ASTRONOMY. 
able ftar arifes from the various luftre of the different parts 
of its furface, fucceflively turned to us by its rotatory mo¬ 
tion.’ Befides, vve perceive a greater fimilarity between 
the Sun and the Ifars, by means of tire fpots that mud now 
be admitted to exift on their furfaces, as well as on that of 
the Sun. Dr. Herfchel farther obferves that the (tars, be¬ 
fides a rotatory motion on their axes, may have other move¬ 
ments ; “ fuch as nutations or changes in the inclination of 
their axes; which, added to bodies much flattened by 
quick rotatory motions, or furrounded by rings like Sa¬ 
turn, will ealily account for many new phenomena that 
may then offer themfelves to our extended views.” 
Dr. Herfchel’s catalogue, as far as it has yet gone, 
comprehends the ftars of the following conftellations, viz. 
Aquarius, Aquila, Capricornus, Cygnus, Delphinus, 
F.quuleus, Hercules, Pegafus, Sagitta, Aries, Canis ma¬ 
jor, Canis minor, Caffiopea, Cetus, Corvus, Eridanus, 
Gemini, and Leo. The relidue of the conftellations are 
intended to be given upon the fame principle, as loon as 
time will permit the completion of them. To this cata¬ 
logue are alfo fubjoined many ingenious notes, which are 
tire refult of conftant and indefatigable attention, and 
which will ferve to alfift future aftronomers in extend¬ 
ing their acquaintance with the celeffial bodies. Thofe 
who are defirous of a farther explanation of this arrange¬ 
ment, are referred to the doctor’s own papers, in the 
Philofophical TranfaCtions of 1796. The practical aftro- 
nomer will alfo derive much information and affiftance, by 
confulting the following publications, from the bulk of 
which this treatife hath been compiled ; viz. the Rev. 
S. Vince’s complete Syftem of Aftronomy. The late in¬ 
genious Mr. George Adams’s Aftronomical and Philofo¬ 
phical Works. Dr. Hutton’s Mathematical Di£tionary. 
Sir Ifaac Newton’s Principles of Natural Philofophy. Dr. 
Adam Smith’s Affronomical Effays. Sir H. Englefield’s 
Elements of the Orbits of Comets. The celebrated French 
authors La Place and La Grange ; Montucla’s Hijloire des 
Mathematiques, and the eloquent M. Bailly’s Hijloire de 
iAJlronomie ancitnne et moderne. All Dr. Herfchel’s Papers 
in the Philofophical TranfaCUons, from the year 1782, to 
the prefent time ; to which we would add the Nautical 
Almanac, and the Obfervations and Aftronomical Tables 
of Dr. Malkelyne, with his other works in feveral folio 
volumes, publifhed under the direction of the Board of 
Longitude, and the Royal Society. Other ingenious au¬ 
thors on aftronomy are, De la Lande, Weidler, Voffius, 
Perrault, Niceron, Chauffepie, De la Hire, Dr. Long, 
Fergufon, Emerfon, Leadbetter, Brent, Keil, Whifton, 
Wing, Street, BonnycalHe, &c. 
Of the ASTRONOMICAL INSTRUMENTS. 
It feems highly probable, that the ancients were not 
wholly unaccquainted with planetary machines, but that 
the fame powers of genius w hich led them to contemplate 
and reafon upon the motions of the heavenly bodies, in¬ 
duced them to realize their ideas, and form inftruments for 
explaining them ; and we may fairly prefume, that tllefe 
were carried to no infignificant degree of perfection, fince of 
one Archimedes was the maker, and Cicero its encomiaft. 
Thofe vve are now going to deferibe, are of more modern 
invention, and require equal nicety and attention in their 
ufe. For this reafon it is necelfary to have a room or 
building conveniently fituated and fuitably contrived. It 
ihould have an uninterrupted view from the zenith down 
to (or even below) the horizon, at lead towards its cardi¬ 
nal points: and for this purpofe, that part of the roof 
which lies in the direction of the meridian in particular, 
fltould have moveable covers, which may eafily be taken off 
and put on again ; by which means an inftrument may be 
directed to any point of the heavens between the horizon 
and the zenith, as well to the northward as fotithward. 
The apartment, thus fitted up-, may be called an obferva¬ 
tory, and fhould contain, according to its magnitude and 
intent, mod, or all, of the following inftruments. 
A pendulum clock, for fliewing equal time. This fhould 
VOL. II. No.80. 
fhew time in hours, minutes, and feconds ; and with 
which the obferver, by hearing the beats of the pendulum, 
may count them by his ear, while his eye is employed on 
the motion of the celeftial objeCt he is obferving. Juft 
before the object arrives at the pofition deferibed, the ob¬ 
ferver ftiould look on the clock, and remark the time, 
fuppofe it 9I1. 15'. 25”. then faying, 25, 26, 27, 28, &c. 
refponfive to the beat of the pendulum, till lie fees through 
the inftrument the objeCt arrived at the polition expeCted ; 
which fuppofe to happen when he fays 38, he then writes 
down 9b. 15'. 38". for the time of obfervation, annexing 
the year and the day of the month. See Clock. 
A rej railing, and a rejlcBing, telefcope, for obferving in- 
vifible and particular phenomena. The invention and im¬ 
provement of this inftrument is one of the nobleft and 
molt ufeful thefe ages can boaft of; by means of it the 
w onders of the heavens are difeovered to us, and aftrono¬ 
my is brought to a degree ufrperfeCtion which former ages 
could’have no idea of. But the rnoft confiderable magni¬ 
fying powers that have ever been ufed, have lately been 
invented by Dr. Herfchel, and are truly aftonilhing. Par¬ 
ticulars of this invention, with an elegant view of Dr. 
Herfchel’s forty-feet refle£ting telefcope, will be given 
under that head. Telefcopes, with all thefe improve¬ 
ments, are now fitted up by W. and S. Jones, in Holborn, 
London, from five guineas to five hundred pounds each. 
A micrometer, which is an inftrument ufually fitted to a 
telefcope, in the focus of the object-glafs, for meafunng 
fmall angles or diftances, as the apparent diameters of the 
planets, Sec. The general principle is, that the inftru- 
ment moves a fine wire parallel to itfelf, in the plane of 
the picture of an objeCt, formed in the focus of a- tele¬ 
fcope, fo as with great exaCtnefs to meafure its perpen¬ 
dicular diftance from a fixed wire in the fame plane-; and 
thus are meaftired fmall angles, fubtended by remote ob¬ 
jects at the naked eye. See Micrometer. 
Agronomical quadrants, mural and portable, for obferv¬ 
ing meridian and other altitudes of the celeftial bodies. 
The mural quadrant is in the form of a quarter of a cir¬ 
cle, contained under two radii at right angles to one ano¬ 
ther, and an arch equal to one fourth part of the circum¬ 
ference of the circle. It is one of the moft ufeful of: all 
the aftronomical inftruments; and as it is fometimes fixed 
to the fide of a ftone or brick wall, and the plane of it 
erected exactly in the plane of the meridian, it in this cafe 
receives the name of mural quadrant or arch. Tycho 
Brahe was the firft perfon who applied this arch to a 
wall ; and Flamftead, the firft in England who with in¬ 
defatigable pains fixed one up in the royal obfervatory at 
Greenwich. 
The portable aftronomical quadrant, is generally made 
from twelve to twenty-three inches, ftrongiy framed to¬ 
gether by croired perpendicular bars. When this inftru¬ 
ment is let up for tile or obfervation, it is necelfary that 
two adjuftments be very accurately made : -one, that the 
plane or furface of the inftrument be truly perpendicular 
to the horizon ; the other, that the line fuppofed to be 
drawn from the centre to the firft line of the limb, be 
truly on a level or parallel with the horizon. See the ar¬ 
ticle Quadrant, where Mr. Ramfden’s new inftruments 
for meafuring horizontal angles are alfo deferibed. 
The ajlronomical or equatorialJ'eBor, for finding the dif¬ 
ference in right afeenfion and declination between two ob¬ 
jects, the diftance of which is too great to be obferved by 
the micrometer. The firft inftrument of this kind was. 
made by Mr. Graham, and is now in the Obfervatory at 
Greenwich. See Equatorial. 
The tranfit injlrument, for obferving objects as they ptifs- 
over the meridian, confifts of a telefcope fixed at right 
angles to an horizontal axis, fo fupported that what is 
called the line of collimation, or line of light of the tele¬ 
fcope, may move in the plane of the meridian. It is ufed 
to take the right afcenlion of the heavenly bodies, and to 
correfl the going of the clock ; for which purpofe it lias 
a fyftem of five parallels and equidiftant wires perpendku- 
5 R hf 
