4®2 ASTRO 
di(k, on the left, much denfer and brighter than the reft ; 
its body round, and reprefenting a very lucid little ftar : 
the nuclei (till encompaifed with another kind of matter. 
February io, the head fomewhat more obfcure, and the 
nuclei more confuted, but brighter at top tlian bottom. 
February 13, tlie head diminiftied much both in fize and 
fplendour. March 2, its roundnefs a little impaired, and 
its edges lacerated, &e. March 28, very pale, and ex¬ 
ceeding thin; its matter much difperfed; and no diftinct 
nucleus at all appearing. Weigelius, who faw the comet 
of 16-64, as alfo the Moon, and a fmall cloud in the hori¬ 
zon illuminated by the Sun at the fame time, obferved, 
that through the telefcope the Moon appeared of a con¬ 
tinued luminous furface: but the comet very different; 
being exactly like the little cloud. And from thefe ob- 
fervations it was that Hevelius formed his opinion, that 
comets are like maculae or fpots formed out of the folar 
exhalations. 
The eftimates that have been given of the magnitude of 
comets by Tycho Brahe, Hevelius, and fome others, are 
not very accurate; as it does not appear that they diftin- 
guifhed between the nucleus and tire furrounding atmof- 
phere. Thus Tycho computes that the true diameter of 
the comet in 1577 was in proportion to the diameter of the 
Earth, as 3 is to 14; and Hevelius made the diameter of 
the comet of 1652 to that of the Earth, as 52 to 100. But 
the diameter of the atmofphere is often 10 or 15 times as 
great as that of the nucleus: the former, in the comet of 
1682, was meafured by Flamftead, and found to be 2', 
when the diameter of the nucleus alone was only 11" or 
12". Though fome comets, eftimated by a comparifon 
©f their diftance and apparent magnitude, have been jud¬ 
ged much larger than the Moon, and even equal to fome 
of the primary planets. The diameter of that of 1744, 
when at the diftance of the Sun from us, meafured about 
1', w hich makes its diamete. oout three times that of the 
Earth : at another time, the diameter of its nucleus was 
nearly equal to that of the planet Jupiter. 
There have been various conjectures concerning the na¬ 
ture of the tails of comets. The opinion of the ancient 
philofophers, and of Ariftotle himfelf, was, that the tail 
is a very thin fiery vapour arifing from the comet. Apian, 
Cardan, Tycho, and others, believed, that the Sun’s rays, 
being propagated through the tranfparent head of the co¬ 
met, were refracted, as in a lens. But the figure of the 
tail does not anfwer to this; and, moreover, there ftiould 
be fome reflecting fubftance to render the rays vifible, in 
like manner as there muft be duft or fmoke flying about in 
a dark room, in order that a ray of light entering it may 
be feen by a fpeCtator (landing fide-ways from it. Kep¬ 
ler fuppofed, that the rays of the Sun carry away fome 
of the grofs parts of the comet which reflects the Sun’s 
rays, and gives theappearance of a tail. Hevelius thought, 
that the thinned parts of the atmofphere of a comet are 
rarefied by the force of the heat, and driven from the fore¬ 
part and each fide of the comet towards the parts turned 
from the Sun. Sir Ifaac Newton thinks, that the tail of 
a comet is a very thin vapour, which the head, or nucleus, 
of a comet, fends out by reafon of its heat. He fuppo- 
fes, that when a comet is defcending to its perihelion, the 
vapours behind the comet, in refpett to the Sun, being ra¬ 
refied by the Sun’s heat, afcend, and take up with them 
the reflecting particles with w hich the tail iscompofed, as 
air rarefied by heat carries up the particles of fmoke in a 
chimney. But, as beyond the atmofphere of a comet, the 
ethereal air {auram atheream) is extremely rare, he attri¬ 
butes fomething to the Sun’s rays carrying with them the 
particles of the atmofphere of the comet. And, when 
'■the tail is thus formed, it, like the nucleus, gravitates to¬ 
wards the "Sun; and, by the projectile force it received 
from the comet, it deferibes an ellipfe about the Sun, and 
accompanies the comet. It conduces alfo to the afeent of 
thefe vapou/s, that they revolve about the Sun, and there¬ 
fore endeavour to recede from it; whilft the atmofphere 
«f the -Sun is either at reft or moves with fuch a flow mo- 
x 
N O M Y. 
tion as it can acquire from the rotation of the Sun about 
its axis. Thefe are the caufes of the afeent of the tails 
in the neighbourhood of the Sun, where the orbit has a 
greater curvature, and the comet moves in a denfer atmof¬ 
phere. The tail of the comet therefore, being formed from 
the heat of the Sun, will increafe till it comes to its peri¬ 
helion, and decreafe afterwards. The atmofphere of tlie 
comet is diminiflied as the tail increafes, and is leaft imme¬ 
diately after the comet has patted its perihelion, where it 
fometimes appears covered with a thick black fmoke. As 
the vapour receives two motions when it leaves the comet, 
it goes on with the compound motion, and therefore the. 
tail will not be turned direCtly from the Sun, but decline 
from it towards thofe parts which are left by the comet; 
and, meeting vvitii a fmall refiftance from the ether, will 
be a little curved. When the fpeCtator therefore is in the 
plane of the comet’s orbit, the curvature will not appear. 
The vapour, thus rarefied and dilated, may be at laft flut¬ 
tered through the heavens, and be gathered up by the 
planets, to iupply the place of thofe fluids which are (pent 
in vegetation, and converted into earth. This is the fub¬ 
ftance of Sir Ifaac Newton’s account of the tails of comets. 
But Mr. Rowning, not fatisfied with this folution of 
the phenomena, accounts for the tails of comets in the 
following manner: It is well known, fays he, that when 
the Sun’s light paflfes through the atmofphere of any bo¬ 
dy, as the Earth, that which pafles on one fide is by the 
refraCtion made to converge towards that which pafles on 
the oppofite fide ; and this convergency is not wholly ef¬ 
fected either at the entrance of the light into the atmof¬ 
phere, or at its exit on going out; but, beginning at its 
entrance, it increafes in every point of its progrefs. It is 
alfo agreed, that the atmofpheres of the comets are very 
large and denfe : he therefore fuppofes, that by fuch time 
as the light of the Sun has palled through a conflderable 
part of the atmofphere of the comet, the rays are fo far 
refraCted towards each other, that they then begin fenfibly 
to illuminate it, or rather the vapours floating in it, and 
fo render that part they have yet to pafs through vifible 
to us : and that this portion of the atmofphere of a comet, 
thus illuminated, appears to us in the form of a beam of 
the Sun’s light, and pafles under the denomination of a 
comet’s tail. Rowning's Nat. Philof. part 4, chap. 11. 
M. Euler, Mem. Berlin, tom. ii. p. 117, thinks there is 
a great affinity between the tails of comets, the zodiacal 
light, and the aurora borealis, and that the common caufe 
of all of them, is the aCtion of the Sun’s light on the at¬ 
mofpheres of tlie comets, of the Sun, and of the Earth. 
He iuppofes, that the impulfe of the rays of light on the 
atmofphere of comets, may drive fome of the finer par¬ 
ticles of that atmofphere far beyond its limits ; and that 
this force of impulfe, combined with that of gravity to¬ 
wards the comet, would produce a tail, which would al¬ 
ways be in opposition to the Sun, if the comet did not 
move. But the motion of the comet in its orbit, and about 
an axis, muft vary the pofition and figure of the tail, giv¬ 
ing it a curvature, and deviation, from a line joining the 
centres of the Sun and comet; and, that this deviation 
will be greater, as the orbit of the comet has the greater 
curvature, and as the motion of the comet is more rapid. 
It may even happen, that the velocity of the comet, in its 
perihelion, may be fo great, that the force of the Sun’s 
rays may produce a new tail, before the old one can fol¬ 
low ; in which cafe, the comet might have two or more 
tails. The poffibility of this is confirmed by the comet of 
1744, which was obferved to have feveral tails while it 
was in its perihelion. 
Dr. Hamilton, in his Philofophical Eflays, urges feveral 
objections againlt the Newtonian hypothelis ; and ohferves, 
that we have no abfolute proof of the exiftence of a folar 
atmofphere ; and, if we had, that, when the comet is mo¬ 
ving in its perihelion in a direction at right angles to the 
direction of its tail, the vapours which then arife, parta¬ 
king of the great velocity of the comet, and being alfo 
fpecifically lighter than the medium in which tliey move, 
muft 
