4<h AST R O 
accuracy, but, as he himfelf informs, hvi calamo. M. 
Clairaut computed the effefts both of Saturn and Jupiter, 
and found that the former would retard its return in the 
laff period ioo days, and the latter 511 days; and lie de¬ 
termined the time when the comet would come to its pe¬ 
rihelion to be on April 15, 1759, obferving that he might 
err a month, from neglecting fmall quantities in the com¬ 
putation. It palled the perihelion on March 13, within 
thirty-three days of the time computed. Now, if we fup- 
pofe the time dated by Dr. Halley to mean the time of 
its palling the perihelion, then, if we add to that 100 days, 
arifirig from the abtion of Saturn which he did not confi- 
der, it will bring it very near to the time in which it did 
pal’s the perihelion, and prove his computation of the ef¬ 
fect of Jupiter to have been very accurate. If he mean 
the time when it would firft appear, his prediction was 
very accurate, for it was firft feen on the 14th day of De¬ 
cember, in the year 1758 ; and his computation of the ef¬ 
fects of Jupiter will then be more accurate than could have 
been expedited, conlidering that he made his calculations 
only by an indirect method, and in a manner proferiedly 
not very accurate. Dr. Halley therefore had the giory, 
firft to foretel the return of a comet, and the event an- 
fwered remarkably to his prediction. He farther obferved, 
that the action of Jupiter, in the defcent of the comet to¬ 
wards its perihelion in 1682, would tend to increafe the 
inclination of its orbit; and accordingly the inclination in 
1682 was found to be 22' greater than in 1607. A learned 
profelfor (Dr. Long’s Aftronomy, p.562) in Italy to an 
Englilh gentleman writes thus : “Though M.delaLande, 
and fome other French gentlemen, have taken occafion to 
.find fault with the inaccuracies of Halley’s calculation, be- 
caufe he himfelf had faid he only touched upon it (lightly ; 
neverthelefs they can never rob him of the honour,—Firft, 
of finding out that it was one and the fame comet which 
appeared in 1682, 1607, 1531, 1436, and 1305. Secondly, 
o.f having obferved that the planet Jupiter would caufe 
tiie inclination of the orbit of the comet to be greater, and 
the period longer. Thirdly, of having foretold that the 
return thereof might be retarded till the end of 1758, or 
the beginning of 1759.” From the obfervations of M. 
Meflier upon a comet in 1770, M. Edric Profperin, mem¬ 
ber of the royal academiesof Stockholm and Upfal, (hew¬ 
ed, that a parabolic orbit would not anfwer to its motions, 
and he recommended it to aftronomers to feek for the el¬ 
liptic orbit. This laborious talk M. Lexell undertook, 
and has (hewn that an ellipfe, in which the periodic time 
is about five years and feven months, agrees very well 
with the observations. See the Phil. Tranf. 1779. As the 
ellipfes which the comets defcribe are all very eccentric, 
aftronomers, for the eafe of calculation, fuppofe them to 
move in parabolic orbits, for that part which lies within 
the reach of obfervation, by which they can very accu¬ 
rately rind the place of the perihelion, its diftance from 
the Sun, the inclination of the plane of its orbit to the 
ecliptic, and the place of the node. 
But, concerning the return of comets, there have been, 
and (till are, different opinions. Sir Ii’aac Newton, Flam- 
ftead, Hailey, and other Englifh aftronomers, feem fatisfied 
of the return of comets: Caffini and fome of the French 
think it highly probable ; but M. de la Hire, and others, 
oppofe it. Thofe on the affirmative fide fuppofe, that the 
comets defcribe orbits prodigioufly eccentric, infomuch 
that we can fee them only in a very fmall part of their re¬ 
volution : out of this, they are loft in the immenfity of 
(pace; hid not only from our eyes, but from our tele- 
fcopes: that little part of their orbit next us pafling fome- 
times within thofe of all the inferior planets. M. Caffini 
gives the following reafons in favour of their return-: 1. 
it is found that they move a confiderable time in the arch 
of a great circle, when referred to the fixed ftars, that is 
a circle whofe plane paries through the centre of the Earth ; 
deviating but a little from it chiefly towards the end of 
their appearance; a deviation however common to them 
with the planets. 2. Comets, like planets, appear to move 
N O M y 0 
fo much the fafter as they are nearer the Earth ; and, wiSiea 
they are at equal diftances from their perigee, their velo¬ 
cities are nearly the fame. By fubtradfing from their mo¬ 
tion the apparent inequality of velocity occalioned by their 
different diftance from the Earth, their equal motion might 
be found: but we (hould not (fill be certain that this is 
their true motion; becaufe they might have confiderable 
inequalities, not diftinguifliable in that fmall part of their 
orbit viiible to us; It is rather probable that their real 
motion, as well as that of the planets, is unequal in itfelf, 
and hence we have a reafon why the obfervations made, 
during the appearance of a comet, cannot give the juft 
period of their revolution. 3. There are no two different 
planets whofe orbits cut the ecliptic in the fame angle, 
whofe nodes are in the fame points of the ecliptic, and 
have the fame apparent velocity in their perigee: confe- 
quently, two comets feen at different times, yet agreeing 
in all thofe three circumftances, can only be one and the 
fame comet. Not that this exadt agreement, in thefe cir¬ 
cumftances, is abfolutely neceffiary to determine their iden- 
dity: for the Moon herfelf is irregular in all of them, fo 
that it feeins there may be cafes in which the fame comet, at 
different periods of revolution, may.difagree in thefe points. 
As to the objedlions againft the return of comets, the 
principal is that of the rarity of their appearance, with 
regard to the number of revolutions affigned to them. In 
1702 there was a comet, or rather the tail of one, feen at 
Rome, which M. Caffini takes to be the fame with that 
obferved by Ariftotle, and again in the year 1668 ; which 
would imply a period of thirty-four years. Now, it may 
feem ftrange that a ftar which lias fo fliort a revolution, 
and of confequence fuch frequent returns, ffiould be fo 
feldom feen. Again, in April of the fame year 1702, a 
comet was obferved by Meflrs. Bianchini and Maraifti, 
which the latter fuppofed was the fame with that of 1664, 
both on account of his motion, velocity, and direction. 
M. de la Hire thought it had fome relation to another he 
had obferved in 1698, which Caffini refers to that of 1652 ; 
which would make it a period of forty-three months, and 
the number of revolutions, between 1652 and 1692, four¬ 
teen. Now, it is hard to fuppofe, that in this age, when 
the heavens are fo narrowly watched, a ftar (hould make 
fourteen revolutions unperceived ; efpecially fuch a ftar 
as this, which might appear above a month together ; and 
confequently be often difengaged from the crepufcula. 
For this reafon M. Caffini is very referved in maintaining 
(he hypothefis of the return of comets, and only propofes 
thofe for planets where the motions are eafy and fimple, and 
are folved without (training, or allowing any irregularities. 
M. de la Hire propofes one general difficulty againft the 
whole fyftem of the return of comets, which would feem 
to prevent any comet from returning as a planet: which 
is this; that, by the difpolition neceflarily given to their 
courfes, they ought to appear as fmall at firft as at laft; 
and always increafe till they arrive at their neareft proxi¬ 
mity to the Earth; or, if they (hould not chance to be 
obferved as foon as they are capable of being feen, it is 
yet hardly poffible but they muff: often (hew themfelves 
before they have arrived at their full magnitude and bright- 
nefs : but, adds he, none were ever yet obferved till they 
had arrived at it. However, the appearance of a comet 
in the month of October 1723, while at a great diftance, 
fo as to be too fmall and dim to be viewed without a tele- 
1’cope, as well as the obfervations of feveral others fince, 
may ferve to remove this obftacle, and fet the comets (till 
on the fame footing with the planets; or at lead to fup- 
pert the prevalent opinion, that the revolution or return 
of the comets is certain. In (hort, the beft way to af- 
certain the fadt feems to be, to compare the elements of 
all thofe comets which have been computed, and, where 
we find they agree, we may conclude that they are ele¬ 
ments of the fame comet, it being fo extremely improba¬ 
ble that the orbits of two different comets (hould have the 
fame inclination, the fame perihelion diftance, and the 
places of the perihelion and node the fame. Thus, know- 
