1301.] Of an Orrery in 
Lo the Editor of the Monthly Magazine. 
SIR, 
HE Hiftory of the Arts holds a place 
of merited diftin@tion in your valut 
able mifcellany. From the notices which 
you give of modern improvements in che- 
miftry and machinery, and of thofe inven- 
tions which tend to utility and the orna- 
ment of life, your readers derive no {mall 
fhare of amufement and infru€tion. With 
refpeét to a knowledge of the Jaws of na- 
ture, and their pratiical application, we 
“feem to be daily advancing with accele- 
rated rapidity: ‘Thus circumftanced, how- 
ever, it may poflibly be profitable—it will 
certainly be pleafant to us—occafionally to 
take a retrofpeét, and confider what pro- 
grefs was made in fcience at various pe- 
riods of former times. I flatter smyfelf, 
therefore, that you will give the honours 
of infertion to a defcription of a kind of 
Orrery or Planetarium, conftructed at Flo- 
rence in the fifteenth century, at the time 
when the arts flourifhed under the liberal 
patronage of Lorenzo de’ Medici. An 
account of this ntachine is to be found in 
a letter from Angelo Politiano* to his 
friend Francefco Cafa. The defcription 
of a complex piece of machinery mutt, 
without the illuftration of engravings or 
drawings, be obf{cure and unlatisfaCtory. 
Of this Politian feems to have been fenfible, 
as he requefts his correfpondent to attri- 
bute the obicurity of his epiftle, not to the 
perplexity of his flyle, but to the peculiar 
nature of his fubjeét. It may, however, 
be juftly fufpe&ted, that his idea of the 
machine was not perfeétly clear, efpecially 
as he wrote his account of it, not from 
immediate obfervation, but from memory. 
Whatever may be the caufe, I mutt can- 
didly confefs, that many particulars of his 
cefcription elude my comprehenfion. ‘They 
will probably be better underftood by thofe 
who have principallydirected their atiention 
to mechanics. I fhall therefore proceed to 
tranflate Politian’s letter,occafionally quot- 
ing the original where I am doubtful re- 
{pecting the accuracy of my verfion. It 
may be proper to admonifh your readers, 
that the machine in queftion was intended 
as an illuftration of the Ptolemaic Syftem, 
which was the orthodox creed of philolo- 
pby at the period of its contruction. 
, * For fome entertaining anecdotes of this 
eminent fcholar, I fhali take the liberty of 
referring the lovers of polite learning to a 
work lately publithed by Cadell and Davies, 
entitled ** Memoirs of Angelus Politianus, 
Petrus Bembus, &c. by John Grefwell.” 
MONTHLY. Mac, No. 72. 
the 15th Century. 297 
Angelo Politigno to Francefco Ccfa, 
I HAVE received your letter, int which 
you inform me, that, you have heard mention 
made of a piece of machinery, conftrudted by 
one Lorenzo, a Florentine, which accurately 
exhibits the courfes of the heavenly bodies ;. 
and you fay, that, as common report is not to 
be trufted, you with me to communicate to 
you on this fubje€t any intelligence for the 
authenticity of which I can vouch. I will 
with pleafure comply with your requeft. As 
I have of late fixed my refidence in the coun- 
try, it is fome time fince 1 faw the machine 
in queftion; but I will endeavour briefly to 
explain from memory its form, principle, and 
ufe. If my defcription fhould appear fomewhat 
obicure, I truft you will attribute this cir- 
cumftance not altogether to my mode of ex. 
preflion, but to the difficulty and novelty of 
the fubject. 
The machine confifts of a pillar of a py- 
ramidal form, about three cubits in height, 
furmounted ty* a flat circle of brafs, orna- 
mented with gilding and colours, on one part 
of which (being lefs thanacubit in diameter) 
the courfes of the planets are defcribed. This 
circle of brafs. is moved by tooth-wheels 
which aé within it, and its edge plays within 
an immoveable circle, which is graduated 
into four-and-twenty divifions, correfponding 
with the four-and-twenty hours of the day. 
On the extremity of the moveable circle are 
engraven, at equal diftances, the twelve figns 
ofthe zodiac. Inthe inner part of this*circle 
are feen eight {mall circles (or wheels) of 
nearly equal magnitude. Two of thefe are in 
the centre, and are conne€ted with each other 
in fuch a manner that the lower, which is 
the larger of. the two, reprefents the fun, the 
higher the moon. An index, extending 
from the fun to the outer and immoveable 
circle, points to the hour of the day; and on 
the circle where the figns of the zodiac are 
delineated, it denotes the’ month, the day, 
the number of degrees, and the trueand {mean 
motion of the fun. 
A ftyle alfo extends from the moon, de- 
figned to act as an index of her time, which 
is marked below on the edge of the greater 
of the imall circles, or wheels. This ftyle, 
paffing through the centre of the lunar 
epicycle§, and reaching the delineation of 
the zodiacal figns, denotes the moon’s mean 
* Planus orbis aheneus: 
+ Orbiculi, which may alfo mean circular 
plates. 2 
{ Medium motum. 
§ Epicycle, in the ancient aftronomy, was 2a 
little circle, whofe centre was in the circum-~ 
ference of a greater circle. It was conceived 
to be a fmall orb or fphere, which being fixed 
on the great circle formed by the revolution 
of a pla. et, was carried along with it, and at 
the fame time caufed the planet to revolve 
round its own centre, 
.Q4q 
mot on, 
