upported by, and revolves upon, a long polifhed 
piece of fteel, {crewed home into the annual bar, and kept- 
faft to its place by atapped nut below i bar, which per- 
eae piece of fteel, or ftem, is turned in a lathe in. 
fuch a way that the two extremities of the revolving tube. 
move ee upon it without fhake, but with as little fric- 
ion as poffaib] r which ala the whole SS of the 
em is a little dimin in diameter, except at the two 
ends, on whic e extremities of the tube revolve. B 
this portion of the mechanifm the ae proportion is rl 
ferved between a folar day. anda folar or tropical year. 
The next portion is that part of the mechamifm by which 
the oa axis is preferved oll to itfelf in every part of 
its annuel orbit; this confifts of two contrate wheels of 
fimilar dimeak one. each containing 62 teeth, one placed with 
the teeth downwards faft to the central ftem above E, and 
_ other revolving pe the bridge G with its teeth upper- 
that its motion may be in a retrograde dire ion, and 
round along tube nade aft to the bridge, which is inter. 
pofed between the tube of wheel 18, and the tube on which 
the wheel 62 is faftened, fo that the fixed intermediate tube 
8 
the annual bar, by means of the conneétion that the interior 
inion 
