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nets : For the Heat of the Sun, 'tis allow 'd, H as the Den- fity of his Rays, /. e. reciprocally as the Squares oF the Dis- tances of Places from the Sun : Wherefore, fince the Dif- tance of that Comet in its Perihelion, (December the Sth, was obferv'd to be to the. Diftance of the Earth from the Sun, nearly as 6 to icoo; the Sun's heat in the Comet, at that time, was to his heat with us at Midfinmiter, as 1000000 to 3(5", or 28000 to r.
And again, rinding by Experiment, that the Heat of boil- ing Water, is little more than three times the heat of our dry Earth, when expos'd to the AUdfummer's Sun; and af- firming the heat of red-hot Iron to be about three or four times as great as that of boiling Water : he concludes, that
' Nature. • — —ghiid autem mirannlr Cometas\ tarn Yaniftt munM fp&aculwm nondum teneri legibus certis, nee initia illorum finefque note/cere, quorum ex ingentibus intervalliS
recurfus efl? Veniet Tempus quo ifia qu<e nunc latent;
in lucem dies extrabat ££ longioris JEvi ddigentia. Veniet Wempus quo pojieri ncjlri tarn aperta nos nefcijje mirentun
■ Brit qui demonjlrct aliquando, in quibus Cometae par-
tibus errent; cur tarn ftdu&i a ceteris eaut, quanti qua- lefque fint ~
This Prediction we have fcen accomplifti'd m our Days, by the great Sir /. Newton; whofe Dottrine is as follows.
The Comets are compact, IbJid, fix'd, and durable Bodies 5 in one word, a kind of Pla
n„»„- — D - ......... ,.„,«, „ , LJ11U u, j.,anets; which move in very oblique
the heat of the dry'd Earth, or body of the Comet in its Orbits, every way with thegreateft freedom; perfevering in
Perihelion, muft be near 2000 times as great as that of red- their Motions, even againft the Courf'e and Direction of the
hot Iron. Planets : And their Tail is a very thin, Ilender Vapour*
Such an Immenfe heat once acquir'd in its Perihelion, the emitted by the Head, or Nucleus of the Comet, ignited
Comet rauft be a long time in cooling again, The fame Au- or heated by the Sun.
thor computes, that a Globe of red-hot Iron, of the Dimen- sions of our Earth, would fcarce be cool in '50000 Years. If then the Comet be fuppos'd to cool ico times as faft as red-hot 'Iron; yet, fince its Heat was 2000 times greater, fuppofing it of the bignefs of the Earth, it would not be cool in a Million of Years.
fames 'Bernoulli, in bis Syjlema Cometarum, fuppofes
This at once folves all the Phenomena : For rft, ' 'Tis evi- ' dent, that thofe which proceed according to the Order of the ' Signs, a little before they difappear, mult move more flowly, Q or appear retrograde, if the Earth be betwixt them and the 1 Sun j andlwifter, if the Earth be in a contrary Part: on the c contrary, thofe proceeding againft the Order of the Signs^ c?rtc.* For fince their Courfe is not among the fix'd Stars, but
fome primary Planet, revolving round the Sun in the Space among the Planets; as the Motion of the Earth either con of four Years and 157 Days 5 and at the diftance, from his fpires with 'em, or goes againft 'em; their Appearance* Body, of 2583 Semidiameters of the magnm Orbis 1 ^This with regard to the Earth, mull be changed; and, like the
Planet, he concludes, either from its vaft diftance, or fmall- nels, to be invifible to us 5 but, however, to have, at vari- ous Diftanccs from him, feveral Satellites moving round him,andfometimes defending as low as the Orbit of Saturn; and that thefe becoming vifible to us, when in their 'Perigce- nm, are what we call Gmets,
Planets,they muft fometimes appear fwificr, fometimes flower*
and fometimes retrograde. See Direction, Retrograda.-
tion, £5?c.
idly, e When the Comets move the fwifteft, they muft
c proceed in ftrait Lines, but in the end of their Courfe de- cline, £Sc.' Becaufe in the end of their Courfe, when they
fZ)es Cartes advances another Opinion : He conjectures that recede almoft directly from the Sun, that part of the appa-
Comets are only Stars, formerly fix'd, like the reft, in the re nt Motion which arifes from the Paraliax, muft bear a
Heavens; but which, becoming by degrees cover'd with greater proportion to the whole apparent Motion.
Mtcute, or Spots, and at length wholly robb'd of their -dly, ' The Comets muft move'in Ellipfis's, having one of
Light, cannot keep their place, but are carried off by the ' the j r p oc j j n the Centre of the Sun.' Becaufe they don't
Vortices of the circumjacent Stars; and, in proportion to wander precariouily from one fictitious Vortex to another;
their Magnitude and Solidity, mov'd in fuch manner, as to but, making a part of the folar Syftcm, return perpetually*
be brought nearer the Orb of Saturn; and thus coming and run a conftant round. See Orbit.
within reach of the Sun's Light, render'd vifible. See Car- Hence, their elliptic Orbits being very long and eccentric^
tesianism. they become invisible, when in that Part molt remote from.
But the Vanity of all thefe Hypothcfes abundantly appears the Sun.
from the Phenomena of Comets; the chief of which we From considering the Curvity of the Paths of Comets, Sir 7;
/hall enumerate : as being the Teft by which all Theories concludes, that when they difappear, they are much beyond
arc to be tried. the Orb of Jupiter; and that in their Perihelion they fre-
Firft, then, thofe Comets which move according to the quently defcend below the Orbits of Mars and the inferior
Order of the Signs, do all, a little before they difappear, ei- Planets.
ther advances lowlier than ufual, or elfe go retrograde, if the Earth be between them and the Sun; and more fwiftly, if the Earth be fituate in a contrary Part : On the contrary, thofe which proceed contrary to the Order of the Signs, pro- ceed more fwiftly than ufual, if the Earth be between them
4thly, * The Light of their Nuclei muft increafe in their ' Rcccfs from the Sun, and vice verfa;' Becaufe, as they are in the Regions of the Planets, their Aecefs toward the Sun, bears a confiderable proportion to ihcir whole diftance.
From Obfervations of the Comet of 16*80, Sir I. Newton
and the Sun; and more flowly, or go retrograde, when the found thatthe Vapour in the Extremity of the Tail, fan*z$i
Earth is in a contrary Parr. began to afcend from the Head before ^December the nth;
adly, So long as their Velocity is encreas'd, they move, and had therefore fpent more than 45 Days in its Afcent :
nearly, in great Circles; but towards the end of their Courfe, but that all the Tail which appear'd (December 10. afcended
deviate from thofe Circles; and as often as the Earth pro- in the Space of thofe two Days, then juft paft, fince its
cecds one way, they go the contrary way. Perihelion. The Vapour, therefore, at the beginnings
jdly, They move in Ellipfis's, having one of their Foci when the Comet was near the Sun, afcended prodigioulfy
in the Centre of the Sun; and by Radii drawn to the Sun, fwift; and afterwards continu'd to afcend with a Motion
defcribe Areas proportionable to the Times. retarded by the Gravity of the Particles; and by that
4thly, The Light of their Bodies, or Nuclei,, increafes in Afcent increas'd the length of the Tails : But the Tail*
their Recefs from the Earth towards the Sun; and, on the con- trary, decreafes in their Recefs from the Sun towards thelEarth.
5thly, The Tails appear the largeft and brighteft, imme- diately after their Tranfit thro' the Region of the Sun.
tfthly, The Tails always decline, from a juft oppofition to
the Sun towards thofe Parts which the Bodies, or Nuclei pais Hence, alfo.
notwithstanding its length, confided almoft wholly of Va- pours, which had afcended from the Time of the Perihelion 5 and the Vapour which afcended firft, and compos'd the Ex- treme of the Tail, did not vani/h, till it was too far from the Sun to be illumin'd by him, and off us to be vifible.
over, in their progrefs thro' their Orbits.
7thly, This Declination, ceteris paribus, is the fmalleft, when the Heads, or Nuclei approach ncareft the Sun : and tefs, ftill, near the Nucleus of the Comet than towards the Extremity of the Tail.
the Tails of Comets that are /hotter, do not afcend with a quick and continual Motion from the Head, and then prefently difappear; but are permanent Columns of Vapours and Exhalations, gathered from the Head by a very gentle Motion, and in a great fpace of time; which yet, by participating of that Motion of their Heads they had 8thly, The Tails are fomewhat brighter, and more dif- at the beginning, continue cafily to move along with their tincf ly defined in their convex than in their concave Part. Heads thro' the eclertial Regions : whence alfo the Vacuity*
9thly, The Tails always appear broader at their upper of thofe Regions is argued. See Vacuum. Extreme than near the Centre of the Comet. 5thly, ' Their Tails muft appear the largeft and brighteft
iothly, The Tails are tranfparent, and the fmalleft Stars ' immediately after their Tranfit thro' the Region of the' appear thro' 'em. ' Sun :' Becaufe, then, their Heads being the moll heated*
Thefe are the chief Pharnomcna of Comets; which, how will emit the moft Vapours, ill they confift with the wild Notions of the Antients, and From the Light of the Nucleus, or apparent Star, we in- thc weak Conjectures of moft of the Moderns, is pretty fer their Vicinity to the Earth, and that they are by no) evident. Indeed, there were fome, Pliny tells us, among means in the Region of the Fix'd Stars, as fome have ima- thc Antients, who ' hadjufter Notions, who took thefe Stars gin'd; fince in that cafe, their Heads would be no more illu-
to be perpetual, and bclicv'd they moved in their proper min'd by the Sun, than the Planets are by the Fix'd Stars. ' Orbs; but were never fecn unlefs when left by the Sun :' dthly, ' The Tails muft ftill decline from a ftrift oppofi- And more fully Seneca, S^tccft. Nat. Lib. VII. ' I am not c tion to the Sun, towards thofe Parts which the Heads pafs ' of the common Opinion, nor do I take a Comet to be a ' over, in their progrefs thro' their Orbits : Becaufe, all ' fudden Fire, but eitcem it among the Eternal \V»rks of Smoke, or Vapour emitted from a Body in Motion, tends up-
A a a a warJs-
