43° ASTRO 
lar to a fixth wire bifefting the field of view. See Tran¬ 
sit Instrument. 
The equatorial, or univerfal dial, an inftrument defigned 
to anfwer a number of ufeful purpofes in practical altro- 
nomy, independent of any particular obfervatory, may be 
made life of in any fteady room or place, and performs 
molt ot the ufeful problems in the fciencc. See Equa¬ 
torial. 
To the above lift of agronomical infiruments, all of 
which appear abfolutely neceflary to the practical aftro- 
nomer, we might add the following more coftly and fu- 
perb apparatus : 
I'll c grand orrery, invented by Mr. Rowley, which at 
once .difplays all the phenomena of the celeftial bodies. 
Alfo the orrery invented by Mr. Fergufon, fhews the mo¬ 
tions of the Sun, Mercury, Venus, Earth, and Moon ; 
and occafionally the fuperior planets, Mars, Jupiter, and 
Saturn, may be put on. Jupiter’s four fatellites are moved 
round him in their proper times by a fmall winch ; and 
Saturn has his five fatellites, and his ring which keeps its 
parallelifm round the Sun; and, by a lamp put in the Sun’s 
place, the ring fhews all its various phafes. For a com¬ 
plete defcription of its conftruCtion and ufes, fee Orrery. 
The planetarium, is a fmaller kind of orrery, contrived 
to reprefent the motions, orbits, &c. of the planets, as 
they really are in nature, or according to the Copernican 
fyflem. A very remarkable machine of this fort was in¬ 
vented by Huygens, and defcribed in his Opufc. Poflh. 
tom. ii. p. 157, edit. Ami 1728. And it is ftill preferved 
among the curiofities of the univerfity at Leyden. In this 
planetarium, the five primary planets perform their revo¬ 
lutions about the Sun, and the Moon performs her revo¬ 
lution about the Earth, in the fame time that they are 
really performed in the heavens. Alfo the orbits of the 
Moon and planets are reprefented with their true propor¬ 
tions, eccentricity, pofition, and declination, from the 
ecliptic or orbit of the Earth. So that by this machine 
the fituation of the planets, with the conjunctions, oppo- 
fitions, &c. may be known, not only for the prefent time, 
but for any other time either part or to come; as in a per¬ 
petual ephemeris. There was alfo exhibited in London, 
in the year 1791, a (till much more complete planetarium 
of this fort; called “ a planetarium or aftronomical ma¬ 
chine, which exhibits the molt remarkable phenomena, 
motions, and revolutions, of the univerfe. Invented, and 
partly executed, by the celebrated M. Phil. Matthew 
Hahn, member of the academy of fciences at Erfurt; but 
fiuifhed and completed by Mr. Albert de Mylius.” This 
rs a moft flupendous and elaborate machine; confiding of 
the folar fyftem in general, with all the orbits and planets 
in their due proportions and pofitions ; as alfo the feveral 
particular planetary fyfteins of fuch as have fatellites, as 
of the Earth, Jupiter, dec. the whole kept in continual 
motion by a chronometer, or grand eight-day clock ; by 
which all thefe fyftems are made perpetually to perform 
all their motions exactly as in nature, exhibiting at all 
times the true and real motions, pofitions, afpeCts, phe¬ 
nomena, &c. of all the celeftial bodies, even to the very 
diurnal rotation of the planets, and the unequal motions 
in their elliptic orbits. A defcription was publifhed of 
this moft fuperb machine ; and it was purchafed and fent 
as one of the prefents to the emperor of China, in the era- 
bafiy of Lord Macartney, in the year 1793. But the pla- 
netariums or orreries commonly ufedare only turned round 
with the hand, to help to give young beginners an idea of 
the planetary fyftem-; and to refolve problems relating to 
the motions of the planets, and of the Earth and Moon, 
&c. See Planetarium. 
Thefe curious and elegant machines are now fitted up 
In the completed manner, by Meffrs. W. and S. Jones, 
in Holborn, exemplifying the motions and phenomena of 
all the planets, the Georgium Sidus included, from forty 
guineas to one thoufand pounds. 
The tellurian, is a contrivance invented by the late in¬ 
genious Mr. Adams, which by an addition to the plane- 
N O M Y. 
tarium, will fhew, in an accurate and clear manner, all the 
phenomena arifing from the annual and diurnal motions of 
the Earth. 
The lunarium, is alfo an inftrument fo contrived as to 
be occafionally affixed to the planetarium, whereby all 
the phafes and phenomena of the Moon are diftinCtly ex¬ 
emplified. 
The cometarium, an inftrument very ingenioufly contriv¬ 
ed, whereby the eccentric motions of comets are clearly 
delineated. 
The terrejlrial and celejlzal globes. No new globes of a 
large fize have been conftrutted fince the time of Senex, 
until the prefent year 1797, in which a very fine fet of 
new and improved globes of eighteen inches diameter have 
been conftruCted, by Meflrs. W. and S. Jones, before- 
mentioned. They contain all the lateft aftronomical and 
geographical difeoveries. The places upon the terreftrial 
globe are laid down by an eminent geographer, with their 
names in Englilh ; and upon the celeftial globe there are 
about fix thoufand ftars very accurately laid down for the 
year 1800, from the bed obfervations ; to which are added 
many clufters of ftars, and nebulas, with the figures of 
the conftellations. The ftars are marked with Bayer’s and 
M. de la Caille’s letters; and an improved apparatus is 
added for the folution of problems. Elegant copper¬ 
plates of thefe globes, with complete directions for their 
application and ufe in all aftronomical and geographical 
problems, will be given under Globe. Similar inftruc- 
tions, with copper-plate figures, will alfo be given, to ex¬ 
emplify the ufe of all the aftronomical and mathematical 
infiruments, under each of their refpeCtive heads. 
Of ASTRONOMICAL CALCULATION and MEA¬ 
SUREMENT. 
There is no part of mathematical fcience more truly 
calculated to intereft and furprize mankind, than the mea- 
furement of the relative pofitions and diftances of inacceflr- 
ble objects. To determine the diftance of a (hip feen on a re¬ 
mote fpot of the unvaried face of the ocean, to afeertain the 
height of the clouds and meteors which float in the invisible 
fluid above our heads, or to (hew with certainty the motions 
and dimenfions of the Sun, and other bodies, in the hea¬ 
vens, are among the numerous problems which to the 
vulgar appear far beyond the reach of human art, but 
which are neverthelefs truly refolved by the incontrover¬ 
tible principles of the mathematics. 
The pofition of any objeCt, with regard to a fpeftator, 
can be confidered in no more than two ways ; namely, as 
to its diftance, or the length of a line ftippofed to be drawn 
from the eye to the object; and as to its direction, or the 
fituation of that line with refpeft to any other lines of di¬ 
rection ; or, in other words, whether it lies to the right 
or left above or below thofe lines. The firft of thefe two 
modes bears relation to a line abfolutely confidered, and 
the fecond to an angle. It is evident that the diftance can 
be direCtly come at by no other means than by meafuring 
it, or fucceflively applying fome known meafure along the 
line in queftion ; and therefore, that in many cafes the 
diftance cannot be direCtly found ; but the pofition of 
the line, or the angle it forms, with fome other aflumed 
line, may be readily afeertained, provided this laft line 
do likewife terminate in the eye of the fpe&ator. Now 
the whole artifice of meafuring inacceflible diftances con- 
fiftsin finding their lengths, from the confideration of an¬ 
gles, obferved about fome other line, whofe length can be 
fubmitted to aCtual menfuration. Every one knows the 
form of a common pair of compalfes. If the legs of this 
inftrument were mathematical lines, they would form an 
angle greater or lefs in proportion to the fpace the points 
would have pafled through in their opening. Suppofean 
arc of a circle to be placed in fuch a manner as to be pafled 
over by thefe points, then the angles will be in proportion 
to the parts of the arc pafled over; and, if the whole cir¬ 
cle be divided into any number of equal parts, as for ex¬ 
ample 360, the number of thefe comprehended between 
