A S T R O N O M- Y. 
ing the periodic time, we may find the major axis of the 
ellipfe; and, the perihelion diftance being known, the mi¬ 
nor axis will be known. When the elements of the orbits 
agree, the comets may be the fame, although the periodic 
times fhould vary; as that may arife from the attraction 
of the bodies in our f'yftem, and whicli may alfo alter all 
the other elements a little. We have already obferved, 
that the comet which appeared in 1759 had its periodic 
time increafed confiderably by the attraction of Jupiter 
and Saturn. This comet was feen in 16S2, 1607, and 
1531, all the elements agreeing, except a little variation 
of the periodic time. Dr. Halley fufpected the comet in 
1680 to have been the fame which appeared in nort, 531, 
and forty-four years before Chrifl. He alfo conjeCtured, 
that the comet obferved by Apian in 1532. was the fame 
as that obferved by Hevelius in 1661; if fO, it ought to 
have returned in 1790, but it has never been obferved. 
But M. Mechain having collected all the obfervations in 
1532, and calculated the orbit again, found it to be fenfibly 
different from that determined by Dr. Halley, which ren¬ 
ders it very doubtful whether this was the comet which 
appeared in 1661; and this doubt is increafed by its not 
appearing in 1790. The comet in 1770, whofe periodic 
time M. Lexell computed to be five years and feven 
months, has not been obferved fince. There can be no 
doubt but that the path of this comet, for the time it was 
obferved, belonged to an orbit whofe periodic time was 
that found by M. Lexell, as the computations for fuel) an 
orbit agreed fo very well with the obfervations. But the 
revolution was probably longer before 1770; for, as the 
comet paffed very near to Jupiter in 1767, its periodic time 
might be fenfibly increafed by the aCtion of that planet; 
and, as it has not been obferved lince, we may conjecture, 
w ith M. Lexell, that, having paffed in 1772 again into the 
fphere of fenfible attraction of Jupiter, a new difturbing 
force might probably take place and deftroy the effect of 
the other. According to the above elements, the comet 
would be in conjunction with Jupiter on Atigufl: 23, 1779, 
and its difiance from Jupiter would be only ^-^of its dis¬ 
tance from the Sun, confequently the Sun’s action would 
be only ^ 4 ^ times that of Jupiter. What a change mult 
this make in the orbit! If the comet returned to its pe¬ 
rihelion in March 1776, it would then not be vifible. See 
M. I.exell’s account in the Phil. Tranf. 1779. The ele¬ 
ments of the orbits of the comets in 1264 and 1556 were 
fo nearly the fame, that it is very probable it was the fame 
comet; if fo, it might be expeCted to appear again about 
the year 1848. 
Sir If. Newton conjeCfures, that, as thofe planets which 
are nearefi to the Sun, and revolve in the lead orbits, are 
the fmalleft; fo, among the comets, fuch as in their peri¬ 
helion come nearefi: the Sun are the (mailed, and revolve 
in the lead: orbits. As to their apparent velocity, the co¬ 
met of 1472, as obferved by Regiomontanus, was fuch as 
to carry it through forty degrees of a great circle in twen¬ 
ty-four hours; and it was obferved, that the comet of 
1770 moved through more than forty-five degrees in the 
laft twenty-five hours. M. Facio has fuggefted, that fome 
of the comets have their nodes fo very near the annual or¬ 
bit of the Earth, that, if the Earth fhould happen to be 
found in that part next the node at the time of a comet’s 
palling by ; as the apparent motion of a comet will be im- 
menfely fwift, fo its parallax will become very fenfible; 
and its proportion to that of the Sun will be given : whence, 
Inch tranfits of comets will afford the belt means of deter¬ 
mining the difiance between the Earth and Sun. The co¬ 
met of 1472, for infiance, had a parallax above twenty 
times greater than the Sun’s: and, if that of 1618 had 
come down in the beginning of March to its defeending 
node, it would have been much nearer the Earth, and its 
parallax much more notable. But hitherto none has 
threatened the Earth with a nearer appulfe than that of 
1680: for, Dr. Halley finds, by calculation, that Nov. 11, 
at ih. 6'. after noon, that comet was not mpre than one fe- 
midiameter of the Earth to the northward of the Earth’s 
Vol. II. No. 79. 
path ; at which time, had the Earth been in that of its 
orbit, the comet would have had a parallax equal to that 
of the Moon. What might have been the confequence of 
fo near an appulfe is at belt uncertain. Mr. Whifion at¬ 
tributes the univerfal deluge in the time of Noah, to the 
near approach of a comet. His opinion was, that the Earth, 
parting through the atmofphere of the comet, attraif- 
ed therefrom great part of the water of the flood ; that 
the nearnefs of the comet railed a great tide in the fubter- 
raneous waters, fo that the outer cruft of the Earth was 
changed from a fpherical to an oval figure ; that this could 
not be done without making fi(lures and cracks in it, thro’ 
which the waters forced themfelves, by the hollow of the 
Earth being changed into a lefs capacious form ; that, a- 
long w ith the water thus fqueezed up on the furface of the 
Earth, much (lime or mud would rife ; which, together 
with the groffer part of the comet’s atmofphere, would, 
after the fubfiding of the water, partly into the fiflures and 
partly into the lower parts of the Earth to form the fea, 
cover all over, to a confiderable depth, the antediluvian 
Earth. Thus he accounts for trees and bones of animals 
being found at very great depths in the Earth. He alfo 
held that, before the fall, the Earth revolved round the 
Sim in the plane of the ecliptic, keeping always the fame 
points of its furface towards the fame fixed ftars. By this 
means, as every meridian would come to the Sun but once 
in every revolution, a day and a year were then the fame: 
but that a comet, ftriking obliquely upon fome part of the 
Earth, gave it the diurnal rotation ; that the antediluvian 
year confided of 360 days ; but, that the additional matter 
depofited upon the Earth from the atmofphere of the co¬ 
met at the flood, fo retarded the revolution thereof round 
the Sun, that it is not now performed in lefs than 365 days 
and about a quarter. The fame comet he thought would 
probably, coming near the Earth when heated in an im- 
menfe degree in its perihelion, be the inftrumental caufe 
of that great cataftrophe, the general conflagration, fore¬ 
told in the facred writings, and conjeftured from ancient 
tradition. 
That this is very poffible, admits not of a doubt; for, 
the heat of the comet of 1680, was to the heat of the fum- 
mer fun as 28,000 to 1. The heat of boiling water is 
about three times greater than the heat which dry earth 
acquires from the fumtrier fun: and the heat of red-hot 
iron about three or four times greater than the heat of 
boiling water. Therefore the heat of dry earth at the co¬ 
met, when in its perihelion, was about 2000 times greater 
than red-hot iron. This heat of the comet mu ft alfo be 
capable of being retained a very long time. For a red-hot 
globe of iron, of an inch diameter, expofed to the open 
air, fcarcely lofes all its heat in an hour; but a greater 
globe would retain its beat longer, in proportion to its 
diameter, becaufe the furface, at which it grows cold, 
varies in that proportion lefs than the quantity of hot mat¬ 
ter. Therefore a globe of red-hot iron, as big as our 
Earth, would fcarcely cool in fifty thoufand years ! 
From the beginning of our era to this time, it is pro¬ 
bable, according to the beft accounts, that there have ap¬ 
peared about 300 comets. Before that time about 100 others 
are recorded to have been feen, but it is probable, that not 
above half of them were comets. And, when we confi- 
der, that many others may not have been perceived, from 
being too near the Sun, from appearing in moon-light, 
from being in the other hemifphere, from being too finall 
to be perceived, or which may not have been recorded, 
we might imagine the whole number to be confiderably 
greater; but it is likely, that of the comets which are record¬ 
ed to have been feen, the fame may have appeared feveral 
times, and therefore the number may be lefs than is here 
fiated. The comet in 1786, which appeared on Auguft 1, 
was difeovered by Mifs Caroline Herfchel, a filler of Dr. 
Herfchel; and fince that time The has difeovered three 
others. A comet was alfo difeovered at Paris, by M. Bou- 
vard, on Aug. 14, 1797, which was feen in England, on the 
18th of the lame month, by the Hon. Capd Loft, of Frof- 
5. L ton, 
