happen, that is, when tlie éarth approaches, near each
node, as before fhown j ari'd a® the nodes lie at oppofité
points óf the earth’ s orbit^thefe feaföns would be at the
diftance of half a year ffÖrn each other, if the nodes were
ftationary ; bilt as the nodes haye a retrograde motion of
about 10° hi a year, qpd the earth moves about a degree
in a day, thé fea&ns of éclipfes will return, at an
interval of about 9 or 10 léfs than half a year; fo
that if "there be ecJfpfès about the middle of January,
the next eclfpfes may be expected about the firft week
<if J u ly - ;
• On the Nature and Motion of Comets.
15J.' Comets me folid bodies revolving .in very ex*
eentric ellipfes about the fun in one of the foci» and are
Subject to the fame laws as the planets are; but they
differ in appearances from them ; for they are veryftynt
bodies, and in fome of them, as they approach the fun,
a tail of light begins to appear, which increafef tSfibp
comet comes- jtpr its perihriion.'and then,ItyitoSMfa
again, and vaniflies. The ancient'plulofbpheia fiippofed,
fhem fi£he-planets. A ris to t le fays, that« fame Ital-
iam dalled Pythagoreans, fay, that a comet isioixe of the-
planets. A pollonius affirms, that the comets were,
by the, Chaldeans, reckoned amongft the pepets, and
had their periods. Seneca having canfideredtiie pfiseuo-
mena of two remarkable comets, believed them to be of
equal duration with the world, though he whs ignoriint
of the laws which governed them: and foretold; that
future ages would- unfold tllefe myfteries. Herecom-
mended it to aftronomers-to keep a catalogue of them,
in order to be able to determine whether they returned
at certain periods. Nótwithftandiligthjs, inöft aftroui-
inert from his time to T yo b o B rak e confideredtHem
only as meteors, exifling in our atmofphère; but he;
finding that they had no diqrnal parallax, pkeed them
above the moon. A t length S ir I. N k-wton having
proved that K epler’ s law, by- which the motions of
the' planets are regulated, was a neceffaiy confequence
of lijs theory of gravity, it immediately-followed, that
iepmets were governed by the fame law-; and tke-ob&fo
various upon them agreed fo accurately with his theory,
as to leave no doubt of its truth. Comets therefore re-
volye in very excentrie ellipfes about the fun in one o f
the foci. Aftronemers, however, for the cafc of calcti-
tion, fuppofe thepi to move in parahoKc orbifs, for that
part which lies within the reach; of obfervation, by which
they can, with greafc -aceoraey, find the place of the
perihelion ; its diftance from the fun; the inclination
of the plane of its orbit to the ecliptic.; and-the place of
the node, but not the periodic time.
' 156. D r. Halley ftippofed that the. comet which
was obferved by A f u s , in W , was the’ fame ns that
which K epler and L ongömöntanus deferibed in
1531 ; and the fame as that which .he obferved in 1682 ;
aiid having computed the effed-.of Japitef upon it at
that time, He found that it would increafe its'pertedid
time above a year ; in confequence of which he'.predicted
its return at the-end of the year lyjtl, or fhs
besinning of- *759.. iHe^fuforms us that he- did'npt
make his computations with the utmoft accuracy; but lu6
prediction was right, for it'was fee»’ on Dec/14, i-yyg5,
and paffed'its perihelion onihjhrejb 13; x
had the glory of firff foretdling thereturisof a- comet.
tfT - Comets are not vifible tffittl>i#y-return inter the
planetary régibus. They are
denfe atmofphere,--and from the fidwoppofite to the fun;
they frequently fend-fortha tail, which increafes géithê
comet approaches its perihelion, immediately after which
it-is longeft and tooft lunjin'oot, and then it a generally a
little bent and-convex towards thofe parts to which the
comet is moving; the tail thgn decreafes, and atlaft it va*
nilhes. The fmalleft: ftars are’ feen through the tail, not*
,-withftanding its great thicknefs, which flioiys that the
matter of it is extrerifelj'rare. Aaistqa-;i>;E.thought the
atpiNobe a dun -filter vwqiif >artfifrg from the cornet,
■ Apian, C^gjANreTif cHjO, and others, fuppofed th"aï:4le'
iun’-s r^ s b^pgpfobpg&ed through the tranfparent
headjof the oómetr-were refra&ed, 1 as by a. lensl^ But
the 'figure of the tail does not; anfwer to this. -KSslSr
fup ppfesL ihaL.thsL fouls. rays. carried-oft, fome-of the
grofs parts of the comet. Sir I. Ne,wtp,n thought'that
the-tail was a very thin vajfeur which the head; uu-
tiScQa of the comet, fendsjïut by rpdfqn ofsSheat. D r.
H alley, in--bis dcfofrption of;the slurora Borealis in
1716, fays, “ the ftreins ( f light-fo muchrefembled the-,
long tails óf comets, that, at- firft ‘fight they might'be
well taken ftp-fac&’fo Arifoafferwaids;, ‘ ‘’Biiaijight
feems. to_ have n great4affinity! to' thattwhtèh -.th e; effluvia,
ofdédtric bodies emit in the dark.” :J9l de-'KIasran
caHs'the, tail- of
cornet.^ This opinion. Dp.: H A :* 1L-uoit Supports if>y the
following aiguirientsUui^sd'AuVdfa Boreah's-'ha* -n<->
effeft:tapó*5<theflars i ^ :;4Sifo:ugtr% «ét has dSbgS&pt:
' a 'coinctiCJ-The afmofphere) isfknown-- tacabhuad* with
eleélric matter; nndrihie Rppeaten^ a^ii|®:^fiÉfe?mgttef
in. vacuo, -iseexaéUy like- t-he, appearaace-eftthe' Atoora
Borealis,:whieH from- its-*great- ahknde, may be-con-'
fidired to-be in a«f perfeift- a vaouufn as we oah-makèï
The-eleétricmatter-ifl vacuo foffers the trAysfof^Ht td
prfs through; withoutitbefog: affeifted-by-. them. ’ The
tail of a comet/doe8 not fpriéaiitfelf 'fidrways,. nor doe3
the; '-eleftfic matter. . Hence, he fu^ofesitKe- -tails of
•komets, the .aurora borealis,' and the ele&ric- fluid, to bd
matter of the fame kind-.
tefpea tothe nature of comets, SiriIïïNsw-,
-jon obferves, that- they. muff , be- &Kd'bo^r
planets. For i f they were tiothbig but-vapours; they
muflr-be diffipated when they- come near Sfeéflfuni : For
the comet in 1680, when in its perihelion, was nearer to
the fun than on© fixth of its diameter, therefore the' heat
óf the comet at that time was to fummer heat, as 28000
to 1. But; the heat of boiling- water is. about' 3 times
greater than the heat which dry earth acquires fromfhe
fummer fun-; and the heat of red -hot iron is about 3
or 4 Limes.greater than the heatrof boiling water. Therefore
the heat of dry earth at- the comet,, when,in-ito perihelion,
>‘--f I N T R O D
bel-ion, was-hbont 3000 times grcateritlian red -hot iron. '
B y fuel! heat, all vapours Ü / J !« * i - a r a i f e 1. ’ ■ , j
ic o . This licat o f the comet mult ,be retained.a long
t ;me; For a; r ed jo S g lo b e ;.qF iron o f an inch diameter,,
expofed to the open air, fca/cely lofcs all t h at in an
hour ; but. a greater glól).é would retain its hcat hinger,
in proportion- to its diameter, beëaufe' The for face, at
which it grows..cold, varies in that proportion lei 3“ than::
th'e quantity fttln o t 'matter. Therefore1 a glob e'of red
hot iron as big as the earth, would' fcarcely.' cool in ■
yoGOO.'years. . . Af, y * 4, - - J r 'lf. J;*
- 1 ’. I ' JkCvJ, " '' '' 1 '' S Ö # ' this time, Jit .is
'probable, according to the bell accounts,g thatihere have ,
appeared abdufj^oo comets.:-,; 1 [ " 11 ” time, abouty
ïó o ptb.ei-s.kre re< been^S?j;kWt it is provable,.
that not above one half o f them were comets. J
. ’ On the fixed Stars.
A l i th e hea enly bodies, bey >1 «1 our fyftem, dre
-ealled fixi l Hars becaufe (,f > e fen excëptédj.théy do
n t pj 1 it h ve-.any pi per motion o f ffi ir own.
1 iin beii imme fe diffanGe; the, 11 ill b bodies o f
verj great-n a.gn t tides, othei & they c ijild; t V >-
ble ; arid when we Gonfider the weakriefs o f reflfedted
light, there can, Be-np'.donb't but |®||)they Ih-ine with their.
own liglit. They "are eafily known’ from the planets,
b y ’Iheir, twinkling. D r. 1I ersc.hiü., by h s .ate improvements
in' tdefcopes, has; di’fcovetied that tliejium
her o f fixed ftars is great- beyond all conception. In the
milky may, he has, in a: qnartei o f an hour; feen 116000
ftars' pafs through his telefcope, the field -of view of
which v is only 15 aperture Theft 11 rs/.which- can
-be < if no tilt -to u , are probably fu s to other fyftems óf
planets. ’ *.
162. Fron-i an- attentivë~exa.minatipri oftjnte'ftars'with
good telcfcppc rfmauy wl <-:h ippear inly fu It to the
naked eye, are found-to ctinfift o f two,, three, or more
ftars- D r., M a sk e lyn e had' obferved « h'crculis to be'a-_
dörible’ ftar ; and olhcrtf&ptio'iricis have (l-ifcóvcrcibmaiiy .
others to be doTililc i f D r. H iel ha rfoi ‘ j
yöp-, of which, npt ah ivi 42 had been before, obfervt 1.
W e willib-eye-mention a few; o f them.
a \Herculi a .beatitiful double fta r; the two ftars
very.unequal^ the largeft is' red,: and. the- fmalleft blue, |
inclining' to gi eea. c J
y JlndronmU, doiiblc;’very unequal-; the' larger red-
•dim-white, the fmaller a fine br ght flcy blue, melt
■ green:./, ' V ■ - ’ " - '- i . r *
a, Geminohm double, a little micqrialj bolli while.
J3 *A«r<s, quadv-ujile, amcqual; yvliite, but tltreeof them
a little inclinèdTq-iplï ■ '
BoOtts, dbtïbie, very unequal; larger;ifeaSifh'Jiimaller
blue; br f a t t e a'feiint lilac.' '**>
treble, very unequaly! d. *'• -in?u anfi-i sf I
liotll d'UÏIcyA;, ; : : . - ; y-;
a’-Lyne, double, vefy-iuiequalj •' ®[|ian I
.-yyiiite, 'fm&ferlifH'flc.y-.
U C. T I O N ; ■ xxxiii
I Thclc are a few"of the principal double, treble and]
qnad'ruiple::'']ft:ji|s®u'eritioried by D r. H eRschel in the
P'Ml T ia n f ,1785,
163, SCvei^^^feentio'n'efti.'by the ancient- aftroiio-J
mers'are ii,Gt rfpiyv;to':;be fourid-.p.-and feveral are now ob-J
ferve'd, .which; do riot appear in their catalogues. Tile mo ill
ancient obfervation o f a new ftar, is that by H 1 p p AUcm s ,1
about 120 years before J. C. which occafioned his mailing
a catalogue o f the fixed ftars; in order that future
aft'T-pnomcrs might- fee! what alterations-had taken place
G&yma, on ]Mov.-8, 1572,
obferved-^i;n;e\yiffar in the chair of !ca]]iipe’a. I t exceeded
Jirius i-n'brightnefs,| and was feen at mid-day. I t . firflj
appeared -'bigger than jupitsr ; but it gradually decayed,!
anrl -'after.'pS-fmontbs it'.eiitirely disappeared, ft- was op- .
■ ferved b y TvcHOj-w-hu found that it had no' fenfible pk*
rallax ; and he;,epneluded-that it wak'ai'iixed ftar. •
164- Many ‘ ftarSpappear and difappear at certain
periods. O n yA d g u ft 1 . 3 , - D avid aF abr ic iu s
obferved a new.ftar in. the neek o f the whale, j .I t * difap-
peared after Oclober iii||jilie- ianw- y-ear. Piip.CYLXipES
iffpLWARDA d-ifeovered- it again in 1637 ’; and after it
had difapperired 1 he faw it again. B u l -
-hi'ALD.y s 'd'etermined the periodic time o f its greateft
.brightnefs. to be 333 days. Its i s '.
that o f a fta-r o f the fecond rijagnitnde, and its leaft, that
-of a ftar o f the fixth *
■ ii( 165. In 1686, Kirchius obferved x >n thefavan to
-be a changeable ftar, and found the period- to be -405
|dp®5 H p
< r66 J. G 0.0Da 1 e k k, Efqa bias determined' the peri -
odic W-riati'drii ‘ ,- soffiperfei, to-.be abSB't:2'd; 21I1.
Its great#Biii|Mncfs:is pf_'-tlie i ^ | g n f Itfalt o f the
fourth m'SgffiitJjJ} . . It.changes from;i-lve fecond to the ;
fourth; 1 1 about 3J. hours,: and back-again in the'liune'
time, and retains its 'full briglltriefs for the remdfmhg
time He alfo .difeoveied that /S lyre, and i cephei, are
fubjeSi. .to- a- .periodic-' yariariat^£ibi^gcit^e®^| flicformer
in I2d; 19b. md'.the latte inyd. 8h 371'.
167- E . P ig > r'T:, cfcp' Sift r Wt 1 oS.^dnt'^ioifQtTtf' va-
ria-hl'c .ftar,.wii'h W-pcribd-o'f 7«!. 4h. 38.'/ '.«
'161!. D r H;erschel in t\ie P hili fK a n fif ’]8 f . has
given ia' large were ft .r ly feen’, .
iB&Sare:nmv ' 'Harsiyand.
ofh.eisjfiars.
S 0 Sl5g. There have, been various corijidures to aeeount
for the-vat-ia'bidkppearanpes'of thc]changeabje-ftars. M.
M aup.ERTuis' fiippofes, that tbdyi Sntt^ttave fo quick a
motiSifCateqri^felriC a*e®> t^'aijdheir. centiafugal forces
mayf'ied Ltl 1 fo flat ob it fob poid , hot mu h iin-
like 1 -millfftone■; and its planel may be inclined to the
j,lai I of theot-fiits o f ils-plan;ets, by wh'ofei attfaiSmn the
pbljilSW: ;of the bo'dy imay bp alteredv ifo it|atSyiS “ in: p;
plane; paffes Mgfcirth it m^y befalmoft ;or en'-
ti-rcly vifib||l againrasrits. broadlide
is turii id-tow rds us ’ O hi rs ft
pa ts of h irfhf'friCes-.arfe'-G verediwitl dark.'fpdfsjijidneK
rendci-i the body invifible fwhen fe j^ a re turric^®|aVd's '
us. ’ Otliii s t-oujediiVp''• ili.at 1 heir .d.ilappearancc may
f . arife