AS TRONOM Y, 
Them a kind of planets or Wandering ftars, difappear'rng in 
the fuperior parts of their orbits, and becoming vilible only 
in the lower parts of them. But Ariftotle held, that co¬ 
mets were only a kind of tranfient fires, or meteors, con¬ 
fining of exhalations raffed to the upper region ot the air, 
and there fet on fire; far below the courfe of the Moon. 
Seneca, who lived in the firft Century, and who had feen 
two or three comets himfelf, plainly intimates that he 
thought them above the Moon; and argues ftfongly againft 
thofe who fuppofed them to be meteors, or who held other 
abfurd opinions concerning them; declaring his belief that 
they were not fires fuddenly kindled, but the eternal pro¬ 
ductions of nature. He points out alfo the only way to 
arrive at a certainty on this fubjeft, viz. by collecting a 
number of obfervations concerning their appearance, in or¬ 
der to difcover whether they return periodically or not. 
For this purpofe (fays he) one age is not fufficient; but 
the time will.come when the nature of comets and their 
magnitudes will be demonfirated, and the routes they take, 
fo different from the planets, explained. Pofierity will 
then wonder, that the preceding ages fliould be ignorant 
of matters fo plain and eafy to be known.” For a long 
time this prediction of Seneca feemed not likely to be ful¬ 
filled ; and Tycho Brahe was the firft among the moderns 
who reftored the comets to their true rank in the creation; 
for, after diligently obferving the comet of 1577, and find¬ 
ing that it had no fenfible diurnal parallax, he afligned it 
its true place in the planetary regions. See his book de 
.Cometa, anni 1577. 
Hevelius, from a great number of obfervations, pro- 
pofed it as his opinion, that the comets, like the folar ma¬ 
culae, or fpots, are formed or condenfed out of the grofler 
exhalations of his body ; in which he differs but little from 
the opinion of Kepler. James Bernoulli, in his Syftema 
Cometarum, imagined that comets were no other than the 
fatellites of fome very diftant planet, which was itfelf iu- 
vifible to us on account of its diftance, as were alfo the 
fatellites unlefs when in a certain part of their orbits. 
Des Cartes advances another opinion : he conjectures that 
comets are only ftars, formerly fixed, like the reft, in the 
heavens; but which becoming gradually covered with ma¬ 
culae, or fpots, and at length wholly deprived of their light, 
cannot keep their places, but are carried off by the vor¬ 
tices of the circumjacent ftars; and, in proportion to their 
magnitude and folidity, moved in Inch a manner, as to be 
'brought nearer the orb of Saturn, and thus, coming with¬ 
in the reach of the Sun’s light, rendered vilible. 
But the vanity of all thefe hypothefes now abundantly 
appears from the obferved phenomena of comets, and from 
the doCtrine of Sir Ifaac Newton. The comets, he fays, 
are compaCt, folid, fixed, and durable, bodies; in faff, a 
kind of planets, which move in very oblique and eccen¬ 
tric orbits, every way, with the greateft freedom; perfe- 
vering in their motions, even againfi: the courfe and direc¬ 
tion of the planets : and their tail is a very thin and (lender 
vapour, emitted by the head or nucleus of the comet, ig¬ 
nited or healed by the Sun. This theory of comets at 
once folves their principal phenomena, which are as follow : 
Firft, Thofe comets which move according to the or¬ 
der of the figns, do all, a little before they difappear, ei¬ 
ther advance flower than ufual, or elfe go retrograde, if 
the Earth be between them and the Sun; but more fwiftly 
if the Earth be placed in a contrary part. On the other 
hand, thofe which proceed contrary to the order of the 
figns, move more fwiftly than ufual, if the Earth be be¬ 
tween them and the Sun ; and more (lowly, or elfe are re¬ 
trograde, when the Earth is in a contrary part. For, fince 
this courfe is not among the fixed ftars, but among the 
planets, as the motion of the Earth either confpires with 
them, or goes againfi them, their appearance, with refpect 
to the Earth, muft be changed ; and, like the planets, they 
mu ft fometimes appear to move fwifter, fometimes flower, 
and fometimes to be retrograde. 2. So long as their ve¬ 
locity is increafed, they nearly move in great circles; but, 
towards the end of their courfe, they deviate from thofe 
Vol. II. No. 79. 
circles; and, when the Earth proceeds one w*y, they go 
the contrary way. Becaufe, in the end of their courfe, 
when they recede almoft directly from the Sun, that part 
of the apparent motion which ariles from the parallax’* 
muft bear a greater proportion to the w hole apparent mo¬ 
tion. 3. The comets move ill ellipfes, having one of their 
foci in the centre ot the Sun; and, by radii drawn to the 
Sun, deferibe areas proportional to the times. Becaufe 
they do not wander precarioutly from one fictitious vortex 
to another; but, making a part of tire folar fyftem, return 
perpetually, and run a conftant round. Hence, their el¬ 
liptic orbits being very long and eccentric, they become 
invilible when in that part which is mod remote from the 
Sun. And, from the curvity of the paths of comets, Sir 
Ifaac Newton concludes, that, when they difappear, they 
are much beyond the orbit of Jupiter ; and that, in their 
perihelion, they frequently deicend within the orbits of 
Mars and the inferior planets. 4, The light of their nu¬ 
clei, or bodies, increafes as they recede from the Earth 
towards the Sun; and, on the contrary, it decreafes as they 
recede from the Sun. Becaufe, as they are in the regions 
of the planets, their accefs towards the Sun bears a con- 
liderable proportion to their whole diftance. 5. Their 
tails appear the largeft andbrighteft immediately after their 
tranfit through the region of the Sun, or after their peri¬ 
helion. Becaufe then their heads, being the mod heateij, 
will emit the moft vapours. From the light of the nu¬ 
cleus we infer their vicinity to the Earth, and that they 
are by no means in the region of the fixed ftars, as fome 
have imagined ; fince, in that cafe, their heads would be no 
more illuminated by the Sun than the planets are by th£ 
fixed ftars. 6. The tails always decline from a juft oppo- 
fition to the Sun towards thofe parts which the nuclei or 
bodies pafs over, in their progrefs through their orbits. 
Becaufe all fmoke, or vapour, emitted from a body in mo¬ 
tion, tends upwards obliquely, ftill receding from that 
parts towards which the fmoking body proceeds. 7. This 
declination, cateris paribus, is the fmalleft when the nuclei 
approach neareft the Sun : and it is alfo lefs near the nu¬ 
cleus, or head, than towards the extremity of the tail. 
Becaufe the vapour afeends more fwiftly near the head of 
the comet, than in the higher extremity of its tail; and 
alfo when the comet is nearer the Sun, than when it is far¬ 
ther off. 8. The tails are fomewhat brighter, and more 
diftiniftly defined, in their convex, than in their concave, 
part. Becaufe the vapour in the convex part, which goes 
firft, being fomewhat nearer and denfer, reflects the light 
more copioully. 9. The tails always appear broader at 
their upper extremity, than near the centre of the comet. 
Becaufe the vapour in a free (pace continually rarefies and 
dilates. 10. The tails'are always tranfparent, and the 
fmalleft ftars appear through them. Becaufe' they confift, 
of infinitely thin vapour. 
The nuclei, which are the heads, or bodies, of comets, 
viewed through a telefcope, fheyv a face very different from 
thofe of the fixed ftars or planets. They are liable to ap¬ 
parent changes, which Sir Ifaac Newton aferibes to chan¬ 
ges in their atmofphere ; and this opinion was confirmed 
by obfervations of the comet in 174+. Hift, Acad, Scienc. 
1744. Sturmius fays, that, by obferving the comet of 
1680 with a telefcope, it appeared like a coal dimly glow¬ 
ing, or a rude mafs of matter illuminated with a dufky 
fumid light, lefs fenfible towards the extremes than in the 
middle; whereas a ftar appears with a round difk, and a 
vivid light. 
Of the comet of 1661, Hevelius obferves, that its body 
was of a yellowifh colour, very bright and confpicuous, 
but without any glittering light: in the middle was a denfi; 
ruddy nucleus, almoft equal to Jupiter, encompaffed by a 
much fainter, thinner, matter. February 5, its head was 
fomewhat larger and brighter, and of a gold colour; but 
its light more dufky than the ftars : and here the nucleus 
appeared divided into feveral parts. February 6, the difk 
was leftened ; the parts of the nucleus ftill exifted, though 
lefs than before: one of them, on the lower part of the 
j K difk 9 
