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WILLIAM L. CLEMENTS
LIBRARY OF
AMERICAN HISTORY
UNIVERSITYOF MICHIGAN

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ESSAY
Ο Ν
C O M E T S.


AN
E S S A Y
ON
COM E TS,
In TWO
W OPAR T S.
P A R T I.
CONTAINING an Attempt to explain the Phänomena
of the TAILS of COME TS, and to account
for their perpetual Oppoſition to the SUN, upon
philoſophical Principles.
PART II.
Pointing out ſome important Ends for which theſe
TAILS were probably deſigned : Wherein it is
fhewn, that, in Conſequence of theſe curious Append.
ages, COMETS may be inhabited WORLDS,
and even comfortable Habitations ; notwithſtand-
ing the vaſt Excentricities of their Orbits.
The Whole interſperſed with Obſervations and Re-
flections on the SUN and primary PLANETS.
By ANDREW OLIVER, JUN. Esq.
In human Works, though labour'd on with Pain,
A thouſand Movements ſcarce one Purpoſe gain ;
In God's, one fingle can it's End produce ;
Yet ſerves to ſecond too ſome other Uſe.
Pope's ESSAY ON MAN.
SALEM, New-ENGLAND:
Printed and fold by SAMUEL HALL, near the Exchange,
MDCCLXXII.

2

TO
JOHN WINTHROP, LL.D.
HOLLISIAN PROFESSOR
Of the MATHEMATICS, and NATURAL PHILOSOPHY,
AND
FELLOW of the CORPORATION,Of HARVARD-COLLEGE:
MEMBER of the AMERICAN PHILOSOPHICAL SOCIETY
OF PHILADELPHIA;
AND
FELLOW of the ROYAL SOCIETY.
The following ESSAY,
AS an Acknowledgment of the Obligations which
reſult from an Initiation, under his Inſtruction, in
the Study of thoſe Sciences which more immediately
belong to his Province: In Reliance upon his Pa-
tronage of a Treatiſe, which owes its Publication
to his Candor and Approbation : And, with all
due Deference to his ſuperior Judgments is
fully infcribed, by
His Obliged Friend,
grate-
And
Moſt Humble Servant,
The AUTHOR.

TOIHI
9020
Box
10
vad
АТО, од 20 ОТ ТЯТ.
YA20 wollotor
bil ogildo si obalnom
til motiunini aid obtido fosisingi na cialo
Viss boridi stocarbidwaldo to you
not goesialgnivo aid old
woon
AA bar roba
gostosa
obsini

qi)
PRE FACE.
TH
'H E deſign of the following Elay was, partly to era-
dicate ſome abſurd notions which have been hand-
ed down from the darkeſt periods of antiquity, and
which are ſtill entertained by ſome, upon the appearance of
a Comet; and to remove the apprehenſions which may have
been excited in the minds of others, even by the writings of
ſome great men among the moderns : And partly to offer to
thoſe who indulge themſelves in more abſtruſe reſearches after
the operations of natural cauſes, a few hints, the proſecution
of which may enlarge the field of philoſophical ſpeculation,
and open to all a new ſource for adoration of the wiſdom
and beneficence of that Being, who has made nothing in
vain, and has diſpoſed the various parts of the Univerſe by
weight and meaſure. It may therefore be preſumed, that,
if the Author is ſo happy as to be underſtood by the gene-
rality of his readers (which he has aimed at throughout the
whole) the gentlemen of ſcience will pardon thoſe minute
diſcuſions, which they may judge unneceſſary, and that they
will weigh with impartiality the arguments which may be
offered for their conſideration, overlook any immaterial
inaccuracies, which may have ariſen through inadvertency,
and point out and refute with candour any miſtakes which
he
may
have fallen into
IT is now well known to aſtronomers that the motions of
Comets are regulated by the univerſal law of gravitation,
and that they regard the Sun as the common center of all
their motions, equally with the Planets, alıbough they are
m201 e

( 11 )
more numerous, and apparently diſtina in ſpecies from them.
The Planets, ſince the diſcovery of the Satellites or Moons
which attend ſome of them, are generally,and with the higheſt
reaſon ſuppoſed to be inhabited Worlds like the Earth.
Whereas it bas been thought even by ſome of the greatest
modern aſtronomers, that the extremes of heat and cold to
which Comets are alternately expoſed in the different parts
of their orbits, are irreconcileable with the notion of their
being fil habitations for any material race of beings what-
ever. They are to this day regarded by the bulk of man-
kind, either as portentous meteors, exhibited only to
threaten war, famine or peſtilence to the inhabitants of the
Earth ; or as fiery globes which move thro' the Heavens
at random, and might fortuitouſly come acroſs the Earth in
its way, to it is no ſmall detriment, if not to the deſtruction
of its inhabitants; or laſtly as penal worlds, ordained to a
perpetual chaotic ſiate, whoſe inhabitants are condemned to
be frozen and burned alternately, at their aphelia and peri-
helia ; agreeable to Milcon's ide of the puniſhment of
apoftate Spirits ; Speaking of whom he ſays,
“ They
feel by turns the bitter change
Of fierce extremes, extremes by change more fjerce,
From beds of raging fire to ſtarve in içe
Their ſoft æthereal warmth, and there to pine
Immovable, infix'd, and frozen round
Periods of time, thence hurried back to fire."
PARADISE LOST, Book II.
93
THE firſt of iheſe opinions was exploded long ago by
Seneca ; who, infiead of regarding Comets as tranſient
neteors, on the contrary ranks them with " the eternal
" orks of Nature.” The ſecond direEtly contradic?s the
idea

(iii)
idea of a divine ſuperintendency, and falls of courſe, in the
minds of thoſe who entertain any juſt apprehenſion of the
providential care of the great Author and Governor of the
Univerſe. And as to the laſt, with what colour of reaſon
can we ſuppoſe that the Creator would provide upwards of
fifty Worlds (for ſo many different Comets and more there
certainly are) ſolely for the puniſhment of the incor-
rigible inhabitants of five or Sex Planets only ? On the
contrary does it not redound more to his honour, to conſider
Sheſe bodies as ſo many inhabited Worlds, provided with
every neceſſary for the comfortable ſubſiſtance of innumera-
ble inhabitants, rational and irrational, like the Earth?
Or as Doctor Young (Speaking of the CREATOR
and his works) beautifully expreſſes himſelf ;
« Darts nor HIS Glory a ſtill brighter Ray,
The leſs is left to Chaos and the Realsns
Of bideous Night?
Night THOUGHTS, Night 9.
THE reader will find (irr the introduction) that the
confideration of the ſubjeet, in this light, was firſt ſtarted
by Hugh Williamfon, M. D. of Philadelphia, in c
treatiſe that was publiſhed in one of the weekly papers;
which, as it contained ſome new thoughts, the purſuit of
which (it was judged) would tend to the improvement of
natural philosophy, first induced the Author to undertake
the following Eſſay, upon a ſubjeEt indeed which, till then,
be bad but little attended to : He therefore hopes that,
if any obſervations or reflections ſhall turn up, in the courſe
of it, which may merit the attention of his readers, and may
have eſcaped the notice of others, they may in ſome meaſure
atone

(iv)
atone for the many defeats, of which he is apprehenfive, and
which are ſubmitted to thoſe who have made greater pra-
ficiency in natural knowledge, than he pretends to, to re&tify.
DIVERS of the Author's friends, who are gentlemen
of Speculation and learning, peruſed the Eſſay in manufcript,
upon whoſe approbation of it, and at whoſe requeſt, he was
induced to conſent to its publication : But when the unex-
peated number of ſubſcribers, and their reſpectable characters
in general, came to his knowledge, bis diffidence of its title to
their favourable reception was naturally increaſed, which
would have excited in his mind a proportionable relu&tance to
its appearance abroad, left their expectations ſhould be diſap-
pointed, had be not been fully perſuaded, that the greateſt
candour was naturally to be expected from thoſe who are
the beſt judges of the ſubject ; eſpecially, as the treatiſe co13-
tains nothing more than an attempt to diſcover ſome phi-
lofophical truths, the knowledge of which may tend to pro-
mote the cauſe of Science, and thereby may poſibly be of ſome
Service to mankind.
IT was with pleaſure He found that Doelor Gowin
Knight, F. R. S. had attempted to account for the
phænomena of the tails of Comets from the ſame principle
which he has cadeavoured to eſtabliſh in the following
Pages, viz. a mutual repellency, ſubſiſting between the
atmoſpheres of the heavenly bodies : As the Doctor,
when he wrote his treatiſe, was engaged in the general
ſolution of all the phenomena in Nature, by the help of two
aniverſal principles, viz. Attraction and Repulfion, be but
ſlightly touched upon the particular phenomena, now under
conſideration ; accordingly what he fays upon the ſubjeét is
very

( )
very short, being included within the compaſs of a fingle
corollary, which ariſes out of a long chain of deduEtions
from the ſuppoſed properties of an imaginary repellent
fluid, uniformly diftributed throughout the infinity of
ſpace ; the exiſtence of which, however probable, has never
been proved. When the author wrote that part of the
Ejay which is confined particularly to this ſubject, Doctor
Knight's ſolution was entirely out of his mind, if he had
ever ſeen it before. He therefore took a different courſe to
eſtabliſh the principle, deducing it from the known pro-
perties of air, a real fluid, the exiſtence of which we are
certain of, from the immediate teſtimony of our ſenſes,
as we are of its properties from innumerable experiments.
This element differs effentially from the Doctor's univerſal
fluid, which indeed be ſuppoſes to conſiſt of particles mutually
repellent, like thoſe of air, but ſo inconceivably rare, that
a quantity of it fufficient to fill the whole ſpace occupied by
the ſolar Syſtem, might not weigh one ſingle grain (Knight's
Eſſay, page 15): Air on the contrary, is a heavy fluid, the
preffure of which, merely from its gravity, is capable of
giving grear pain to thoſe who may attempt to ſuſtain its
weight, in pneumatical experiments.
HOWEVER, it does not follow, that if air, cople
ſidered as a fluid ſui generis, is ſufficient immediately to
account for the phenomena before us, we ſhould therefore
exclude the agency of another, upon whoſe exiſtence and
properties (if proved) the properties of air itſelf may poſſible
depend. By ſuppoſing ſuch an univerſal repellent fluid,
Doctor Knight has very curiouſly accounted for many of
the phenomena of Nature, which are so common as to be
but little attended to ; ſuch as fluidity, elaſticity, mag-
netiſm, &c. And, whether ibe mutual repellency of the
particles
و

( vi
particles of air does (as be ſuppoſes) or does not depend upon
the preſence of the ſame fluid, the conclufion will equally
follow that, The aerial atmoſpheres of the heavenly
bodies are mutually repellent.
પણ છે. '
Ti
HE reader is defired to make the following corrections, the neceffity and
viz.
In Page 22, Line 4, for, by the incumbent weight of the whole atmoſphere, to
read, by the weight of the whole incumbent atmoſphere. In Page 38, Line 7,
from the bottom, for, all fluids which tend, &c. to read, every fluid the ſeveral
parts of which tend, &c. In Page 45, Line 8, from the bottom, for, atmoſphere,
co read, atmoſpheres. In Page 78, Line 3, for, decreaſe of its atmoſphere, to
read, decreaſe of the denſity of its atmoſphere. In Page 80, Line 17, to read,
whereby the preſence of the Sun by day; and in Line 19 of the fame Page,
after the word Comet, to inſert theſe words, viz, tban wbex it is is tbe remotar
parts of its orbit.
ESSAY

(1)
INTRODUCTION
TO THE
ans
2002
E SS A Y.
mido
N the month of September, 1769, during the ap-
pearance of a remarkable Comet, Doctor Hugh
Williamſon of Philadelphia favoured the publick
with a treatiſe on the ſubject of the following Eſſay,
which was read before, and publiſhed by order of the
Philoſophical Society in that city. This piece con-
tained fome curious hints, well deſerving the attention
of the friends of ſcience : Particularly, that Com-
ets, as well as the Planets, may be habitable Worlds ;
that the comfortable ſtate the inhabitants of the various
globes of the ſolar ſyſtem enjoy, may not depend merely
upon
their ſeveral diſtances from the Sun; and, that
although the rays of the Sun may be abſolutely ne-
ceſſary to the very exiſtence of planetary heat, yet the
temperature of that-heat may depend upon the denſities
of the atmoſpheres ſurrounding the ſeveral globes ;
whereby the Comets, even in their aphelia, may
be
rendered comfortable habitations, by means of the vaft
atmoſpheres which attend them, and which, when they
might become detrimental to their inhabitants, as when
they are in the neighbourhood of the Sun, are, by the
momentum of the Sun's rays, or by fome other cauſe,
thrown off to immenſe diſtances behind them.
B
THE
30
00

(2)
The peruſal of this treatiſe occaſioned the following
attempt to eſtabliſh the doctrine of the habitability of
Comets, advanced therein by Doctor Williamſon.
We differ indeed in our hypotheſes to account for
the phænomena of their tails; but whichſoever may
be the true one, all bis concluſions, relative to the den-
ſities of their atmoſpheres in the various parts
of their
orbits, may equally follow.
The Doctor's hypothelis, which is the ſame with
Kepler's, ſuppoſes the atmoſphere of a Comet, by the
velocity and conſequent momentum of the Sun's rays,
to be propelled through immenfe ſpaces behind its
body, whereby the tail is formed, which is rendered
viſible by reflecting thoſe rays.
This, it muſt be confeſſed, ſeems to be as natural
and eaſy a ſolution as any one hitherto offered ; and
might be embraced as ſuch, had any one experiment
ever been produced to prove the rays of light endued
with
any ſuch
power, even upon the moſt minute cor-
puſcles, or the moſt rarefied vapor. Sir Iſaac Newton
paſſes it by with little more than a bare mention,
(Princip. under Prop. XLI. Book III.) which, as it is
ſo plauſible, is difficult to account for ; eſpecially as
he fills pages to refute others, that are almoſt felf-
evidently abſurd. This hypotheſis, however, has ne-
ver been confirmed by any one experiment, although
an age has elapſed ſince Kepler's time, during which
experimental philofophy has been at its zenith. There-
fore, as Mr. Profeſſor Winthrop, in his lectures on
Comets, obſerves, « much ſtreſs ſhould not be laid
upon
*

(3)
upon this
upon it, as we know of no parallel inſtance in na-
ture to ſupport it.” The hypotheſis contained in
this Eſſay is founded upon a mutual repellency, which
is ſuppoſed to fubfift between the ſeveral atmoſpheres
of the heavenly bodies ; which is deduced from the
repellency of the particles of air, of which the atmoſ-
phere of the earth, and thoſe which ſurround the other
globes, are compoſed. That our atmoſphere conſiſts
of particles thus repellent, is certain from the whole
ſyſtem of pneumaticks, as no one experiment, in that
uſeful, inſtructive and entertaining branch of natural
philoſophy, can be performed, which does not depend
property of AIR.
That the Sun and
Planets have ſimilar atmoſpheres, will appear in the
following pages.
Sir Iſaac, after having refuted ſeveral hypotheſes,
which had been raiſed by others, to account for the
aſcent of Comets tails from the Sun, at length rather
hints at, than ſubſtitutes one of his own; as he aſſerts
nothing poſitively, but only offers a query for the con-
fideration of his readers. Therefore it is hoped that
nothing here advanced will be conſidered as a vain,
raſh attempt to confute that illuſtrious author.
• aſcent of ſmoke in a chimney (ſays he) is owing to
< the impulſe of the air with which it is entangled.
“ The air, rarefied by heat, aſcends becauſe its ſpecific
gravity is diminiſhed, and in its aſcent carries along
“ with it the ſmoke with which it is engaged.” (Prin-
-cip. ubi ſupra). To which he adds the following query:
“ And why may not the cail of a Comet riſe from the
66 Sun after the ſame manner ?" Previous to an an-
{wer to this query, let us conſider more particularly
the
66 The
65

( 4 )
the cauſes of the aſcent of vapors in our atmoſphere.
It is indeed beyond doubt, that to the cauſe above
affigned, by Sir Iſaac Newton, is in a great meaſure
owing the riſe of ſmoke from common fire ; from
which, if enkindled in the open air, it will curl up in
broad volumes, ſpreading as it riſes ; but if confined
in a chimney, the rarefied air having but one way to
expand itſelf, viz. through the funnel, the more con-
denſed air in the rooms below driving it upwards, to-
gether with the ſmoke engaged with it, it afcends in
a ftrait courſe, with a greater velocity, and apparently
to a greater height than when unconfined.
Sololi
But this is not the fole, nor yet the principal cauſe
of the aſcent of vapors in general, but rather an acci-
dental one, which increaſes their velocity when already
on the wing. For, really, the caufa fine qua non of
their aſcent, in a quieſcent atmoſphere, is the difference
of the ſpecific gravities of thoſe vapors, and the air
they float in*; the former, being generally lighteſt,
continue riſing till they arrive at that region of the at-
moſphere in which the denſities of both are equal..
There they gather and form into clouds ; there they
remain ſuſpended, or are driven by winds in directions
parallel to the ſurface of the Earth, until fome caſual
rarefaction of the air, the meeting of contrary winds,
or of electrified clouds differently charged, cauſe them,
either to condenſe imperceptibly, and fall in dews or
gentle rains, or to ruſh more violently together, and
precipitate in ſhowers, according to the nature of the
cauſe by which they are actuated. The froke of a
chimney
How they are generated, and detached from the ſurfaces of bodies, is a proper
ſubject for an enquiry by itſelf, but foreign here.

( 5 )
chimney is for the moſt part lighter than the circum-
ambient air it aſcends through, otherwiſe it would
deſcend again as ſoon as it could diſengage itſelf from
that column of air which carried it up ; in conſequence
of which, life would be very uncomfortable in large,
populous cities, for, at times, the air with us is ſo
rarefied that the ſmoke does in fact thus deſcend, and
hover juſt above the ſurface of the Earth, to the no
ſmall annoyance of the eyes and lungs of thoſe who
breathe in ſuch an impure medium. Were it neceſſary,
experiments in pneumaticks might be recited, which
prove that air, near the ſurface of the Earth, is in ge-
neral denſer or heavier than the ſmoke and other vapors
which float therein; which are here omitted, as it is
preſumed the fact will not be diſputed.
SOOS
Now, if the tail of a Comet riſes from its head, or
rather from the Sun, in the fame manner as ſmoke
does from fire with us, and from a ſimilar cauſe, the
thereal medium, thro' which it aſcends, muſt be nearly
of the ſame denſity with the vapors of the tail ; other-
wiſe the latter could not Aoat in, nor get ſo entangled
with it as to be carried up thereby through diſtant re-
gions of the Heavens ; but that the tails may aſcend
to ſuch amazing heights, as ſome of them do, we muſt
neceſſarily ſuppoſe their ſpecific gravities much leſs than
that of the æther itſelf.
WHAT then becomes of the free celeſtial ſpaces ?
in which Sir Iſaac ſays, -_~ not only the ſolid bodies of
" the Planets and Comets, but alſo the extremely rare va-
pors of Comets tails, maintain their rapid motions with
great freedom-----without reſiſtance." (Ibid.) Soon
after
56

( 6 )
after he adds, «_-the tails--retaining their own pro-
6 per motion" (i. e. the motion they had in common
with their heads) " and in the mean time gravitating
" towards the Sun, muſt be revolved in ellipſes round
" the Sun in like manner as the heads are, and hy that
“ motion always accompany their heads." Now, al-
though the æther may be ſuppoſed fo extremely rare,
as that the ſolid globes may revolve thro' it for many
ages, without any ſenſible impediment, or, as Sir Iſaac
ſays, “ above ten thouſand years ;" yet, as the tails
muſt, upon this hypotheſis, be nearly of the fame fpe-
cific gravity with that æther, how can they “ maintain
" their rapid motions through it without reſiſtance,and
« revolve in ellipſes round the Sun together with their
“ heads”? That this is utterly impoſſible, appears
from the reaſoning of Sir Iſaac himſelf, throughout the
whole of his VII Section Principia Book II. intituled,
Concerning the motion of fluids, and the reſiſtance made te
projeEled Bodies : According to which reaſoning the
projectile motions of theſe vapors would be very
foon
deſtroyed, and the Comet in its regreſs from the Sun
would neceſſarily leave them behind as they roſe from
the head, in conſequence of the reſiſtance of this æthe-
real medium : Therefore, whenever the Comet be-
came viſible, after the perihelion, it would have a tail
as before, but reverſed, appearing, like a lucid beam,
to ſtream away from the head towards the Sun, till it
were confounded with the twilight. But this is con-
trary to all obſervations; for the projection of a Com-
et's tail, after its perihelion as well as before, is both
really and apparently from the Sun, excepting a ſmall
deviation towards the parts from whence the Comet
laſt came : Which deviation is by no means owing
to

r 7)
to the reſiſtance of any medium it paſſes through, but
is perfectly conſiſtent with its ſeveral parts retaining
the motion they had in common with their head when
they began to aſcend, and “revolving freely in ellipſes
" round the Sun together with their head." For, the
remote parts of the tail making a larger ſweep through
the Heavens than their head, while the latter takes a
ſhorter turn round the Sun at its perihelion, muſt ne-
ceſſarily be longer in deſcribing the ſame angular mo-
tions round the Sun than the globe itſelf; conſequently
muſt be deflected from the direct oppoſition to the
Sun, and the whole tail be incurvated towards the
pe:
rihelion poſition lately paſſed ; and this upon the ſup-
poſition that both head and tail move freely through a
perfect vacuum. This incurvation of the tail lefſens a6
the Comet recedes from the Sun, 'till at length it re-
covers its oppoſition as at firſt. Now, as theſe phæ-
nomena agree perfectly with the motion of a Comet's
tail through ſpaces void of reſiſtance, they are utterly in-
conſiſtent with the ſuppoſition of its moving through
a medium of equal or greater denſity than itſelf, or in
any other reſiſting medium whatever ; for according to
this celebrated author (Princip. Sect. VII. Book II.
Prop. XXXVIII. Cor. 4.) if the head or ſolid globe
of the Comet moved in a fluid of the ſame denſity
with itſelf, it would loſe half its motion before it could
deſcribe the length of two of its diameters, conſe-
quently it would ſoon loſe the whole. The vapors
of the tail riſing (upon his hypothefis) in a medium of
nearly the ſame denſity with themſelves, would in like
manner, from the reſiſtance of that medium, loſe all
the projectile motion they derived from the head, full
as ſoon as the head would loſe its own motion in the
former caſe.
SIR

( 8 )
Sir Iſaac concludes faid VII Section with the follow-
ing words, “ The reſiſtance in every fluid is as the mo-
« tion excited by the projectile in the fluid ; and can-
s not be leſs in the moſt fubtile æther, in proportion to the
denſity of that æther, than it is in air, water, and quick-
5 filver, in proportion to the denſities of thoſe fluids."
Upon the whole, then, it neceſſarily follows, that the
æthereal ſpaces through which the extremely rare vapors
of a Comet's tail revolve freely with its head in ellipſes round
the Sun, muſt be perfect vacuums as to all the purpoſes
of reſiſtance, and conſequertly, that the rays of light
chemſelves in no wiſe impede the freedom of their mo-
tion, even when in the neighbourhood of the Sun.
All that is deſired of the reader is, that he will pe-
ruſe the following ſheets with candour, and not pro-
nounce ſentence until he has fairly weighed the evi-
dence produced in ſupport of the ſeveral propoſitions
therein contained ; upon the ſtrength, or through the
deficiency of which, they muſt ſtand or fall.
Contando id
CVXXX.19
ons noisono sto bitew begira en 100
ESSAY

(9)
**
像
​*****
E Е.
S S A Y
ON 002 do
tohned logo
COM E T S.
PART
1.
A
COMET conſiſts of two parts which
fall under our obſervation, viz. a ſolid,
ſpherical, opaque body, which, like the
planets, ſhines by reflecting the light of
the Sun, and no way differs in appearance from a
planetary globe; this is by aſtronomers called the
nucleus : Alſo a very extenſive atmoſphere ; which,
from the form it uſually exhibits to us, is called the
tail, and commonly exhibits a faint beam of light, which
diverges as it recedes from the head, and often appa-
rently extends thro' diſtant conſtellations, when it nearly
reſembles thoſe momentary corruſcations which ſhoor
upwards from the horizon towards the zenith, during the
appearance of an aurora borealis,or northern light. This
tail increaſes in length as the Comet approaches the
C
Sun,

( 10 )
Sun, et vice verſa ; and its direction is always nearly
in oppoſition to the Sun. Sometimes indeed the
cometic atmoſphere aſſumes a different form, ſur-
rounding the nucleus equally on every fide like a
thin cloud or miſt, or, as ſome have fancied, like a
buſh of hair ; whence the Comet has been denomi-
nated crinite or hairy. The latter is the uſual appear-
ance of a Comet when firſt diſcovered in its defcent
toward the Sun, provided the Sun and Comet be in
oppoſite hemiſpheres ; and may take place in fome
other ſituations, agreeable to the rules of optics, even
when to ſpectators in other parts of our Syſtem, the
exhibit a tail of an enormous length.
Comet may
It has been ſufficiently demonſtrated by Sir Iſaac New-
ton, Dr.Halley,and others, that theſe bodies are, in com-
mon with the other globes of the folar ſyſtem, ſubject
to the law of mutual gravitation, and that they regard
the Sun as their common center of gravity, and con-
ſequently move round him in conic-ſections, carrying
their atmoſpheres or tails along with them. Their
orbits differ widely from circles on the one hand ; on
the other, ſince the diſcovery of the Newtonian method
of computing their trajectories from obſervations, they
have never been found to deviate into byperbolas; and
though their obſerved places, in the ſmall parts of their
orbits in which they are viſible to us, agree with their
computed places, upon the ſuppoſition that they move
in parabolas ; yet, as the periodical revolutions of ſome
of them have been aſcertained by their regular returns
after certain intervals, it is agreed by aſtronomers, that
their paths are truly elliptical, though very excentric ;
fuch

( 11 )
fuch ellipſes, near the extremities of their tranfverfe
axes, differing but infenfibly from parabolas. All that
is attempted in this Eſſay, is,
1. To account for the phænomena of the tails of
Comets, upon philoſophical principles : And then,
2. To point out ſome ends to which they feem
adapted, and for which they are probably defigned.
Sir Iſaac Newton has ſufficiently proved that theſe
tails conſiſt of a fluid matter, extremely rare, emitted
from their heads upon their approach to the Sun,
which is rendered viſible by reflecting his rays *: But
what cauſe there may be exiſting in nature, capable of
projecting the cometic atmoſpheres through fuch im-
menſe ſpaces, is a queſtion which ſtill remains to be
folved, no fatisfactory account having hitherto been
offered to the publick. In order therefore, to a ra-
tional ſolution of this curious phænomenon, the tail of
a Comet, the following propofitions, obſervations, &c.
are ſubmitted to the candid perufal of the reader :
Firſt premiſing, that when a ſubject, under conſider-
ation, is in its nature purely phyſical, it is to be pre-
fumed that ftrict, mathematical demonſtration will not
be expected
We ſhall now endeavour to prove the following
propoſitions.
First, THAT the primary Planets, the Comets,
and the Sun, are all ſurrounded with atmoſpheres.
SECONDLY,
事 ​*
Princip. Book III. under Prop. XLI.

( 12 )
SECONDLY, THAT theſe atmoſpheres conſiſt of the
fame fiuid with the atmoſphere of the earth, viz. Air.
THIRDLY, That they are mutually repellent to
each other ; as the globes they ſurround are in a ſtate
of mutual attraction or gravitation.
1. As the proof of the exiſtence of the at-
moſpheres of the Sun, Planets and Comets depends
upon a variety of aſtronomical obſervations, it is ne-
ceſſary to be particular as to each of them.
AND, 1. That the Earth we inhabit (which is a
Planet vaſtly inferior, both in bulk and fituation, to
ſome others) has one, we have the beſt evidence pofii.
ble, viz. the teſtimony of our fenfes ; beſides, we all
know, that the air, which every where ſurrounds the
Earth, is eſſential to the breath of life.
2. That Mars has an atmoſphere, and that very
extenſive, has been demonſtrated from the occultation
of a fixed Star by that Planet *; as the Star vaniſhed
at a diſtance from, or without ever arriving at a viſible
contact with his limb; as was obſerved by Caffini,
October iſt, 1662. The like was obſerved after the
ſame oçcultation, by M. Roemer, at Rone, the Star
not being viſible after the tranſit till at a diſtance from
his limb.
3. JUPITER's belts are in a fluctuating ſtate, as
they frequently vary their form, fize, and ſituation ; t
which cannot be accounted for, unleſs we ſuppoſe them
to
* See Smith's Opticks, Vol. II, Page 430m
7 Idem, Page 433.

(13)
to be clouds and exhalations, floating in an atnoſphere
which ſurrounds his globe. +
VO
4. Some belt-like appearances were diſcovered in
Saturn by Mr. Hadley through his five-foot reflecter,
and by Mr. Pound through Hugenius's glaſs, though
they appeared very faint, as he is ſo remote $; which
belts are probably in the ſame fluctuating ſtate as
thoſe of Jupiter, and ariſe from ſimilar cauſes.
5. That Venus has an atmoſphere may alſo be
inferred from the variable ſpots which have been ob-
ſerved upon the face of that Planet, || which are pro-
bably of the ſame kind with the belts of Jupiter, or
the clouds which float in our own atmoſphere, and,
conſequently, have a ſimilar one to ſuſtain them. But
for proof of its exiſtence we need go no further back
than the late tranſit of Venus over the Sun's diſc, anno
1769 ; when the atmoſphere itſelf was viſible, and
that in different ſituations, to obſervers in diſtant
parts.
6. MERCURY is too near the Sun to favour us
with ſuch obſervations; but if we ſuppoſe his globe
inhabited, he doubtleſs ſtands as much in need of an
atmoſphere as any other Planet, and has as important
purpoſes to be ſerved thereby.
7. THAT
+ Sir Ifaac Newton ſays theſe belts « are formed in the clouds of that Planet."
Princip. Book III. Lemma Prop. XXXIX.
I S. Opt. Page 441. || Idem, Page 421.
Were theſe beits, ſpots, &c. really adhering to, and parts of their reſpective
gloves, they would always appear invariably the ſame, in the ſame ſituations,
as thoſe of the Moon do ; in which no atmoſphere has yet been diſcovered.
*See Tranſactions of the American Philoſophical Society of Philadelphia, Page
42. Alſo Obſervations by Mr. Benjamin Weſt at Providence, N. E, Page 16.

( 14 )
19. That the Comets are ſurrounded with at-
moſpheres is already taken for granted, becauſe felf-
evident. Sir I. Newton, from various obſervations,
concludes that their diameters are, one with another, at
leaft equal to ten diameters of their nuclei or ſolid
globes. +
8. THAT the Sun has an atmoſphere, and that
proportioned to his amazing magnitude, is rendered
highly probable by the macular appearances, or ſpots
frequently diſcovered upon his diſc : Which, as they
often ſuddenly break out, and as fuddenly diſappear,
and ſometimes vary their ſhapes, even when under
the
eye of the obſerver ; can be no other than huge
clouds of ſmoke, or other vapours floating in ſuch
atmoſphere ; huge indeed! as they frequently ex-
ceed the whole ſuperficies of the Earth. Mr. Derbam,
who was peculiarly aſfiduous in obſerving them, has
aſſigned a cauſe, to which their phænomena accurately
agree. He ſuppoſes them to be immenſe volumes
of ſmoak, belched forth by volcanoes or fiery erup-
tions, which are frequently breaking out upon his
ſurface 1. Theſe ſpors, when large, fometimes con-
tinue during a whole revolution of the Sun round his
axis, or about twenty-five days ; and it is from the
regular returns of ſuch ſpots to the ſame part of
his diſc again that this revolution has been determined.
But ſuch clouds, were they not ſupported by an at-
moſphere, would tumble down again upon the Sun
prelently after the exploſions which raiſed them were
over,
+ See Princip. Book III. Prop. XLI.
† See Jones's Abridgment of Philofoph. Tranſ. Vol. IV. Page 238.

( 15 )
over, as we ſee vapors emitted in an exhauſted receiver,
for want of air to ſuſtain them, ſink down to the bot-
tom of the receiver.
II. We ſhall now endeavour to prove that the
planetary, cometary, and ſolar atmoſpheres, conſiſt of
the ſame elementary Auid with the atmoſphere of the
Earth, viz. AIR.
Air is a fluid, which when pure is tranſparent, and
in the higheſt degree elaſtic, being indefinitely com-
preſſible and dilatable ; which qualities are truly cha-
racteriſtic of it, as we know of no other fluid to which
they belong
It may be neceffary to obſerve here, that upon our
globe, which (as already obſerved) is itſelf a Planet, the
preſence of air is neceffary, both to the preſervation of
animal life, and to the exiſtance of flame : For in a
glaſs receiver, ſmall animals die, and candles go out,
immediately upon the air being exhauſted; upon air
alſo ſeems to depend the exploſive power of enkindled
vapors; for gun-powder itſelf, one of the moſt powerful
agents
hitherto invented by the art of man, if fired by
an hot Iron in vacuo, will conſume away,but never flaſh,
nor explode; and it is generally agreed by thoſe who are
acquainted with experimental philoſophy, that this vi-
vifying, inflammating quality of air, depends upon its
elaſticity, or the active, contrifugal power of its par-
ticles, to be conſidered under the next general head.
This being premiſed, we proceed to prove,
Firſt, That the atmoſpheres of the feveral globes
of
droh der Rei

( 16 )
of the Syſtem confiſt of tranſparent fluids : The truth
of which will be ſufficiently evident, if we conſider the
ſeveral phænomena by which they were diſcovered.
It has already been ſhewn that Mars is ſurrounded
with a very extenſive atmoſphere ; which, had it not
been tranſparent, muſt have eſcaped the notice of af-
tronomers during the time of the occultation of the
ftar before nientioned ; for were it opaque, it would,
by reflecting the Sun's rays, as well as by its want of
tranſparency, have hid the Planet itſelf, and by the ob-
ſervers have been confounded with it †; in which cafe
the moments of the occultation of the ftar, and of its
viſible contact with the limb of the Planet, would have
been the ſame ; whereas the diſappearance of the ſtar
before any fuch contact could take place, is a demon-
ftration of the refraction of its rays in a tranſparent
medium, with which the Planet is ſurrounded.
The changes which are obſervable in thoſe fluctu-
ating collections of heterogeneous matter, of which
the belts and ſpots in the other Planets, and the Sun
confift, for the ſame reaſon, could never have been
diſcovered, were they not ſuſtained in tranſparent fluids,
above the ſurfaces of their reſpective globes.
10
The tranſparency of the cometic atmoſpheres is un-
deniable, as their nuclei are frequently ſeen thro' them,
although they occupy ſpaces equal to many diameters
of the Earth ; and the ſmalleſt ſtars are viſible through
the
و
+ In like manner, as Sir I. Newton concludes, (Prin. B. III. Lemma IV.) « that
" the Earth, if it was viewed from the Planets, would without all doubt fine
66 by the light of its Clouds."

( 17 )
the tails which proceed from, and are only expanſions
of them ; in which tails we have ocular demonftration
of a moſt ſurprizing dilatability ; from whence their
recompreſſibility and elaſticity may be juſtly inferred.
Secondly, ALTHOUGH there are no obſervations in
the records of aſtronomy which prove this elaſticity in
the planetary atmoſpheres; yet, as the Planets are with
the higheſt reaſon fuppoſed to be inhabited Worlds,
the preſumption is at leaſt very ſtrong, if ſhort of a
demonftration, that their atmoſpheres are defigned to
anſwer purpoſes every way ſimilar to thoſe which are ef-
fected by the atmoſphere of the Earth, and that they are
endued with all thoſe properties, which are, with us, ne-
ceſſary to the preſervation of animal Life; conſequently
that they are elaſtic, as well as tranſparent, and alto-
gether like our air.
The atmoſphere of the Sunis,by his exceſſive luſtre,
hid from our view ; but if we conſider him as an im-
menſe flaming globe, kindled up to warm and en-
Jighten the whole fyftem, we may well ſuppoſe that he
has a large ſhare of that fluid, without which (as before
obſerved) ſcarcely any flame can ſubſiſt with us
for a moment.
Some Authors indeed are of
opinion, and not without reaſon, that the Sun is
noi a body of fire, as commonly ſuppoſed ; Doctor
Knight in particular, in his curious treatiſe of at-
traction and repulfion, as two univerſal principles, by
which he endeavours to ſolve all the phenomena in
nature, ſeems to be of opinion, that the inhabitants
of the Sun (if inhabited) are in as much danger of
D
ſuffering

( 18 )
luffering from cold, as from exceſſive heat *. But of
whatever ſubſtance the body of the Sun may conſiſt,
Mr. Derham has, by his obſervations, put it beyond
all reaſonable doubt, that the macule and faculæ, or
the darker and brighter ſpots obſervable at times upon
the Sun's diſc, are really owing to the burſting of vol-
canoes ; " the faculæ being only the appearance of
“ the flames, after the denſe ſmoke attending the ex-
“ploſions is diſſipated, or removed to a diſtance from
" them +.” Now there is no reaſon to doubt but
that, upon our globe, air is as neceſſary to the flames
of Ætna and the eruptions of Veſuvius, as to the
finaller blaze of a candle, or to the exploſion of gun-
powder ; may we not then rationally conclude, that it
is equally neceſſary to thoſe aſtoniſhing volcanoes in
the Sun ?
It is now proved, as far as the propoſition is in the
nature of it capable of proof, that the celeſtial bodies
are ſurrounded with atmoſpheres, and that theſe at-
moſpheres conſiſt of tranſparent elaſtic Auids, like the
air ſurrounding this Earth. It remains that we prove,
* See ſaid treatiſe, Page 58; in which are theſe remarkable Words : Speak-
ing of the Sun and fixed Sars, he ſays, “ Their Globes are no longer
“ frightful Gulphs of Fire, but inhabitable Worlds : Thoſe Philoſophers
“ who thought them too hot for the Habitation of Salamanders, and
6 thoſe ſublimer Genii, who thought them to be Hells, will now per-
“ haps be in Pain, left the Inhabitants fhould freeze with Cold”.--
However, Doctor Knight concludes from his own principles, that the Sun has
an immenſe aerial atmoſphere condenſed round him ; for in the preceding
Page 57, he ſays, “ The vaſt Weight of the Sun's Atmoſphere muſt make
* the Denſity of the Air ſo great near the Sun's Surface, that what would
« create a Sound ſcarce audible with us, would there produce a very loud
* Noiſc.
+ See Jones's Abridgment of Phil. Tranſ. before cited.
III. THAT

( 19 )
III. That the atmoſpheres of the Sun, Planets,
and Comets are mutually repellent to each other; as
the folid globes they ſurround are in a ſtate of inutual
attraction or gravitation.
Ir muſt here be obſerved ; that in reaſoning
upon gravitation, the great Author of the preſent phi-
lofophical ſyſtem of the Heavens argues downward
from the greater to the lefſer, from Worlds to Atonis ;
thus, finding by mathematical deductions, compared
with aſtronomical obſervations, that all the globes of
the ſolar Syſtem, Sun, Planets and Comets gravitate,
or have a mutual tendency towards each other, and
that this reciprocal attraction is proportional to the
quantities of ſolid matter they reſpectively contain
he very juſtly concludes, that every ſingle particle in
any one of their folid maſſes both attracts, and is
attracted by, every other particle of matter, contained
in every globe throughout the ſolar Syſtem.*
3
3
But as the Heavens exhibit no phænomena from
which we can, directly and with equal certainty, infer
the exiſtance and univerſal extent of the contrary
principle of repulſion, as fubfiſting between the at-
moſpheres of the heavenly bodies we are here
obliged to uſe a contrary method, and to reaſon up-
ward from the powers and properties which, by their
effects, we diſcover to belong to thoſe parcels of air
upon which we can make experiments ; to the effects,
which the fame powers and properties would naturally
produce, in thoſe vaſt collections of air, which con-
* Principia Sparfim.
ſtitute

( 20 )
ſtitute the atmoſpheres of the ſeveral globes; and if
by tracing the neceſſary operations of theſe powers,
ſtep by ſtep, we can at length arrive at any of the
grand phænomena of nature, we may with the higheſt
reaſon conclude, that theſe phænomena are the effects
of thoſe powers.
We ſhall therefore endeavour to prove from authors
of the beſt credit, or from experiments which any one
may try at his leiſure,
1. That there is a mutual repellency fubfifting
between the particles of air, whereby they continually
endeavour to recede from each other ; in conſequence
of which, that Auid is indefinitely dilatable from the
centrifugal activity of its particles, as well as com-
preſſible by any foreign power.
The honourable Robert Boyle, Efq; found by expe-
riments, that air might be ſo rarefied as to occupy
13769 times the ſpace it fills when in its natural
ftate near the furface of the Earth I ; other experi-
ments prove, that it may be ſo condenſed, as to be
contained in to part of the ſame ſpace*, therefore
multiplying 13769 by 60, it appears, that the cubic
ſpace it is capable of filling, under different circum-
ſtances, may be as 826140 to 1.
But the cube root
of 826140 is 94 nearly; therefore the central diſtances,
of the particles from each other, as diſcoverable by
actual experiment, may be as one to ninety-four, and
that from their centrifugal activity.
See Shaw's Abridgment of Boyle, Vol. I. Page 551.
* Sre Martin's Philoſophical Graminar, Page 178.
MOREOVER

( 21 )
Moreover, Sir 1. Newton from experiments con-
cludes this repulfive power to be ſo great, as that a
cubic inch of air, condenſed as it is with us,if removed
one femidiameter of the Earth above its ſurface, where
it would be free from the preſſure of an incumbent ac-
moſphere, would fo expand itſelf by virtue of this
power, as to fill the whole ſphere of Saturn's orbit ;
he adds, " and far beyond it”t
و
nay
2. This mutual repellency of the particles of air
is greatly increaſed by heat. For if a bladder nöt niore
than half filled with air,be tied up tight and laid before
the fire, the additional expanſive power, which the air
contained therein acquires from the heat, will ſwell the
bladder to its utmoſt extent, and will at length burſt it
with an exploſion.
7
3. The particles of air, although mutually repel-
lent amongſt themſelves, are with regard to other mat-
ter, in the common ſtate of gravitation or attraction.
This is evident from their being condenſed in the form
of atmoſpheres round the ſolid globes of the Syſtem,
and attending them through all their revolutions.
4. The mutual repellency of the particles of air
is indefinitely greater, in proportion to the quantities
of repelling matter, than the mutual attraction, ſubfift-
ing betwixt the folid particles of attracting matter.
The truth of this propoſition will appear, at leaſt
highly probable, if we conſider that the quantity of
+ Princip. Book III, under Prop. XLI.
matter

( 22 )
matter contained in our air near the ſurface of the
Earth, and which gravitates in common with other
matter towards its center, is ſo ſmall, tho' condenſed
by the incumbent weight of the whole atmoſphere,
that one quart of it weighs no more than eight grains,
as appears from experiments ; yet ſo great is the re-
pulſive power of its particles, that a ſmall quantity of
it, condenſed by art in the barrel of an air-gun, of the
ſize and bore of a common fowling-piece is ſufficient,
when permitted ſuddenly to expand itſelf through the
tube, to diſcharge a muſket ball, in the ſame manner
as fired gun powder, the noiſe excepted, and with the
like fatal effects.
We ſhall now proceed to prove the following pro-
poſition, viz. Iftwo corpuſcles of matter be in a ſtate of
mutual repulſion at any given diſtance, as A. B. Fig. 1.
where they repel each other with a given force, fay
= 1. and this repulſion decreaſes, either as the diſtances
Ac, Ad, A2, Af, &c. or as the ſquares, cubes, or any
higher powers of thoſe diſtances, increaſe, the extent
of that repulſion is indefinite.
For ſuppoſe A and B to be two ſuch particles re-
pelling each other at the diſtance AB with a force
= 1. continue the line A B indefinitely through the
equidiſtant points c, d, e, &c. ler the particle A be
fixed, B movable : Let us ſuppoſe this power to de-
creaſe, ift, fimply, as the diſtances increaſe ; then, if
we ſuppoſe the particle B to move ſucceſſively through
[ d, e, &c. this power,at thoſe ſeveral diſtances, will be
as

( 23 )
as follows, viz. at c= 1, at d= j,ate= 4,&c.in infinit.
2. If it decreaſe
as the ſquares > c= i, at d = ate=it, &c. in inf.
increaſe,itisat
3.Ifasthe cubes, 2
it is at -
(= šgat d=279ate= 4, &c. in inf.
4.Ifasthe biqua-7
Cricat d=1,ate=256.&c. in inf.
drates, it is at
&c. onwards indefinitely.
Now whatever the diſtance aſſigned between the
two particles may be, and whatever may be the index
of the power which expreſſes the ratio of the decreaſe
of their repulſive force, this force will in every cafe
be expreſſible by a fraction whoſe numerator is 1,
and its denominator equal to the given diſtance, in-
volved according to the index of the given power, as
is evident from inſpection of the foregoing fractions
as they ftand. It can never, therefore, become equal
to nothing, until the denominator of the fraction be-
comes infinite, which never can be at any aſſignable
diſtance, however great ; therefore, the extent of this
mutual repuljon is indefinite. Q. E. D.
In mathematical reaſoning, the ſmalleſt quantities
are not to be diſregarded, unleſs ſuppoſed ſmaller than
any aſſignable, as in Auxions, &c.; for, as all the
huge maſſes of the ſolar Syſtem,are compoſed of par-
ticles of matter inconceivably ſmall, ſo their me-
chanical effects upon each other are proportional to
the numbers of ſuch component particles they re-
{pectively contain ; and though the mutual effe&ts of
two

( 24 )
two ſingle particles, at any given diſtance, might be
indefinitely ſmall, yet we may eaſily conceive, that,
when indefinite numbers of ſuch particles are conſoli-
dated into one maſs, and exert their influence from
a common center, that that influence may be as exten-
five as the ſolar Syſtem, and perhaps as the material
univerſe. Thus the Sun, by the united attractive
force of his conſtituent particles, regulates the motions
of the Comets, even at their aphelia, where their
diſtances from him confound the human imagination ;
amounting in ſome of them to many thouſands of
millions of miles ; yet, at thoſe amazing diſtances
from the Sun, they are by his influence retained in
their proper orbits, without any deviation, till at length
they are brought back to him again, many of them
after excurſions of ſome hundreds, and poſſibly ſome
of them after thouſands of years ; in effecting which
every particle of matter in the Sun, however ſmall,
bears a part.
On the other hand, the atmoſpheres of the heavenly
bodies conſiſt of particles mutually repellent; not
conſolidated indeed, as that would be incompatible
with the nature of the fluid compoſed of them, but
each, in conſequence of the mutual attraction between
its conſtituent particles and the globe it ſurrounds,
condenſed into a fluid maſs.
In this caſe alſo, though the mutual repellency be-
tween two ſingle particles might at a given
diſtance
be indefinitely ſmall, yet the influence of two ſuch
maſſes upon each other, when thus condenſed, may
be

( 25 )
be as extenſive as in the former caſe of attraction.
The united repulſive force of the particles in each,
would in like manner exert itſelf as from their com-
mon center.
Let us, for illuſtration of the ſubject, fuppoſe this
repellent power between the particles A and B, Fig. 1,
to decreaſe as the ſquares of the diſtances increaſe,
then at a diſtance equal to 100000 000 A B, or, in
other words, ſuppoſe the particles A and B removed
one hundred millions of times further aſunder than
repreſented in the figure, the repulſive force would
then be equal to 1000000000OODIJO, or to one ten-
thouſand-million-millionth part of what it is at the ſimple
diſtance A B; which, though ſmall indeed, is yet
fomething, for the fame diſtance remaining, viz.
100000 000 A B, if ten thouſands of millions of millions
of fuch particles of equal bulk and repellency were
condenſed round the point A as a center, then the
repulſive force between the maſs at A and the particle
B would be equal to that between the two ſingle par-
ticles A and B, at the diſtance AB; once more,
ſuppoſe the fame diſtance, viz. 100 000 000 A B to
remain, and the particle B to have the fanie number
of repellent particles condenſed round it, as we have
already ſuppoſed, to be condenſed round A ; then the
repulſive power ſubſiſting between the two maſſes
would be ten thouſands of millions of millions of times
greater than between the two particles A and B, at the
diſtance AB.
We have ſeen that air conſiſts of particles thus re-
pellent; nor can we diſcover any bounds to that re-
E E
pellency

( 26 )
و
pellency by any experiments we can make ; and if te
other experiments we add Sir Iſaac Newton's reaſoning
from fome of his own, we muſt conclude them inde-
finitely ſo, and conſequently that their repulfive force
decreaſes regularly according to ſome certain ratio of
the increaſe of their diſtances, viz. either as the diſtances
Samply, or as their Squares, cabes, or fome other powers
thereof.
It ſeems moſt agreeable to mathematical reaſoning,
to fuppofe, that all powers, whether attractive or repel-
lent, which act in right lines, to, or from the centers of
the attracting or repelling bodies, are proportioned to
the inverſe ratio of the ſquares of the diſtances of their
centers. For if we conſider theſe powers as repreſented
by lines,or rays,converging from every point of the viſi.
ble concave of the Heavens to a given point as 'a center
in caſe of attraction, and diverging equally from faid
center in caſe of repulfion; and ſuppoſe a corpuſcle of
matter to be placed in any part of that ſpace, the num-
ber of ſuch rays, intercepted by, and falling upon faid
corpuſcle, whether they were converging or diverging,
would be in that ratio, viz. inverſely as the ſquares of the
diſtances of faid corpuſcle from ſuch central point.
The truth of this aſſertion is capable of the moſt fim-
ple mathematical demonftration ; which may be here
omitted, as it has been already fufficiently demonſtrated
by others : Upon it indeed depends the truth of Sir I.
Newton's poſition, viz. that “the denſity of the Sun's
rays is reciprocally as the Squares of the diſtances
es from the Sun”, from whence he deduces the propor-
tions of light and heat, enjoyed by the ſeveral Planets
of the Syſtem.
LE

( 27 )
If we conceive fuch fuppoſed rays to move with a
uniform velocity, and to be attended with any mo-
memtum whatever, whereby, when converging, they
impel the corpuſcle to, or repel it, when diverging, from
faid central point ; as the whole momentum would
be proportional to the number of the intercepted rays,
the tendency of the corpuſcle to, or from the center,
that is, its attraction or repulſion would alſo be in the
aforeſaid ratio. That the attraction of gravitation is
regulated by this law, aſtronomical obſervations ſuffi-
ciently demonſtrate.
Bur Sir 1. Newton concludes from experiments,
to which all theoretic opinions muſt ever give way,
that the mutual repellency between the particles of air
is reciprocally as their diſtances only, or " nearly fo.”
If this be the caſe, the phyſical effects of two par-
ticles, and conſequently of two fluid maſſes compoſed
of ſuch particles, upon each other, muſt be yet vaftly
greater than if it were reciprocally as the ſquares of
the diſtances, as we before ſuppoſed, as will evidently
appear upon inſpection of the following ſcheme ; in
which the firſt ſeries repreſents the diſtances, increaſing
in arithmetical progreſſion ; the ſecond, the repellent
force, decreaſing as the diſtances increaſe only ; the
third, the ſame force conſidered as decreaſing accord-
ing to the increaſe of the Squares of the diſtances
AB
Diſtances
0-1-2-3-4-5-6-7-8
Inverſe ratio of dift. O
/ 를
​&c.
--of the ſquares of dift. o
To 2'5 36
* Newt. Prinsip. Book II. Prop. XXIII.
HERE
9
thus :
I
I
2
1
5
I
6
I
8
ol - ww
I
1
I
I
1
4
I

( 28 )
Here it is evident that at the diſtance 3 AB, if
the repellency decreaſed only as the diſtances increaſe,
that that force would be equal to } ; buc if it decreaſed
as the ſquares of the diſtances increaſe, it would at the
fame diſtance be equal only to ; ; at the diſtance
5 A B, in the former caſe, it would be š, in the latter
t's; if at the diſtance 8 AB, in the former it would
be s, in the latter 74, &c. But 3 is the ſquare-root
of g or ; of , f of z's, and of ++, &c. .
IT therefore follows, that if this repellent force
does actually decreaſe ſimply as the diſtances increaſe,
the quantity of it at any given diſtance is greater,
in
the direct ratio of that diſtance, than if it decreafed in
the ratio of the ſquares of the diſtances; conſequently,
the fentible effects of two ſuch fluid maſſes, condenſed,
as we have already ſuppoſed, round the particles A
and B, would be proportionably more extenſive.
Now, what are the atmoſpheres of the Sun, Planets
and Comets, but vaſt collections of theſe repellent
particles of air, condenſed round their reſpective
globes, by the mutual gravitation fubfifting be-
cween them, in like manner as we have already ſup-
poſed them to be condenſed round the corpuſcles A
and B (Fig. 1.)? But if, according to Sir I. Newtow,
fo ſmall a quantity as a cubic inch of our air, if left
to itſelf at the height only of one femidiameter of the
Earth, might exert this force, fo as to fill the whole
ſphere of Saturn's orbit; it demonftrably follows, that
the atmoſpheres aforeſaid, which confit of huge maffes
of the fame fluid, may extend their influences as far
30

( 29 )
at leaſt ; and though the condenſation of each round
its own globe, by virtue of their mutual gravitation,
would prevent the ſcattering ofits component particles,
and confounding itſelf with the atmoſpheres of the
neighbouring Planets (as would undoubtedly be the
cafe, however great their diſtances might be, did the
cauſe of that condenſation ceaſe) yer it is by no means
impoffible, nor yet improbable, that under fome cir-
cumſtances, the phyſical effects of two ſuch atmoſpheres
upon
each other may become very apparent : For if
we ſuppoſe two equal Planets with their atmofpheres
(as A and B Fig. 2) to paſs near to each other, theſe
atmoſpheres being fuid, and that in the higheſt con-
ceivable degree of fuidity, would give way to the
leaft degree of external force, and in conſequence of
their mutual repulſion would recede from each other s
but the attractions of their reſpective globes would
prevent their leaving them wholly, while each would
ſo far retire, as to be depreſſed in the parts next the
other, and to ſwell out in the oppoſite parts, changing
their ſpherical figures to two oblong ſpheroids as at C
and D (Fig 2.) But this is ſaid upon the ſuppoſition
that both the globes and their atmoſpheres are equal,
each to the other reſpectively. Whereas, if we ſup-
poſe the body A to be an immenſe globe like the Sun,
having an atmoſphere proportionably large, and
**
conſequently
* The folar atmoſphere is denſe enough at the height of ſeven or eight thouſand
miles, or 1-137th part of the Sun's diameter above his ſurface, to ſuſtain
thoſe huge clouds of ſmoke which appear to us like ſpots upon his diſc. See
Mr. Profeſſor Winthrop's Cogitata de Cometis, Page 25. If therefore the
atmoſphere of the Sun conſiſts of air, as we have endeavoured to prove it does,
and the ſolar clouds and vapors are ſimilar to our own, and, like them, require
a certain denſity of air to ſuſtain them in equilibrio, it follows, that the denſity
of the air in the Sun's atmoſphere is as great- at the height of 7000 miles
above

( 30 )
conſequently containing many thouſands, and perhaps
fome millions of times the quantity of this repellent
fluid contained in the atmoſphere of B; which we may
now conſider as a Comet, of equal magnitude with the
Earth, but ſurrounded, like other Planets of the fame
fpecies, with an atmoſphere of great extent, when
compared with the magnitude of the globe itſelf ;
and if we ſuppoſe alſo the viſible effects of the mutual
repellency of theſe atmoſpheres to be reciprocally as
the quantities of the repellent fluid contained in ther,
the atmoſphere of the Comet might be prodigiouſly
lengthened when in the neighbourhood of the Sun,
and repelled to great diſtances behind the nucleus,
while at the ſame time, the natural ſpherical form of
the Sun's atmoſphere would not be fenfibly diſturbed
by that of the Comet. This reaſoning may be illuſ-
trated by conſidering the effects of the contrary prin-
ciple of mutual attraction or gravitation ſubliſting
between the globes themſelves, which effects are ſub-
jects, both of mathematical computation, and of aſtro-
nomical obſervation: For it is very certain that Comets
perform their revolutions round the Sun, in confe-
quence of this mutual gravitation, in orbits very ex-
centric, and nearly parabolical ; whereas it is proba-
bie, that the joint efforts of all the Comets which ever
appeared, would ſcarcely difturb the repoſe of the Sun
in the center of the Syſtem.
A Comet, in its deſcent through the planetary orbs,
approaches the Sun with an accelerating velocity, un-
above his furface, as that of our own air, at the height of but 3 or 4 miles
above the ſurface of the Earth ; a height which our clouds, probably, never
exceed. How vaſt then muſt be the extent of the Sun's whole atmoſphere !
til

( 31 )
til it arrives at its perihelion ; entering deeper and
deeper within the ſpheres, both of the repulſion of the
Sun's atmoſphere, and of the activity of his rays; in
conſequence whereof the cometic atmoſphere is con-
tinually rarefying during this defcent, both from
the expanſion it undergoes, by means of that re-
pulfion, and from the increaſing heat, which it ac-
quires, as it approaches the Sun, which heat, as
already obſerved, contributes greatly to the repellency
of the particles of the serial fluid. By the con-
currence of theſe cauſes, if admitted, a tail muſt ne-
ceſſarily be formed, the length of which would depend,
partly upon the quantity of air contained in the cometic
atmoſphere, and partly upon its proximity to that of
the Sun, and conſequently would increaſe until the
Comet arrived at its perihelion, or rather, from the
continuance of the cauſes, until a few days after, which
is moſt agreeable to obſervation. When a Comet is
at this ſtage of its revolution, if it paſs near the Sun,
the tail uſually extends from its head through vaſt
regions of ſpace, exhibiting a curious ſpectacle to
diſtant Worlds.
Ir a cometic atmoſphere conſiſted of an unelaſticfluid,
and were thus repelled by the atmoſphere of the Sun,
it would put on the form of a very oblong ſpheroid,
both ends of which would be terminated by a regular
curvilinear ſurface as at A and a (Fig. 3) ; but as it is
an elaſtic fluid, whoſe conſtituent particles are, amongſt
themſelves, mutually repellent, as ſoon as ever the at-
traction of the nucleus, which before condenſed them
ſpherically round itſelf, is diminiſhed in any part of that
atmoſphere, by this oppoſite repulſion, the particles
in

( 32 )
in that part become more and more at liberty to exert
their own inherent repellency, which they would at
length do quaquaverſum, were it not that the repellent
power of the Sun's whole atmoſphere, ſo far predomi-
nates over their own mutual repellency, as conſtantly
to keep them in a direction nearly oppoſite to the Sun's
center ; but as this is not ſufficient totally to prevent
a lateral dilatation, the tail grows broader and broader,
as it extends from the nucleus, while the longitudinal
expanfionis rather increaſed thanimpeded thereby, until
the whole train, in a fair view of it, wears nearly the
form of a parabolic curve ; the ſeveral parts
of which
are leſs and leſs diſtinct, as they are further diſtant
from the head, until at length, ſpectators ſhall, at the
ſame time, differ ten or twenty degrees in their eſti-
mation of its apparent length, according to the differ-
ent acuteneſs of their ſight. * (See Fig. 4. B b.)
After the perihelion, as the Comet recedes from
the Sun, the mutual repulſion of the two atmoſpheres
gradually decreaſes, while the mutual gravitation of
the Comet and its own atmoſphere proportionably gets
the better of it, until at length the exiled atoms are
drawn home, and that perhaps without the loſs of a
ſingle one ; unleſs the tail ſhould happen to ſweep
the ſphere of ſome Planet in its way ; in which caſe,
if the atmoſphere of the Planet were large, the mutual
repellency of the two would rather occaſion a bifurca-
tion of the cometic atmoſphere than a union or con-
* Sir I. Newion in his Princip. ſays, “the Comet of 1680, in the month of
“ December, emitted a notable tail, extending to the length of 400, 500)
" 60° or 700 and upwards”: The disjunctive (or ) intimates an uncer-
tainty, of at leaſt 30 degrees, in its apparent length.
fuſion

( 33 )
fuſion of them; but were that of the Planet very ſmall,
and did we ſuppoſe that in conſequence thereof, that
of the Comet might leave a ſmall portion of its tail
behind ; ſtill no detriment to us could reaſonably be
expected or feared, upon that account, conſidering
the amazing rarity of the Comet's Tail. The cometic
atmoſphere being gradually re-condenſed round its
nucleus as before, would provide it with a ſuitable
garment for winter quarters in the remote parts of
its orbit ; agreeable to the ingenious hypotheſis of
Doctor Williamſon
The thickneſs of a Comet's tail, or the diameter of
a ſection made perpendicularly thro' it, is amazingly
great towards its extremity ; the apparent breadth of
the tail of the Comet of 1680, where its diſtance from
the Earth was equal to the diſtance of the Earth from
the Sun, was equal to above three apparent diameters
of the Sun *; conſequently the real thickneſs of that
part of the tail was about three millions of miles, or
between three and four hundred diameters of the
Earth, yet, ſays Sir I. Newton, “ the ſmalleſt Stars were
« viſible through it without any diminution of their
** luſtre.” How inconceivably rare then muſt it have
been ! Perhaps for ſome thouſands, not to ſay millions
of miles, reckoned from its extremity towards its
head, it might not have contained more air than a well
blown bladder does with us : And that this is not only
poſſible, but even probable, appears from Sir I. New-
ton's computation of the expanſion which a cubic inch
* The Author remembers to have met with this Obſervation ſomewhere, but
cannot at preſent recolle&t where. But it was not far from the tsutb, if ap-
plied to the Comet of 1769.
of

( 34 )
of our air is capable of, already repeatedly referred to.
So much for the third general propoſition, which
the reader will receive or reject, according to the
weight of the evidence offered in ſupport of it. It is
however hoped that candor will be exerciſed, and that
the whole will not be exploded, merely, for want of
accuracy in the method, or of perſpicuity in handling
a ſubject ſo difficult of acceſs.
A ſhort digreſſion here may not be amiſs, in order
to prevent the uncomfortable impreſſions, which the
appearance of a Comet is apt to make upon the
minds of ſome, and which were greatly increaſed dur-
ing the appearance of the Comet of 1769, by an in-
judicious publication in one of the ſouthern news-
papers,
wherein it was inſinuated, that if that Comet
ſhould paſs between the Earth and the Sun, the Earth
would paſs through the tail of the Comet ; the con-
ſequence of which, the writer ſuppoſed, might be fatal
even to che World itſelf. But from the foregoing
obſervations it is evident, that ſuch apprehenſions
were groundleſs. The ingenious and learned Mr.
Wbiſton has indeed endeavoured to prove, in order
to ſupport his Theory, that a Comet paſſing near the
Earth in its deſcent towards the Sun, occaſioned the
univerſal deluge, by leaving behind it a fufficient
quantity of vapors from its atmoſphere and tail, (in
which he ſuppoſed the Earth to be inveloped,) when
condenſed into rain, to drown the World. He fur-
ther ſuppoſed that a Comet, in its aſcent from the
Sun, might be ſufficiently heated to cauſe a general
Conflagration, ſhould it find the Earth in the fame
ſituation.

( 35 )
ſituation. In conſequence of which the fears of
many
are uſually alarmed, at the appearance of a Comet,
by the apprehenſion of ſome grand cataſtrophe. Buc
if the tails of Comets are ſo extremely rare, as we find
they muſt neceſſarily be, and as every one muſt be
convinced they are, who ever ſaw the Stars ſhining
through them; how can we conceive that the Earth
in pafling through one of them, could carry off oceans
fufficient to ſubmerge “all the bigb bills under the
whole Heaven"? to effect which, many of our oceans
would, doubtleſs, be neceſſary. On the contrary, it ſeems
probable, from the foregoing conſiderations, that the
tails of all the Comets which ever appeared, could nor,
jointly, furniſh water fufficient for one ocean only.
If ſo, the near approach of a Comet muſt be found ut-
terly inſufficient to account for the fuperior waters of
the deluge, or the forty days rain mentioned by Moſes;
whatever diſorders might otherwiſe be occaſioned in
both globes, by their murual gravitation, in ſuch vi-
cinity. And had the Earth paſſed directly through
the midſt of the tail of the Comet of 1769, or of
any
other, it is not in the leaſt probable, unleſs the Comet
had come near enough to injure us upon other ac-
counts, that the Earth could have carried off with it,
or that the Comet would have imparted to us, vapors
fufficient, even for a common thunder-ſhower. But
this is ſubmitted to the reader ; who, perhaps, upon
the peruſal of the lecond part of this eſſay, may be of
opinion, that a Comet, in its afcent from the Sun, is
no more calculated to ſet che World on fire, than
to drown it in its defcent. But to return to our
ſubject.
HERE

( 36 )
Here a material objection may naturally ariſe,
viz. why ſhould the Comets appear with tails, and
thoſe, generally, of enormous lengths, while the
Planets, which have atmoſpheres as well as the Com-
ets, are always ſeen without them, even when at equal
diſtances from the Sun !
To this it may be anſwered ; that the atmoſpheres
of the Planets are ſo ſmall, in proportion to the globes
they ſurround, compared with thoſe of Comets, that
whatever may be the cauſe of the tails of the latter ;
if the fame cauſe act upon their ſeveral atmoſpheres,
proportionably to the ſpaces they reſpectively occupy,
the tails of Comets may appear of aſtoniſhing lengths,
while thoſe of the Planets ſhall be totally inſenſible.
In other words; the Planets in ſimilar ſituations
ſhall have no tails at all.
The Earth, as has been repeatedly obſerved, is a
Planet, with which, as it is the place of our abode,
we are well acquainted: The height of its atmoſphere
is generally computed at about fifty miles ; beyond
which, its denſity is not fufficient to reflect the rays
of the Sun in the crepuſculum or twilight: The
diameter of the Earth is about 8000 miles, the dia-
meter then of the Earth and atmoſphere together is
about 8100 miles ; from whence it appears by com-
putation, that the ſpace occupied by the atmoſphere
alone is to the ſpace occupied by the Earth alone,
as i to 26 nearly
The diameter of the atmoſphere of a Comet, as
before obſerved, is, at a medium, equal to ten times
che

( 37 )
the diameter of its nucleus ; and, ſpheres being as
the cubes of their diameters, the magnitude of a
Comet and its atmoſphere together is equal to one
thouſand times the magnitude of the Comet alone
conſequently, the ſpace occupied by the atmoſphere,
is to the ſpace occupied by the nucleus as 999 to 1.
Let us ſuppoſe a Comet of equal bigneſs with the
Earth, having an atmoſphere as above deſcribed,
to be ſo ſituated, as that the Sun, the Earth and
the Comet, may be in the Angles of an equi-
lateral triangle : Let us ſuppoſe alſo the viſible
effect of the repelling power of the Sun's atmoſphere
upon
thoſe of the Earth and the Comet, to be in
pro-
portion to the ſpaces they reſpectively occupy : The
magnitudes of theſe atmoſpheres being one to the
other as 999 to io, that is as 25974 to 1 ; when a
ſpectator upon che Earth might ſee the tail of the
Comet extend through an arch in the Heavens of
60°; had the atmoſphere of the Earth a tail, ariſing
from the ſame cauſe, in proportion to the ſpace it oc-
cupied, to a ſpectator upon the Comet it would ſub-
tend an angle of no more chanoº o' 8" 81" or 60° di-
vided by 25974, an angle which the beſt inſtruments
could never diſcover in an object ſo dubiouſly defined :
In other words, when the Comet would have a tail
60° in length, a Planet at the ſame diſtance from the
Sun would have none at all.
The atmoſpheres of the inferior Planets are pro-
bably leſs in proporcion, and thoſe of the ſuperior,
larger than that of the Earth. But this ſubject may
more

( 38 )
more properly be introduced in the ſecond part of this
Effay.
The principle of repulſion by which we have en-
deavoured to account for the tails of Comets, and their
oppoſition to the Sun, may receive further confirma-
tion by conſidering ſome of the phænomena which the
atmoſphere of a Comet would exhibit in the neigh-
bourhood of the Sun, were the Sun diveſted of his
atmoſphere, or were his atmoſphere deprived of its
repellent principle : We ſhall therefore endeavour to
prove the following propoſition, viz.
Were the Sun without an atmoſphere, or fome
other appendage in its nature repellent to the atmof-
pheres of Comers, and the æthereal ſpaces void of re®.
fiſtance; inſtead of one tail, and that always turned
from the Sun, every Comer would have 1900 ; the
direction of one being towards, the other from the
Sun, of which the former would be the moſt confi-
dérable, and boch would increaſe as the Comet ap-
proached the Sun, from the increaſing gravitation
song
towards him.
18 ob
For chough a Comet’s atmoſphere in its natural
ftare furrounds che nucleus in a ſpherical form, (a
form shot all fluids which tend to a common center
of gravity muſt put on) yet if we ſuppoſe this admo-
phere to be affected by its gravitation towards the
Sun, and to conſiſt of an unelaſtic fluid, like water ;
thoſe parts of this fuid which were neareſt to, and
moſt remote from the Sun, would have their gravita-
tion towards che center of the Comer leffened by the
Sun's
G
E
Eh!

( 39 )
Sun's attraction, and thereby, from a ſpherical form
the whole would be changed to an oblong ſpheroid.
This is in fact the caſe with our ocean, which is
changed from a ſphere to a ſpheroid by the gravita-
tion of the Moon ; the axis of which ſpheroid, revol-
ving round the axis of the Earth in conſequence of
the diurnal motion, cauſes the flux and reflux of our
fea. But the atmoſphere of a Comet is indefinitely
elaſtic, and thence capable of an unlimited expanſion ;
therefore, in proportion as it is in any part elevated
above the ſuperficies of the ſphere it occupies when in
its natural quieſcent ſtate, ſuch parts are proporcionably
at liberty to expand themſelves, by virtue of the mu-
tual repulſion of their component particles, which are
now in great meaſure freed from that conſtraint which
before condenſed them ; * which would change both
ends of the ſpheroidal figure,to forms ſimilar to the tails
which commonly attend the Comets, being broader as
chey become rarer towards their extremities, on one ſide
and the other : But the tail next the Sun would be the
moſt conſiderable, the effects of his attraction on that
fide being greateſt ; and, ſhould the Comet approach
near enough to the Sun, a continued aerial ſtream would
be formed from one globe to the other, whereby the
Sun, by his ſuperior attractive power, would by de-
grees rob the Comet of its atmoſphere, condenſing the
fame round his own globe, without leaving the Comet
a fufficiency for the purpoſes of life and vegetation
in its folitary retreat without the planetary ſpheres.
But as the Sun has an atmoſphere, repeilent to the
atmoſpheres of the other heavenly bodies, theſe in-
* See Page 31.
conveniences

( 40 )
conveniences are thereby prevented, and every globe
of the ſyſtem retains its own, unimpaired, through
every ſtage of its revolution.
That the projection of the tail of a Comet from
its nucleus, and its perpetual oppoſition to the Sun,
do ariſe from the mutual repellency of the atmoſ-
pheres of Sun and Comet, as we have endeavoured to
prove in the foregoing pages; may be confirmed and il-
luſtrated, if not demonſtrated by electrical experiments.
The electric fluid is an element, as diftinct from all
others which we are acquainted with, as air is from
water ; and if there be any ſuch thing exiſting in na-
ture, as pure elementary fire, this claims that character,
in preference to all others. For fire, in its common
form, is ſo far from being a pure element, of itſelf,
that the preſence of air is neceſſary to its very being.
Whereas the electric fire, in many experiments, acts
with greater freedom in vacuo, than in the open air.
But, different as this wonderful ſubſtance is from all
others, we find, that ſome of its properties greatly
reſemble thoſe of air, already treated of : And, if the
following propoſitions, which are introduced to prove
that reſemblance, can be cofirmed by experiments ;
and it be allowed, that ſimilar cauſes, naturally, pro-
duce fimilar effects ; it is preſumed, that the ſolutions
of the cometic phænomena, offered in this eſſay, when
compoſed with thoſe experiments, will be found not
inadequate to the phænomena.
Prop. I. THERE is a mutual attraction, ſublifting
betwixt che electric fluid and common matter ; in
conſequence

( 41 )
conſequence of which, the former is capable of, and
liable to a condenſation round the latter ; in the ſame
manner as air, by common gravitation, is condenſed
round the heavenly bodies, in the form of atmoſpheres,
The truth of this propoſition has been abundantly
proved by Doctor Franklin, and other writers upon
this fubject : Indeed, the ſucceſs of all our electrical
experiments depends upon this property : We may
add further, that all the phænomena which have ever
been obſerved, in which that element is concerned,
from the attraction of ſmall hairs, duft, &c. in
conſequence of the attrition of amber, to the moſt
fevere blaze and irreſiſtable force of lightning; de-
pend in a great meaſure, upon the ſame property.
Prop. 2. Air, as we have ſeen, is a fluid, the
par-
ticles of which mutually repel each other : So, alſo,
is the electric element.
The truth of this propoſition is demonſtrated by
the following
Ε Χ Ρ Ε R Ι Μ Ε Ν Τ. Ι.
Suspend a plate of metal, by wire, from the prime-
conductor generally uſed in theſe experiments, and
electrify it. On another plate, placed under the for-
mer, at the diſtance of three or four inches, put a
ſmall quantity of dry fand, meal, bran, four, or even
the moſt impalpable powder ; the particles upon the
lower plate, being attracted by, and themſelves at-
tracting the electric atmoſphere, condenſed round the
G
under

( 42 )
under ſurface of the upper plate, a beautiful ſhower
will enſue: Theſe particles will afcend to the upper
plate ; each, by virtue of the aforeſaid attraction, will
receive, and condenſe round itſelf, in the form of an
atmoſphere, a quantity of this fluid proportional to
its capacity ; be immediately repelled from it, and
deſcend towards the lower plate : But theſe corpuſ-
cles deſcend, diverging from each other, many of
them falling wide of the plate, upon the table be-
neath, to return no more: Which divergency, not
only indicates, but is the neceſſary conſequence of, the
mutual repellency of the electric atmoſpheres, now
condenſed round them. Thoſe that recover the
lower plate, having diſcharged their fire, re-afcend to-
gether with thoſe that were left behind at firſt ; with
them receive a new charge, and return diverging, as
before. This operation continues, until moſt of the
particles are ſcattered on the table below, few being
left between the two plates. Now, as the quantity of
the fluid condenſed round each corpuſcle is, probably,
proportional to the ſuperficies of the corpuſcle ; * and
as this experiment will ſucceed with an impalpable
powder, if perfectly dry, we may conclude, that the
ſmalleſt conceivable portions of the electric matter,
when once fevered, are mutually repellent ; from
whence we may infer the repellency of its conftituent
particles themſelves.
This property is alſo largely treated of by Doctor
17
See Do&tor Franklin’s printed letters page 55 $ 15. wherein he aſſerts, thal
" The form of the electrical atmoſphere is that of the body it ſurrounds.”-
This he proves by experiments, and from this principle has given a probable
{olution of the operation of points, at a diſtance from the electrified bodies.
Franklin,

( 43 )
Franklin, * and is as neceſſary to moſt electrical ex-
periments, as the former is to all ; and, doubtleſsly
contributes one half to the amazing rapidity of light-
ning, to which nothing, ſhort of that of the rays
of
light, can be compared.
Frop. 3. When two ſmall ſpherical bodies have
electrical atmoſpheres, condenſed round them
3
thoſe
atmoſpheres are mutually repellent : As we have
endeavoured to prove, from the known mutual re-
peilency of the particles of air, that the atmoſpheres
of the heavenly bodies are, amongſt themſelves.
This propoſition naturally reſults from the laſt,
and is, in ſome meaſure, involved in it; but, as the
experiments now to be produced in ſupport of it,
may ſhew us in what manner the electric atmoſpheres
act upon, and affect each other, it deſerves a more
particular conſideration. For, as theſe atmoſpheres
are inviſible, in themſelves, and are diſcoverable, only
by their effects, it is neceſſary to charge them with
fome fubſtances which reflect the rays of light, in or-
der to render them proper ſubjects of obſervation.-
But the duſt, powders, &c. uſed in the proceeding ex-
periment, will not anſwer that purpoſe, as they fly
off, inftantly, upon their receiving, and condenſing
* See Franklin's letters påge 37. $ 5. where he ſays, • Every particle
" of matter electrified, is repelled by every other particle, equally
“ electrified. Thus the ſtream of a fountain, naturally denſe and continual,
** when electrified, will ſeparate and ſpread in the rm of a bruſh, every drop
" endeavouring to recede from every other drop. But on taking out the
" electrical fire, they clofe again.” To which may be added, that, if a ſmall
cup, filled with water, having a ſpout juft big enough to let out the water
by diftin&t drops, be electrified, the water will iſſue from the mouth of the
ſpout in a diverging miſt, ſo long as the cup remains charged, but as ſoon as
diſcharged again, will fall in diftinct drops as before.
found

( 44 )
round themſelves a certain portion of the fluid : In-
ſtead of which, if fine limber threads are run through
the balls, in various directions, and their ends cut off
at equal lengths, theſe threads, as they cannot eſcape,
ſhew, by their ſeveral directions, the tendencies of the
electric atmoſpheres of the bodies they adhere to.
2.
This being premiſed, we proceed to the following
tomu
E XPE R Ι Μ Ε Ν Τ. ΙΙ.
owolone
yangi
Let a pellet of cork or pith, thus prepared, be
ſuſpended, and electrified ; the threads will diverge,
equally, every way, from its center as at A Fig. 6. the
electric atmoſphere being equally condenſed, uponi
every part of it, and equirepellent from its center
outward. But if two ſuch balls be ſuſpended and
electrified, and caufed to approach each other, the
threads of each, which happen to be next to the other,
inſtead of ſtanding out every way from the center, as
before, will be incurvated towards the line of oppoſi-
tion, as at B and C; whereby the reciprocal repulſion
of their atmoſpheres is rendered viſible, as the direct-
ions of the threads faew the tendencies of thoſe at-
moſpheres reſpectively ; from which it appears, that
they recede from each other, as far as the mutual at-
traction fubfiſting between each atmoſphere and its
own ball, (which at firſt occafioned a condenſation of
the former round the latter,) will permit. But that the
atınoſpheres do not wholly fly off, in conſequence of
this repulſion, is evident, as the threads continue in the
fame directions, until one, or bosh be diicharged.
Query

( 45 )
Query. Does not this experiment abundantly con-
firm and illuſtrate the reaſoning in page 29,
relative to
the paſſage of two Planets, or Comets, attended with
large ærial atmoſpheres, by each other ; whoſe mu-
tual effects are reprefented at C and D Fig. 3 ?
We ſhall adduce one experiment more, before we
cloſe this ſubject, which may prove as entertaining to
him who will be at the trouble of making it, as it is
demonftrative of the principle under conſideration.
Ε Χ Ρ Ε R Ι Μ Ε Ν Τ ΙΙΙ.
Provide a wooden ſphere, of four or five inches
in diameter (call it A); let it be gilt, as the metal
will better condenſe the electric fluid upon
its ſurface;
alſo a fmall pellet, of cork, or pith of elder (which
call B), ftrung, as in the laſt experiment, with a few
threads, of three or four inches in length : Suppoſe
A to repreſent the Sun, B, a Comet; fix A on the wire
of an electric bottle, and ſuſpend B by a filken thread
from a point, directly over the center of A, ſo as that,
when neither of them is electrified, B may
reſt againſt
A, a little below the level of its center ; and charge
both ball and pellet. The mutual repellency of their
electric atmoſphere is ſo great, as that that of B is
thrown off as far from A, as the mutual attraction
between the pellet and its atmoſphere will permit ;
which attraction is at the ſame time ſo ſtrong, as that,
rather than ſuffer a ſeparation, the pellet flies off with its
atmoſphere, to a certain diſtance from A, where its na-
tural gravitation to the Earth is juſt balanced by the re-
pellent force of the two atmoſpheres: There B remains
36

( 46 )
ac reft, while its atmoſphere retires as much farther as
pollible from A, without quitting it wholly ; inſtead
of which it undergoes a longitudinal dilatation, in op-
poſition to the center of A, as the atmoſpheres of Co-
mets do, with regard to the Sun : This is evident
from the directions of the threads, which, obferved in
a fide view, bear a near reſemblance to the tails of
Comets. When the pellet is in this ſituation, blow it
gently with a bellows, in a direction perpendicular to
the line connecting the centers ; this will give it a
projectile motion, which will be regulated by its com-
mon gravitation ; and as the center of A is directly
under the point from which B is ſuſpended, the lat-
ter will be carried round the former, as the center of
its motion. Thus will this little Comet perform many
revolucions round its electric Sun, and in every one,
and through every part of each, the tail of threads
will conftantly maintain its oppoſition to it, as the
tails of Comets do to the Sun in the Heavens.
This experiment was very agreeably repeated with
an artificial Comet, conſiſting of a ſmall, gilt cork ball,
with a tail of leaf-gold, about two inches and an half
in length ; when, during the whole time of the expe-
riment, in which it performed, at leaſt, twenty revolu-
tions, the tail, as nearly as the eye could judge, was
conſtantly projected in the line of the oppoſition of the
centers of the two balls ; the thread, by which the
ſmaller one was ſuſpended, either twiſting, or unt wilt-
ing, the ſame way, during the whole time. When
thus in motion, if we raiſed the globe, the tail was de-
preſſed ; when it was lowered, the tail was elevated ;
maintaining its oppoſition, in every fituation.
BUT

( 47 )
Bur now, from theſe experiments, fome may, per-
haps, be inclined to think, that the electric fluid is the
fole cauſe of the phænomena of the tails of Comets :
That the Sun, as it is the grand ſource of light and
heat, throughout our fyftem, may be the fountain,
from which this element, alſo, is ſomehow derived to
its ſeveral globes: That Comets, for wiſe ends, un-
known to us, have a larger ſhare of this fuid than the
Planets ; and that, when they approach the Sun's
electrical atmoſphere, their own are thereby repelled,
as in the foregoing experiments; and appear as lucid,
diverging beams, like thoſe which, in the dark, we fee
îtreaming from electrified points.
و
This hypotheſis might indeed ſolve the phenomena
of the cometic tails, provided the electric atmoſpheres
or bodies, charged with that fluid, were viſible in any
of our experiments, even when made in the dark:
But as that is never the caſe, unleſs the fluid bein mo.
tion; and as the proof of the exiſtence of ſuch atmof.
pheres, when a reſt, depends ſolely upon their
effects upon other ſubſtances, the hypotheſis can-
not be admitted. Were this indeed the true and
ſole cauſe of the phænomena, a Planet, near which a
Comet might happen to paſs, would be in a ficuation,
truly hazardous ; for it is evident that the Planets
(upon that ſuppoſition) are not accommodated with,
ſuch vaſt electric atmoſpheres as attend the Comets
confequently, have not the fame proportion of char
fluid condenſed round them, but far ſhort of it..
Therefore, whenever the electric tail of a Comet
paffed near a Planet, it would be attracted by it, be
drawn afide, from its oppoſition to the Sun, towards it,
and

( 48 )
it diſcharge the ſurpluſage of its fire, that
both might have equal, or proportional ſhares; which
diſcharge, if we conſider the ſnap of a ſmall ſpark be-
tween two cork pellets, and enlarge the idea, propor-
tionably, to that inſtantaneous cataract of fire which
would neceſſarily take place between two Worlds in
ſimilar ſituations, we may well imagine would give
an exploſion, which nothing could equal, ſhort of the
final voice of an archangel; and, if it were not ſuffi-
cient to rouſe the aſhes of the dead, nsight reduce the
living to their primitive duft. But ſuch a cataſtrophe,
we have not the leaſt reaſon to dread, from the neigh-
bourhood of a Comet, unleſs we can ſuppoſe, that in-
finite wiſdom and goodneſs would create one world,
merely for the deſtruction of another ; as we cannot
conceive of any other ends, to which ſuch huge
electrical atmoſpheres could be adapted. Indeed the
diſcharge would be equally fatal to both worlds ; as
it is certain from electrical experiments, that the ef-
fects of a ſtroke of lightning are the fame, whether
the flaſh proceeds from the cloud to the Earth, or is
diſcharged from the Earth into the cloud, both of
which have happened during the ſame thunder-guft ;
as appears from obſervations made by Mr. Kinnerfly
and Doctor Franklin,* communicated to the Royal
Society. But to put this matter beyond all reaſonable
doubt, we may further obſerve, that if the phænome-
na of Comet's tails aroſe from the fame cauſe, which
renders the electric ſtream viſible, when proceeding
from a ſteel point, theſe tails would ſhine by their
own light, as the electric fluid does, when in motion ;
whereas, an apparent obſcuration or defect, in one of
and
upon
# Franklin's Letters, page 116 and 129.
thoſe

( 49 )
thoſe tails, has been obſerved, which evidently aroſe
from the ſhadow of its nucleus, which occaſioned a
partial eclipſe of the tail, by intercepting the Sun's
rays *: Conſequently, the tail, as well as the head,
ſhines with a borrow'd light, and both are viſible,
only by reflecting the rays of the Sun.
As the conſideration of the properties of the electric
element was introduced, merely, to illuſtrate the feve-
ral propoſitions, and the concluſions reſulting from
them, contained in the foregoing pages ; it was
thought proper to add thus much upon that ſubject,
in order to prevent the framing an hypotheſis from
thoſe properties, which, inſtead of removing the fears
of the timorous, upon the appearance of a Comet,
(which was one deſign of this eſſay,) would naturally
tend to increaſe their apprehenſions. Whereas, upon
the principles we have endeavoured to eſtabliſh, the
tails of Comets appear to be nothing more than air,
immenſely expanded and rarefied; through which
the Earth might paſs, with the utmoſt fafety to its
inhabitants. And, inſtead of theſe bodies being
heralds, fent forth to denounce the wrath of Heaven,
in which light they have been conſidered by the igno-
rant and ſuperſtitious, of all ages ; or the immediate
executioners of divine vengeance, as others have ap-
prehended ; we ſhall endeavour to prove, in the fol-
lowing pages, that Comets deſerve to be conſidered
in a more reſpectable light; and that their tails, how-
ever awful and portentous they have been eſteemed
* Hevelii Cometographia Lib. XII. Page 898, quoted by Mr. Profeſſor Winthrop
in his ad Lecture on Comets.
H
by

(50)
by fome ; may be deſigned for, and are wiſely adap-
ted to, the truly god-like purpoſes, of rendering ha-
bitable a vaſt variety of Worlds ; and of affording a
comfortable ſubſiſtance to innumerable ſpecies of beings,
by which they are, probably, inhabited.
英​英​英​英​英​英​英​英​英​英​英​英​英​英​英​英​英
​跑​,跑​就​成​原貌​,来​成双​成​地​求成​成就​,成
​义​的​成​地​。
ESSAY

( 51
E
S
S A
Y
ON
COM ET S.
Ρ Α R T II.
A
S ancient geographers imagined the polar and
equatorial regions, or the frigid and torrid
zones of the earth, were uninhabitable, in
conſequence of the extremes of heat and cold, to which
thoſe climates are expoſed : So, modern aſtronomers
have paſſed a ſimilar judgment upon the ſuperior and
inferior Planets, eſpecially on Saturn and Mercury ;
concluding, that our water would always boil upon
the latter, and be frozen upon the former ; and that
merely in conſequence of their different diſtances from
the Sun*. Whence it has been naturally concluded,
that the textures of their various fluids, and of their
inhabitants, to whoſe uſes theſe fluids are adapted, are
very different from what they are found to be
upon our Earth : And, conſidering the near ap-
proaches of moſt Comets to, and the vaſt elongations
of all their orbits from the Sun, it has been generally
* See Newt. Princip. Book III. Prop. VIII. Cor. 4.
ſuppoſed

( 52 )
ſuppoſed, that no material race of beings could fubfiſt
under ſuch amazing viciſſitudes of heat and cold, as
thoſe bodies muſt, from their different ſituations, ne-
ceſſarily be expoſed to; conſequently that they are
uninhabited.
But the concluſiveneſs of this reaſoning depends
upon the truth of the following Propoſition ; ad-
vanced indeed by Sir Iſaac Newton; but not ſupported
by experiments, which were, with him, the criterion
veritatis ; viz. that, “ The heat of the Sun is as the
denſity of bis rays, that is reciprocally as the ſquares of
" the diſtances from the Sun.*
66
Here, we are again reduced to the diſagreeable
neceſſity, of diſſenting from the opinion of the greateſt
GENIUS that ever dignified human reaſon ; which,
conſidering the juftly celebrated fame of that illuftri-
ous author, may be ftigmatized as ignorance or
vanity : But it is hoped that the reader will wave that
imputation, if he ſhall judge, upon the whole, that
Sir Iſaac himſelf would have altered his opinion, upon
the evidence which we ſhall produce in ſupport of the
contrary poſition : We may, however, lay down this
as a maxim, that, in the profecution of any ſcience,
the progreſs of the mind muſt neceſſarily be retarded,
in proportion to the implicit aſſent we give to the de-
ciſions of any man, however great. We ſhall there-
fore, without further apology, endeavour to prove that
the heat of the Sun, as perceived by us, and as dif-
coverable by its effects upon other ſubſtances expoſed
to his rays, does not depend upon the denſity of thoſe
* Princip. under Prop. XLI. Book III.
oqa
rays

( 53 )
rays only, though they are neceſſary to the very ex-
iſtence of that heat ; but, equally upon the concurrent
operation of another cauſe, which we ſhall preſently
conſider ; from whence it will follow, that theſe
cauſes, wherever they co-exiſt, whether upon the
Earth, or upon the heavenly bodies, will naturally
produce fimilar effects.
و
In the mean time, before we engage in the diſ-
cuſſion of planetary heat, as depending upon the ſeve-
ral diſtances of the Planets from the Sun; it may
throw ſome light upon this ſubject if we conſider the
portion of that heat which falls to our own ſhare, and
the diſtribution of it throughout the various climates
of the Earth.
The ſurface of the Earth has,by geographers, been
divided into five zones, viz. one torrid, including all
the regions between the tropics, upon every part of
which the Sun ſhines perpendicularly twice every year:
Two frigid, which are ſituated between the polar-circles
and the poles, and endure the rigors of perpetual
winter, as the former is always balking in a ſummer
Sun: And two temperate, which experience the viciſſi-
tudes of winter and fummer, and, in ſome parts of
them, in their extremes ; theſe are ſituated between
the frigid and torrid zones, in both hemiſpheres. In
the firſt of theſe, the ſeaſons are much more uniform
than in the others, the days and nights being nearly
of equal lengths, the year round ; and although the
heat
may,
for a conſtancy, be greater therein than in
any other climate, yet it is not liable to ſuch great and
ſudden changes as are experienced in the temperate
zones;

( 54 )
zones ; for, during a whole annual revolution of the
earth, the difference of the degrees of heat, experienced
in the torrid zone,as determined by the thermometer, are
not ſo great as thoſe which, ſometimes, happen in the
temperate zones, within the compaſs of a few hours* :
Much more do they fall ſhort of the extremes which
are endured in the latter, in the oppoſite ſeaſons of the
year. † But what is above aſſerted of the torrid zone
is to be underſtood only of the low, inhabited and
cultivated countries, the mountainous regions with
which thoſe climates abound being excepted, for rea-
fons which will hereafter appear.
The axes of the ſeveral Planets whoſe diurnal ros
Cations have been diſcovered are inclined, more or
leſs, to the planes of their reſpective orbits ; conſe-
quently, their ſuperficies are diviſible into zones and
* One morning, in the winter of 1768, the mercury in Farenbeit's thermometer
was 5° below oº; by 11 o'clock the ſame day it had riſen to 300, and the
next day to above 60° ; the difference being 65° in little more than 24
hours. Again, May 30, 1764, when the general election for the choice of
councellors, for this province, was held at Concord, (a town about 20 miles weſt
from Boſton) the weather was (for the ſeaſon) extremely hot ; but on the
morning of the ift June following, a ſevere froſt cut off all the indian corn,
beans and other tender annual vegetables, in that and the towns adjacent, for
miles round. And on one Sunday morning, in the winter of 1759-60, &
tranſition was made, inſtantaneouſly, from ſevere cold to ſummer heat ; to
the great ſurprize of every one, and to the no ſmall terror of many. The
buildings ſuddenly ſmoked to ſuch a degree, that, in ſome of the worſhipping
aſſemblies at Bolton, the people ſuſpected that the neighbouring houſes were
on fire; and there was ſcarcely a perſon, who did not recoil from the heat, at
the church doors, at the cloſe of the ſervice,
+ One ſummer's afternoon in the year 1760, the thermometer, being expoſed to
the open air, in the ſhade, the Mercury ftood at 1020. At another time, viz.
in the winter of 1766-67, the Sun being an hour high in the morning, it was
at gº below oo. Theſe were probably, as great extremes as have been ob-
fcrved in this climate, the difference being 11 no by the thermometer.
climates,

( 55 )
climates, correſponding with thoſe of the Earth: And
it is, at leaſt, highly probable, that the various cli-
mates of each globe, during its periodical revolution
round the Sun, experience as great viciſſitudes of heat
and cold as thoſe of the Earth : Nor is it unlikely
that, in the equatorial regions of the different Planets,
there may be at the ſame time as great varieties in the
degrees of heat they reſpectively enjoy, as there are in
the temperate zones of the earth in the different fea-
fons of the year ; but ſuppoſing all this, the inequalities in
the diſtribution of heat to the ſeveral Planets and their
various climates would vaniſh, when compared with
thoſe extremes which they would neceſſarily be expo-
fed to, at their ſeveral diſtances from the Sun, upon the
ſuppoſition that the Sun's heat were as the denſity of
his
rays;
for were that really the caſe, the heat of
Summer upon Mercury would be about ſeven times as
great as upon the Earth, and above twice as hot as
boiling water with us. On the other hand our fum-
mer heat would be above ninety times greater than
that of Saturn, the difference being more than ſeven
times as great as that between our ſummer heat and
the heat of red hot iron* : For it is undoubtedly cer-
tain, that the denſity of the Sun's rays is reciprocally
as the ſquares of the diſtances from the Sun ; from
whence the above concluſions muſt neceſſarily follow,
if the heat be proportional to that denſity.
.
It is certainly then a queſtion well deſerving a phi.
loſophical enquiry ; whether there be not ſore me.
* Sir I. Newton concludes from experiments that boiling water is three times,
and red hot iron about twelves times hotter than our ſummer heat. Principe
Prop. XLI. Book III.
dium

( 56 )
dium provided in nature, which, being diftributed in.
different proportions to the feveral Planets, may fo
attemper the heat of the Sun to their refpective dif-
tances from him, as that the inhabitants of all may be
equally happy in the enjoyment of it ;.and that one
globe may receive as much benefit, and be expoſed to
as little injury, from that heat, as any other through-
out the fyftem. This indeed ſeems to be an object fo
worthy of the attention and providence of the great
PARENT of the univerſe, that a philofophic mind
would naturally embrace ſuch an hypothefis, had it,
but, the moſt flender evidence to fupport it. This
medium, we ſhall find, is actually provided in the
element of air, which is, in various proportions, con-
denſed round, and conſtitutes the atmoſpheres of the
Earth and the heavenly bodies.
As air is an element, to which we, and probably
the inhabitants of the other Planets, are more indebted
than is generally imagined ; a ſhort differtation upon
ſome of the advantages which accrue to us, and pro-
bably to them, from its preſence, may be acceptable
to the reader.
AIR is a grand medium in nature, through which
an all-bountiful providence conveys to us many of the
conveniences, comforts, and delights of life. Upon
Air depends the aſcent of vapors, and their condenſa-
tion into clouds, whence they deſcend in dews and
grateful ſhowers, to refreſh and fructify the Earth.
Upon Air we depend for the twilight, which affords
us an agreeable gradation of ſhades from day to night;
without which we ſhould inſtantaneouſly plunge from
the

( 57 )
the light of the Sun to midnight darkneſs; and again
emerge from total darkneſs to the full luftre of day;
which would be unſufferable to our organs of ſight,
upon their preſent conſtructure. Air is alſo the ve-
hicle of ſounds, whether articulate or inarticulate ;
conſequently without it, we ſhould not only be de-
prived of the artleſs melody of the woods, and of the
raptures which accompany the maſterly execution
of muſical compoſition ; but, which is of infinitely
greater importance to us ; there could be no language,
no communication of ideas, but by dumb figns; no
liberal arts nor ſciences in the world. Therefore if
we could fubfiſt without this element, all mankind
would be like the unhappy few among us who are
ſaid to be born deaf and dumb.
It has already been ſhewn from experiments, that
Air is neceſſary both to the ſupport of animal life, and
to the ſubſiſtance of fame : And how far the very
being of fire, in any ſhape, and even of heat itſelf,
may depend upon it, the reader may judge from the
following experiments and obſervations of Mr. Boyle,
related in Shav's Abridgement of his Works.
« COALS,” ſays Mr. Boyle, “ being put glowing
« into a receiver, in three minutes after beginning
" to pump, the fire totally diſappeared.--Other coals
being ſuſpended, in the open air, at the ſame time,
continued burning 'till a great part was reduced
* to aſhes *Lighted match was found more diffi-
“cult to put out by exhauſting the air than kindled
* charcoal, nevertheleſs in about ſeven minutes the
Shaw's Boyle, page 419.
I
66 fare

( 58 )
66
ss fire was extinguiſhed, beyond the poſſibility of re-
covery by re-admitting freſh air.” Here we ſee that
air is neceſſary to the ſubſiſtance, not only of flame, but
of fire that emits no flame at all. To theſe we may add
an eaſy experiment, which any perſon may try at his
leiſure, without the affiſtance of a pneumatic engine, viz.
A compoſition may be made of allum and flour, which
being well mixed together, reduced to a cinder in a
crucible, pulverized, and otherwiſe prepared by a
ſecond heat, in a phial ſecured from the free com-
munication of the external air, acquires an igneous
quality, which, if the phial be kept ſtopped, it will
retain unimpaired, even for many years ; nor will it
fhew any appearance of fire more than any other
matter confined in the ſame manner ; but if at any
time a few grains of it be let out upon any com-
buſtible ſubſtance in the open air, it will by the freſh
air be inſtantly changed into fire, and kindle the fub-
ſtance upon which it falls. This powder is com-
monly called the black phoſphorus. From a ſmall
quantity of it the experiment may be repeated
with ſucceſs for years together, provided care be taken,
whenever the phial is opened to let out any of the
powder, to ſtop it again immediately, to prevent the
too free acceſs of the external Air. Here we have a
ſubſtance which has all the qualities of fire inherent
in it, and retains them for a long time, and yet can
never exhibit them but upon the admiſſion of freſh
Air. But to return to Mr. Boyle : In page 603 he
concludes from experiments, which he had been mak-
ing in condenſed Air, that the conſumption of mat-
ter by fire is greater in proportion to the quantity
of Air contained in the (fame) receiver ; or rather
66
66 in

( 59 )
«in a ſtill greater proportion," as he found by ſome
ſubſequent experiments. Therefore, as the conſump-
tion of matter by fire, without flame, muſt be pro-
portional to the intenſity of the heat which conſumes
it
, we may conclude from this laſt obſervation, that
the intenſity of the heat in any enkindled ſubſtance,
is nearly proportional to the denſity of the ſurrounding
Air, and depends in a great meaſure upon it. In page
604 Mr. Boyle concludes from other experiments, that
“ fire is more eaſily kindled in Air much compreſſed,
“than in common Air. Now it is certain that, the
more intenſe the heat, the quicker the fame combuſti-
bles are kindled by it ; thus bodies, expoſed to the
foci of different burning-glaſſes, will take fire ſooner
from ſome than from others, according as
the
powers
of thoſe glaſſes to condenſe the Sun's rays in their foci
(cæteris paribus) are greater, by which condenſation the
heat is proportionably increaſed : But as in theſe ex-
periments, made in condenſed Air, Mr. Boyle kindled
his fire with the rays of the Sun, thus collected in the
focus of a burning-glaſs, and found, as above, that the
fame glaſs would more eaſily kindle ſubſtances, in
compreſſed Air, than in common Air, it follows, that,
the denſity of the Sun's rays remaining the ſame, the
heat with which they were accompanied was increaſed
by increaſing the denſity of the Air ; in like manner
as if the denſity of the Air had remained the fame, and
the denſity of the rays had been increaſed, by uſing
glaſſes of ſtronger powers : To which may be added
that Mr. Boyle always found it extremely difficult, and
ſometimes impoffible to kindle any
fubftance whatever
in an exhauſted receiver, either by the rays of the Sun,
or even by red-hot iron in contact with gun powder
itſelf. The concluſion is obvious to every capacity.
HAVING

( 60 )
Having thus exhibited to the reader an imperfect
ſketch of the principal uſes to which our Air is ſubſervi-
ent; wherein among other things, we have ſeen the ne-
ceffity of its preſence and co-operation in the product-
ion of heat by the rays of the Sun, in common expe-
riments : We ſhall now in further proſecution of
the ſubject, proceed to prove that the heat of the
Sun, as enjoyed by the inhabitants of the Earth in
general, depends, not only upon the denſity of the
Sun's rays, but, equally, upon the denſity of the fur-
rounding atmoſphere. This we ſhall prove from the
teſtimonies of travellers of the moſt undoubted repu-
tations who crofled the feas, and undertook the moſt
dangerous and fatiguing jcurnies which, perhaps,
have ever been performed by man, with no other view
than to promote the cauſe of ſcience ; particularly
Don George Juan and Don Antonio de Ullou, and their
attendants who were ſent by the Courts of France and
Spain to South-America, to meaſure a degree of the
meridian under the Equator: In the execution of
! which commiſſion, they were obliged to take their
ſtations, and make their obfervations upon ſome of
the higheſt mountains upon the Earth, viz. the Andes
in the neighbourhood of Quito, under the Equinoctial.
It is well known that the tops of high mountains
are at all ſeafons very cold, and are, for the most part,
covered with ſnow the year round. But thoſe above-
mentioned, exhibit a ſcene truly curious ; for from
their ſummits to the plains below, incluſively, may
be
found at the ſame time, all the varieties of heat and cold,
which are to be met with in every climate of the Earth,
at all ſeaſons of the
year.
* See Ulloa's voyage to South-America, Book VI. Ch. VII.
ONE

( 61 )
66
One obſervation made by Don de Ulloa upon the
ſpot, is very remarkable, and much to the preſent point :
He ſays, “ the region of continual congelation began
“ upon the ſeveral mountains at the ſame height
« above the level of the Sea, as determined by equal
heights of the mercury in the Barometer.” But Sir
Iſaac Newton aſſerts, and deduces the certainty of it
from “ actual experiments," that “ as to our own air,
" the denſity of it" (i. e. at any height) “is as the
weight of the whole incumbent air, that is, (ſays he)
" as the height of the mercury in the barometer". * Ιε
therefore follows, that the region of “ continual con-
gelation,” or perpetual froſt commenced upon all
thoſe mountains, where the air was of the fame den-
ſity. Above that certain height, the denſity of the air
leſſen'd, and the cold increaſed accordingly in ſeverity,
till the tops of the mountains preſented all the horrors
of winter, which are to be found in the polar regions.
Whereas below that height, as the denſity of the air
increaſed, from the increaſe of the incnmbent preſſure,
the heat of the Sun alſo increaſed; till the inhabitants
of the plains below ſuffered all the inconveniencies of
the torrid zone.
Now, the denſity of the Sun's rays being the ſame
in the ſeveral cafes, and the tops of the mountains be-
ing above the common region of the clouds, and confe-
quently enjoying the preſence of the Sun much more
chan the plains below ; it follows, that although the
fays of the Sun may be the
fine qua non, without which
the inhabitants of the Earth would enjoy no heat at
all, yet, the degree or quantum of their heat depends
upon the denſity and co-operation of the aerial medium
through which thoſe rays are tranſmitted to them.
* See Princip. Buok II. Prop. XXII. Schol.
As

( 62 )
As the proportionality of the Sun's heat to the den.
ſity of his rays, is a point, upon which, as proved or
diſproved, many curious queſtions in natural philofo-
phy may turn, every argument which tends to deter-
mine that point will doubtleſs be acceptable to the
reader ; and if he ſhould be already ſatisfied in his
own mind, from the foregoing obſervations, that the
heat of the Sun is not as the denſity of his rays ſimply, yet
it is hoped that he will patiently attend to one argument
more, which is drawn from Sir Iſaac Newton's own
principles, and naturally reſults from his computation
of the amazing, inconceivable degree of heat, which
muſt have been acquired by the Comet of 1680, at its
perihelion, upon that ſuppoſition ; which, though it
has never been controverted, but generally allowed to
be juſt, and quoted accordingly, muſt, if true, have
occaſioned the exhibition of ſome phænomena, which
could not have eſcaped the notice of the many curious
aſtronomers of that day. According to this great au-
thor's calculation, this Comet, by its near approach to
the Sun at its perihelion, “ acquired a degree of heat
two thouſand times greater than the heat of red-hot
iron*"; but from previous experiments he concludes,
that the heat of red-hot iron is but twelve times greater
than that which dry earth acquires when expoſed to the
ſummer's Sun: With what an amazing luftre then muſt
the Comet haveglowed, merely from the heat it acquired
during its proximity to the Sun ! Therefore at its
firſt appearance after the perihelion, it muſt have ſhone,
not with a borrowed or reflected light, as it did before
it arrived at that ſtage, but by its own newly acquired
luſtre, far exceeding (perhaps) the brighteſt Star in
tie Heavens ; for if iron heated but twelve times more
* Princip. Book III. under Prop. XLI.
than

( 63 )
than dry earth expoſed to a ſummer's Sun, becomes
red-bot, and from that heat emits a fplendor, indepen-
dent of the Sun's rays, we may defy the human ima-
gination to conceive of that fplendor, when yet in-
creaſed two thouſand fold, and exhibited by a globe of
equal dimenſions with the Earth. Furthermore, Sir
Iſaac computes that a globe of red-hor iron equal to
the Earth, or the Comet, ſuppoſed of equal bigneſs,
would ſcarcely cool in fifty thouſand years ; therefore
the Comet, being two thouſand times hotter, if it were
a globe of iron, would require 50000 multiplied by
2000, or one hundred millions of years to cool in. But
if we ſuppoſe the Comet to cool a hundred times faſter
than an iron globe of the ſame magnitude, equally
heated it could not loſe all its heat under 6
; nay, at the end of five hundred thouſand
years it would ſtill be a thouſand times hotter, and
conſequently brighter than red-hot iron. But as the
period of this Comet is ſuppoſed to be ſhort of fix
bundred
years, [ſeveral appearances of Comets in paſt
ages being ſuppoſed to be different viſits from the
fame Comet, after intervals of five bundred and ſeventy
five years ;] if at every perihelion it acquired a degree
of heat which it could not loſe under a million years,
and had ſhort of ſix hundred years to diſcharge itſelf of
the heat acquired at each revolution, how aſtoniſhing
upon theſe principles muſt be the accumulations of
its heat during thoſe ſeveral revolutions ! ſo great!
million years
* This is the duration of heat generally affigned to this Comet, after its perihes
lion. But why a globe of earth ſhould cool an hundred times faſter than a
globe of iron, as here ſuppoſed, no where appears, nor fhould the fuppofition
be admitted, unleſs we ſuppoſe at the ſame time, that a globe of iron is one
hundred mes denſer than a planetary globe; if ther.
chac

( 64 )
that one would imagine, that for ages after its peri-
helion it would be viſible merely from its own luftre ;
and that when, from its diſtance, its diameter would
become inſenſible, it would ſtill be ſeen as a lucid
point, twinkling among the Stars : Yet fo far was this
from being the caſe, that in three months from its
pe-
rihelion, viz. from 8th December to gth March it totally
diſappeared, though the Earth was in a ſituation to
view it for a confiderable time after : Sir Iſaac Newton
ſays " on the ninth and tenth of February to the naked
eye the head appeared no more."
22
66
This Comet, at leaſt its tail, was diſcovered by Mr.
Flamſtead two days after its perihelion, viz. on the
Joth December; from which time till its total diſappear-
ance, it was conſtantly obſerved by aſtronomers; but
none of their obſervations take notice of any extraor-
dinary brightneſs it exhibited, more than is uſual in
the appearance of other Comets. Sir Iſaac indeed
obſerves, that "in the month of December, juſt after
“ it had been heated by the Sun, it did emit a much
longer tail, and more splendid than in the month of
“ November before, when it had not yet arrived at its
“ perihelion." But this he aſſerts of the tail only,
for foon after he adds, or the head of this Comet at
equal diſtances from the Sun and from the Earth,
appeared darker after its perihelion than it did be-
« fore" : It is true he accounts for it by ſuppoſing
the “ nucleus to be environed by a denſer and blacker
66 ſmoak than before." But this is difficult to recon-
cile with what he ſays a page or two back, when
ſpeaking of the fame Comet, viz. “ by fo fierce a
“ heat, vapors and exhalations, and every volatile
® matter
66
6

( 65 )
matter muſt have been immediately conſumed and
“ diffipated*". May we not add, and the whole
folid maſs calcined or vitrified ? Therefore, if that
dulneſs in its appearance, after the perihelion, was
owing to clouds and vapours, it is evident, from that
great author's own reaſoning, that the Comet could
not have been expoſed to ſo great an intenſity of heat,
in that vicinity to the Sun; ſince all ſuch heterogeneous
exhalations muſt have been conſumed and diſſipated
thereby, as faſt as they aroſe from the head ; if indeed
any volatile or evaporable matter, or any degree of
moiſture whatever could have remained in the head
after ſuch an inconceivable ignition. Finally, had
the Comet ever acquired ſo great an intenſity of heat,
it is probable the inhabitants of the Earth would never
have loft fight of it to the end of time; much leſs
would it have totally diſappeared in three months
after its perihelion.
* If at the perihelion diſtance of this Comet from the Sun, viz, about 760,000
miles, or 1-6th of the Sun's diameter above his ſurface (according to Sir J.
Newton), the Sun's heat is fo fierce, as that “ all kinds of vapours and ex-
halations muft be immediately conſumed and diffipated thereby", how is it
poffible for thoſe clouds, (for ſuch they undoubtedly are, ſee page 18) which
appear, more or leſs, every day, like ſpots upon the Sun's diſk, and float in his
atmoſphere, at the height of but 7 or 8000 miles above his ſurface (ſee note
page 29) to remain undiſſipated for above twenty days together, as ſome of
them moſt certainly do (fee page 14)? Or rather, does not the continuance of
thoſe clouds, for fo long a time, amount to a demonftration, upon Sir Iſaac's
own principles, that no ſuch heat exiſts, even within the denſer regions.of the
Sun's atmoſphere.
+ If the fixed Stars be fuppofed equal in magnitude to the Sun, and the Comet
above-mentioned, equal to the Earth ; the period of the Comet to be 575
years, and its body be ſuppoſed ſufficiently luminous to render it viſible in all
parts of its orbit ; it appears from Mr. Bradley's determination of the pasal-
lax of the fixed Stars, after a long ſeries of obſervations (ſee Smith's Opticks,
Vol. II. page 449 and onwards), that the apparent diameter of the nucleus of
this Comet, muſt be much greater than that of the Stars of the firſt magnitude,
syen when moſt remote from the Sun and from the Earth.
K
SIR

( 66 )
Sir I. Newton, in making his computation of the
heat ſuſtained by this Comet, firſt took it for granted,
có that the heat of the Sun is as the denſity of his rays,
(as we have ſeen before): In the next place, he con-
ſidered the denſity of theſe rays with us, at the mean
diſtance of the Earth from the Sun, as a fixed and
certain ſtandard, with which the denſity of the rays at
every other diſtance might be compared : Then, after
expoſing dry earth to the ſummer Sun, he compared
the heat contracted thereby, with that of boiling water
and red-hot iron, and found by experiments the pro-
portional degrees of heat in them to be nearly as 1, 3,
and 12 reſpectively : In the laſt place he conſidered
the heat acquired by the dry earth aforeſaid, as the
ſtandard of our ſummer heat, and annexed it to the
mean denſity of the Sun's rays with us : And upon this
foundation he feems to have conſtructed his general
ſcale of heat for the ſolar ſyſtem. But it is preſumed
that the reader is by this time ſatisfied, that the main
propoſition upon which that great author's reaſoning
was founded muſt fail for want of ſupport, however
juſt his concluſions drawn from it may be : For we
now find regions of eternal froſt under the equinoctial
itſelf, the rigors of which are ſcarcely exceeded in the
polar regions ; and this, at the height of but two or
three perpendicular miles above the common furface
of the Earth, and where but few clouds interpoſe to
hide the beams of the Sun. If therefore we ſuppoſe
ourſelves carried up forty or fifty miles higher, or
to the very top of the atmoſphere, we may well fhud-
der at the idea of ſuch a ſituation, even if expoſed to
the unclouded rays of a perpendicular Sun.
We

( 67 )
We are now naturally led to conſider fome exten-
five purpoſes which the great Author of Nature pro-
bably had in view, when he formed the atmoſpheres
of, and annexed them to, the ſeveral Planets and
Comets of our Syſtem. We have ſeen from indiſpu-
table authority, that the denſity of the Sun's rays alone
does not produce a competency of heat, for the com-
fortable ſubſiſtance of the inhabitants of the Earth,
even in its hotteſt climates, but, that a certain denſity
of air is equally neceſſary, for theſe rays to operate
upon, and to co-operate with them, in promoting the
various purpoſes of life and vegetation. This air we
are abundantly furniſhed with from the atmoſphere
which ſurrounds us, the denſity of which at every
height, is proportional to the preſſure of the incum-
bent fluid. We may conclude from analogy that the
atmoſpheres of the other Planets, and of the Comets,
are deſigned for, and adapted to the fame purpoſes for
which the atmoſphere of the Earth was originally pro-
vided : And if we ſuppoſe the general ſtock of heat,
which falls to the ſhare of any Planet, to be in a ratio
compounded of the denſity of the Sun's rays and the
denſity of the air upon the ſurface of the Planet, it is
eaſy to conceive, that theſe globes may be ſeverally
furniſhed with ſuch atmoſpheres, as may render them
comfortable habitations, whatever their diſtances from
the Sun may be.
That the heat of the Sun is actually
thus diſpenſed to the Planets neceſſarily follows from
the experiments and obſervations contained in the
foregoing pages, upon the ſuppoſition that they are
ſurrounded with aerial atmoſpheres, ſuited to their ſe-
veral diſtances from the Sun ; that they have ſuch at-
moſpheres is already proved, and the Creator, has,
doubtleſs

( 68
doubtleſs wiſely proportioned their reſpective quanti-
ties and denſities according to thoſe diſtances.
The tails of Comets are nothing more than expan-
fions of their atmoſpheres, whoſe lengths depend upon
their nearneſs to the Sun (as before obſerved) and de-
creaſe as they recede from the Sun, becoming inviſible
to us (generally) before their heads diſappear : There-
fore we have the higheſt reaſon to conclude that when
theſe globes are in the moſt remote parts of their or-
birs, or at their aphelia, their tails wholly ſubfide, and
their atmoſpheres reſume ſpherical forms, like thoſe of
the Earth and of the other Planets, ſurrounding their
nuclei at equal altitudes in every part ; the Sun's at-
moſphere being too remote to have any ſenſible effect
upon them by its repellency. The air muft of con-
ſequence be prodigiouſly denſe near the furfaces of
their globes, being compreſſed by the weight of ſuch a
vaſt incumbent fluid ; whereby the Sun's rays, though
weaker, or leſs denſe than with us in the ratio of the
ſquares of the diſtances, may, upon our principles,
be rendered as active with them, and as productive of
duch degrees of heat as are neceſſary for the purpoſes
of animal and vegetable life, as with us, or
Planet of the Syſtem.
But, (as was obſerved by Doctor Williamſon,)
ſhould theſe atmoſpheres continue of the fame denſity,
through all parts of their orbits, the degrees of heat
which their inhabitants muſt undergo at their peri-
helia would be unſufferable : To prevent which, the
great Author of Nature has made ſufficient proviſion ;
for as they approach the Sun, they are by the repulſion of
his atmoſphere (or ſome other cauſe equivalent to fuch
fuppofed repulſion)gradually eaſed of that incumbrance,
the
any other

( 69 )
the cometic atmoſphere being gradually rarefied and
driven behind its body through vaft ſpaces of the
Heavens ; what remains from time to time being
more and more rarefied by the increaſing action of
the Sun's rays upon it, and repelled as rarefied; till
at length, if they come near enough to the Sun, the
inhabitants may have little more than pure æther to
breathe in. Thus the Comet of 1769, (than which
but few have gone nearer to the Sun) before it ar-
rived at its perihelion, although it projected a moſt
aſtoniſhing tail, yet the remaining atmoſphere was
denſe enough to hide the nucleus it ſurrounded; the
Comet, when viewed through the beſt teleſcopes, pre-
ſenting only a dubiouſly defined luminous appearance:
But when it made its re-appearance in the evening
about the latter end of Oztober, its atmoſphere had
undergone ſo
great a degree of rarefaction in paſſing
its perihelion, that it was ſufficiently pellucid to dif-
cover the nucleus, which appeared plainly and diſ-
tinctly through it.
MAY we noc then conclude, even with certainty,
that as a Comet is perpetually varying its diſtance
from the Sun, ſo the denſity of its atmoſphere is con-
tinually changing through the various ſtages of its
revolution ; and thence, that its inhabitants may at
all times enjoy as much benefit, and receive as little
injury, from the Sun's rays, as the inhabitants of any
other Planet in the ſolar Syſtem ?
As the primary Planets revolve in orbits nearly cir-
cular, they have no occaſion for ſuch vaſt atmoſpheres
as are neceſſary for the Comets in the remote regions
of the Heavens to which they retire ; but are ſur-
rounded

( 70 )
rounded with ſuch, as infinite wiſdom faw beſt ſuited
to their ſeveral diſtances from the Sun; ſuch as might
have no redundancies to be thrown off in tails at one
time more than another ; the nearly equal diſtances
of each from the Sun, in the ſeveral parts of its orbit,
requiring nearly an equal denſity of atmoſphere at all
times.
From the premiſes we may conclude, that the at-
moſpheres of the inferior Planets are ſmaller, and
thoſe of the ſuperior ones larger in ſome proportion, than
that of the Earth, in order that their denſities near
their reſpective fuperficies, may be fo proportioned to
their ſeveral diſtances from the Sun, as that they may
equally ſhare the benefit of his rays. For want of
proper aſtronomical obſervations, to determine this
point with regard to the other Planets, we can pro-
nounce with certainty only concerning Mars : As that
Planet is further diſtant from the Sun than the Earth,
his atmoſphere, for the reaſons above aſſigned, ought
to be larger than our own ; accordingly it appears
(from the obſervations referred to in Part I. Note *
Page 12) that the height of his atmoſphere above his
furface is at leaſt equal to two thirds of his diameter;
which is much greater than that of the Earth, though
it falls vaſtly ſhort of thoſe of the Comets. Obſerva-
tions of future occultations of fixed Stars by Jupiter,
Saturn and the other Planets, made with better inftru-
ments, may poſſibly determine this point with regard
to them alſo.
Several objections may be raiſed againſt the prin-
ciples advanced in this Effay, to all which we hope to
give ſatisfactory anſwers. As
1. IT

( 71 )
1. It may be objected, that, as the light of the Sun
is confeſſedly proportional to the denſity of his rays,
that is inverſely as the ſquares of the diſtances from
the Sun, the inconveniencies ariſing therefrom to the
inhabitants of the Comets, at their perihelia and aphelia,
might be nearly as great as thoſe which would ariſe
from the Sun's heat were it diſtributed in the ſame pro-
portion ; or at leaſt, if it did not render the Comets
uninhabitable, would make the conditions of their in-
habitants, at times very uncomfortable. For example,
were the light of the Sun adapted to their various
pur-
poſes, at their mean diſtances, at their aphelia, they
might not enjoy a fufficiency; and on the contrary
at their peribelia, the ſplendor would be unſufferable.
This objection, it is preſumed will vaniſh upon a
careful attention to the ſtructure of the eyes of terreſtrial
animals, whoſe pupils contract or dilate involuntarily,
according as the denſity of the rays which paſs thro'them
and fall upon their retinas is greater or leſs, whereby
more or fewer of thoſe rays are admitted, as may be re-
quiſite for diftinct, inoffenſive viſion : Thus moft
per-
fons can ſee to read by candle light near as well as by
day light, whereas the quantities of light reflected
from objects in the two caſes ſcarely bear any pro-
portion one to the other. But the aperture of the
pu-
pil is much greater in the former than in the latter,
and more rays in proportion are conſequently admitted.
Moreover, there are ſome animals with us which re-
tire to their holes and caves at the approach of day,
whoſe purpoſes are as well anſwered by the glimmer-
ing

( 72 )
ing light of the Stars, as thoſe of others are by the
preſence of the Sun ; there are yet others which can
behold the Sun in his meridian ſplendor, without of
tence. Now if we only ſuppoſe that the inhabitants
of Comers in general have, in the original formation
of their optic organs, the power of contracting and
dilating their pupils, according to the ſtrength or
weakneſs of the light which is tranſmitted through
them, we may eaſily conceive, that the rays of the
Sun might be no more offenſive to them at one time
than at another ; for at their aphelia their pupils might
be dilated to their utmoſt extent ; on the other hand,
at their perihelia, they might be contracted to phyſical
points, if the ſplendor of the Sun ſo required, whereby
a proportionally ſmaller quantity would be admitted.
The light of the Sun which the Cometarians enjoy at
their aphelia is indeed much greater than we ſhould be
apt to imagine ; for let us conſider the Comet of 1680,
whoſe period is the longeſt and its aphelion diſtance
the greateſt of any one known, being (according to
Dr. Halley) to the mean diſtance of the Earth from
the Sun nearly as 138 to r; but from the reaſoning
of Doctor Smith, in his optics it appears, that the pro-
portion of our day light to moon light with a full
moon, is nearly as 90 000 to 1 ;* and the light of the
Sun upon the Comet at its laſt mentioned ſtage is to
his light with us but as about i to 19000, therefore
if we divide 90 000 by 19 000 we ſhall find that Moon-
light with us is to Sun-light upon the Comet, nearly
as 1 to 41, and conſequently that the light of the Sun
* Smith's Opticks, Vol. I. Page 29.
enjoyed

( 73 )
enjoyed by the inhabitants of that Comet at its aphe-
lion is nearly five times as great as the light of our full
Moon. But it is ſtill much greater upon account of
the largeneſs and denſity of the atmoſphere ; for it
is certain, that our day light, which is equally diffu-
fed upon
all terreſtrial objects, and renders them vi-
fible ; depends upon the reflection of the Sun's
rays, from the atmoſphere, together with the hete-
rogeneous corpuſcles floating in it ; without which all
fuch objects would be as obſcure as at midnight, even
with the Sun ſhining in full ſplendor above the hori-
zon ; excepting thoſe upon which the direct rays of
the Sun might fall, or ſuch as might be faintly illu-
minated by the reflection of thoſe
rays
from neigh-
bouring objects : The Heavens would appear perfectly
black, and the ſmalleſt Stars would appear, at noon-
day, which is prevented, only, by the illumination of
the atmoſphere. The beautiful azure, which we ob-
ſerve in the Sky, after the atmoſphere is purged of its
vapors, by a ſtorm or thunder-guſt, ariſes from the
appearance of this black Sky, through the air, which
is now become more tranſparent, than when charged
with a heterogeneous collection of opaque corpuſcles.
The atmoſpheres of Comets, being much larger and
denſer than that of the Earth, reflect a much greater
proportion of the Sun's rays; their hemiſpheres next
the Sun muſt therefore be more illuminated, and their
day light increaſed, in the ſame proportion ; although
the light ariſing from the direct rays of the Sun would
be conſiderably weakened thereby.
L
THIS

( 74 )
This reaſoning may be illuſtrated by calling to
mind the effects of two great eclipſes of the Sun, one
of which happened on the 5th or 6th day of Auguſt,
1766 ; the other on the 19th day of January, 1768 ;
which effects, moſt perſons among us, whoſe attention
was turned that way, may recollect ; during the for-
mer the air was very clear, and the ſky'cloathed in a
fine blue, excepting, here and there, where a few ſum-
mer clouds were ſcattered : In the midſt of this eclipſe
the air was darkened to ſuch a degree, that, although
the Sun ſhone unclouded, a ſickly gloom ſeemed to
ſpread over the face of nature. In the latter, (though
a much greater eclipſe) the air was full of vapors, the
reflection of the Sun's rays from which was ſo copious,
as to render it offenſive to the eyes to look at the
Heavens, before the eclipſe began,and in the middle of
it, the darkneſs occaſioned thereby would ſcarcely
have been noticed, had not the eclipſe been known
beforehand.
The inhabitants of Comets enjoy another advana
tage from their great atmoſpheres, which is peculiar
to them alone ; for, in the hemiſphere turned from
the Sun, they can have no dark nights like thoſe of
the Planets ; but, in conſequence of the reflection of
the Sun's rays from thoſe atmoſpheres, muſt be fa-
voured with perpetual twilight if not day light ; for a,
cometic atmoſphere is enlightened by the Sun by night,
as well as by day, excepting only a column, which is
nearly cylindrical at the aphelion, whoſe bafe is a great
circle of the Comet, and whoſe alcitude is equal to the
height of the atmoſphere ; which, (conſidering the
great

( 75 )
great extent of the latter,) bears but a ſmall propor-
tion to the whole atmoſpheric hemiſphere; this column
includes that part of the atmoſphere which is eclipſed
by the ſhadow of the globe itſelf, and, if the diameter
of the atmoſphere is equal to ten diameters of the
globe*, does not contain sth part of the whole viſi-
ſible hemiſphere; and is ſtill leſs upon account of the
refraction of the Sun's rays, which ſhortens and con-
tracts the cone of the ſhadow. Whence it is probable
that the darkeſt nights of the Cometarians, at their
aphelia, are much lighter than our brighteſt moon-
light nights. But this is ſubmitted to the judgment
of the reader.
2. IT
may further be objected ; that if the atmoſ-
pheres of Comets undergo fuch amazing rarefactions
and condenſations, as they neceſſarily muſt, from the
alternate projections and retractions of the tails, it
is difficult to conceive that they can at all tin:3 be fit
for the reſpiration of their ſuppoſed inhabitants.
This objection might, perhaps, have remained un-
anſwerable, had it not been for the genius of that truly
great philofopher Doctor Edmund Halley ; who, if he
was not the original inventor of the diving beil, yet,
by a fagacity peculiar to himſelf, improved it to a de-
gree of perfection, which might, before, have rather
been wiſhed for, than expected.t. In this bell perſons
may be let down with ſafety to the bottom of the ſea ;
but the included air differs in denſity at every depth
below the ſurface of the water. At the depth of
* See Page 14
+ See Jones's Abridgment of Philos. Trans. Vol. IV. Page 188, and onward.
thirty-two

( 76 )
thirty-two or thirty-three feet, as the air may occa-
fionally change, or leſs, in proportion as falt water is
heavier than freſh, the denſity of the air within the
bell is double the denſity of the external air; at double
that depth the denfity is triple ; at three times, four-
fold, and fo on.
Now if the bell be let down without
proper precaution, the too ſudden condenfation of the
air within, would give the adventurers extreme pain,
as they ſometimes found by experience ; and ſhould
the bell fink ſuddenly to the bottom of the fea, the
conſequence might be faral to them ; for the fame
reaſon that a ſquare caſe-bottle, filled only with com-
mon air above the ſurface of the water, and corked
tight, if it were let down with the divers in the bell,
would from the increafing preſſure of the condenfing
air without it, be cruſhed inwards, and broken in
pieces, in the fame manner as if the air in the bottle
had been exhauſted by an Air-Pump above the fur-
face of the water ; which effect would be prevented
by the ſmalleſt hole in the cork, provided the bell
were let down leilurely, ſo that the air as it condenſed
without, might gradually inſinuate itſelf through the
hole into the bottle : On the contrary were the bottle
corked at the bottom of the fea, and the bell drawn
up from the depth of nine or ten fathoms; before the
bell could arrive at the ſurface of the water, the bot-
tle would burſt outwards, from the expanſive force of
the condenſed air within it ; which might alſo be
prevented by a ſimilar precaution. And it is doubt-
leſs from the latter cauſe that ſome perſons who have
afcended to the tops of high mountains, have been
ſeized with reachings, vomitings and other inconve-
niences

( 77 )
niences related by travellers ; the external air of the
atmoſphere at fuch heights being too rare to counter-
act by its preſſure, the expanſive force of the denſer
air which is interſperſed throughout the various veſſels
and organs of animal bodies.
66
But Doctor Halley teſtifies from his own experience,
that “ if the diving-bell be lec down (or drawn up)
gradually, about twelve feet at a time with an in-
“ terval of but a few minutes between, no inconve-
“ nience would follow”; as the ſeveral organs of the
body would by degrees be inured to the denſity of the
air, as it increaſed or decreaſed at the ſeveral depths.
The Doctor tells us, that he himſelf was hours toge-
ther at the bottom of the ſea in nine or ten fathoms
of water; and felt himſelf as well as if he had been
all the time on board the ſhip : But the denſity of the
air he then breathed, muſt have been more than three
times as great as that of the air above the ſurface of
the water : In other words, he then breathed in air,
compreſſed by the weight of between three and four
of our atmoſpheres inſtead of one, and of ten or a dozen
ſuch as Don de Ulloa breathed in, when upon the tops
of the mountains of Quito, without
any
inconvenience.
Therefore, if time ſufficient be allowed for the air, in-
cluded in the ſeveral veſſels of the human body, gra-
dually to contract, expand or otherwiſe acconimodate
itſelf, to the increafing or decreaſing denſity of the
exterior air ; no bad conſequences or even inconve-
niencies are to be apprehended, although the difference
of denſity be exceeding great.
But by the gradualand regular approach of any Comet
.Co, or receſs from its perihelion, (at and near which its
velocity

( 78
velocity is greateſt and the conſequent changes in its
atmoſphere are moſt ſudden of all) the increaſe or
decreaſe of its atmoſphere, is much more regular,
uniform, and inſenſible to its inhabitants, than any
increaſe or decreaſe of the denſity of the air can be in
the diving, bell by Doctor Halley's method ; for by
the latter, a degree of rarefaction, or condenſation, is
effected in a few minutes, which might not take place
in the cometary atmoſpheres under fome days.se
WOO 103
Perhaps enough has been already ſaid to remove
every material objection; but if any difficulties yet
remain, in the mind of the reader, on account of the
vaſt changes which the ſeveral climates in each, and
the atmoſpheres of all Comets muſt neceſſarily under-
go, in the various parts of their orbits ; the following
additional obſervations are ſubmitted to his conſide-
ration ; which may tend to leffen thoſe difficulties, if
they do not wholly remove them, viz.
As a Comet approaches its perihelion, that hemiſ-
phere of its atmoſphere which is next to the Sun, be-
ing mpre immediately expoſed to his rays, will feel the
effects of his neighbourhood ſooner than the oppoſite
hemiſphere, and conſequently will be warmed, rarefied,
and thrown off behind the Comet by the repulſion of the
Sun's atmoſphere, ſooner than the other ; the colder and
denſer
parts
of the fluid will of courſe continually flow
in from the other ſide of the Comet to ſupply its place,
in order to preſerve, as near as may be, an equilibrium ;
in conſequence of which there will be a conſtant fuc-
ceſſion of the cooler air from thence ; whereby the
inhabitants on the hemiſphere next the Sun may be
botaniuga continually

( 79 )
continually refreſhed with gales of wind during
that vicinity, which would increaſe till the Comet
arrived at its perihelion, when their velocity would be
greateſt of all; but even then they would not (from
this cauſe) blow in ſudden violent guſts like our
hurricanes, but ſteadily, unleſs diſturbed by cauſes
from within the Coniet's atmoſphere ; beſides,
as the velocity of the current increaſed, the den-
fity of the Auid would leſſen from the increaſing
rarefaction, whereby its momentum might continue
nearly the ſame ; for this momentum would be in a
ratio compounded of the velocity of the fluid and its
denſity together ; and as the violence of our high
winds, and their conſequent effects depend, not upon
the velocity, merely, but upon the momentum of the
current, this briſk circulation of the cometic air may,
(however great we ſuppoſe its velocity) be rather grate-
ful than injurious to the Cometarians : And how unfit
foever the air in ſuch a rarefied ſtate might be for
their uſe, if ftagnant, yet, when thus put in motion, it
may be rendered ſufficiently active to anſwer all the
purpoſes of reſpiration. This reaſoning is confirmed
by daily experience : For it is not an uncommon
thing for people of tender frames to faint in a
cloſe hot and rarefied air ; and as the fan is generally
near at hand, it is as common for the by-ſtanders to
apply it to their faces, which, by giving a briſk
motion to the air, without any alteration of its
denſity, generally revives them, in a ſhort time,
even when no other remedy is at hand.-- This
briſk motion of the air would alſo remove or pre-
vent the diſagreeable ſenſations of heat which the
conietary

( 80 )
cometary inhabitants might otherwiſe ſuffer from an
expoſure to the Sun's rays at their perihelia : For, if a
perſon fit with his face uncovered before the ſcorching
blaze of a common fire, the motion of the air excited
by a common fan, even without hiding the blaze from
the face, is ſufficient, not only to make the ſituation
comfortable, but to change the painful ſenſation to an
agreeable coolneſs : As any one will find upon trial.
If we ſuppoſe that every Comet has a diurnal ro.
tation round an axis of its own, the inhabitants
may enjoy grateful viciffitudes, from the alternate ab.
fence and preſence of the Sun ; and if we further
ſuppoſe this diurnal motion to be performed contrary
to its apparent heliocentric motion ; the returns of day
and night would be quickened as it approached the
Sun, from the increaſe of its angular velocity round
that globe, whereby the preſence of the Sun, in that
neighbourhood, would be of ſhorter duration, upon
any one part of the Comet, and his heat might be
rendered ftill leſs irkſome to the inhabitants, on
that account. It is true, no diſcovery has been made
of
any
ſuch diurnal motion, but as all the primary
Planets, ſo far as our obfervations can reach, are dif-
covered to have ſuch motions, we may well be allowed
to fuppoſe that the Comets are not without them; efpe-
cially now we are endeavouring to prove their habi-
tability, to which this motion is perhaps as neceſſary
as to the habitability of the primary Planets. Theſe
rotations have been already diſcovered and determined
in Venus, the Earth, Mars and Jupiter. Saturn, though
a vaſt globe in itſelf, is ſo remote, and Mercury is ſo
near the Sun, and ſo very ſmall, that this motion has
never

( 81 )
never been diſcovered in either, by our beſt inſtru.
ments, but is juftly inferred by analogy ; which me-
thod of reaſoning will equally extend to the Comets
of the Syſtem. The diurnal motions of the Planets
indeed are performed nearly in the ſame directions
with their annual, both motions in all, as far as they
have been diſcovered, being direct, or from Weſt to
Eaſt, whereas the diurnal motions of Comets, accord-
ing to the foregoing fuppofition, are performed con-
trary to this rule : But this is no objection againſt
the hypotheſis ; for Planets and Comets differ as
widely, in almoſt every other particular ; the annual
motions of the former (as now obferved,) are all direct,
and are apparently confined within the limits of the
zodiac, the latter move indifferently in all directions
through the Heavens ; the periodical revolutions of
the former are made in orbits nearly circular, thoſe of
the latter are prodigiouſly excentric, and nearly para-
bolical ; all which ſeem wifely to be ordered, that a
multitude of Worlds may exiſt at the ſame time, and
be enlightened, warmed, and rendered prolific, by the
Fays of the fame Sun, without interfering in their mo-
tions, or diſturbing the harmony of the Syſtem.
To illuſtrate the reaſoning in pages 78, 79, Fig. 5
is added ; in which let S repreſent the Sun ; to which
Comets in general, though perhaps equal in magni-
tude to our Earth, are, without a figure, but as drops
of the bucket.* Let C repreſent a Comes with its at-
* In the figure the ſpeck at X and the dots tound it, upon the ſurface of the
Sun S, will give a pretty juſt idea of the comparative magnitudes of the Sun
and Comet, and of their atmoſpheres ; the dots at X repreſenting the Comet's
atmoſphere, A A A, &c. the atmoſphere of the Sun,
M
moſphere

( 82 )
moſphere and tail, the dark curve line ckdgh on one
ſide, and cia ef on the other, may ſerve to give an
idea of the motion of a parcel of the cometary air
from its more condenſed ſtate behind the Comet at c,
through its various ſtages of rarefaction and repulſion;
part of the atmoſphere next the Sun, viz. that
in or near the line Sb which connects the centers
of the Sun and the Comet, is rarefied, the denfer
air from behind at c muſt neceſſarily flow in to pre-
ferve as near as poſſible an equilibrium, and continue
ſo to do as long as the rarefaction continues to increale.
The air next the Sun being thus rarefied, that at o
would take a turn round the nucleus through k and i,
but before theſe ſeperate parcels came to d or a the
rarefaction would fo increaſe, that they would begin
to aſcend, and as they afcended, at a and d they would
repel each other ; they would ſtill keep riſing by their
increaſing rarefaction, through S b as through a fun-
nel, and increaſe in their mutual repellency as they re-
ceded from the center of the Comet, till at length at
I and m the repellency of the Sun's atmoſphere would
compel them to retire through g bor ef whence they
would proceed to the extremity of the tail, the remain-
ing parcels of air in the ſame cometic hemiſphere would
take a ſimilar courſe (as repreſented by the faint ſtrokes
in the figure) whether their diſtances from the Comet's
ſurface were greater or leſs, till at length the rarefaction
of the Comet's atmoſphere would become as great, as
the repulſive power of the Sun's atmoſphere could
effect, or the Comet's vicinity to the Sun, require.-
We ſhall offer one obfervation more, for the conſide-
ration of the reader before we cloſe the ſubject, viz.
as that
WHEN

( 83 )
When the air of our atmoſphere appears by the ther-
mometer to be extremely cold, it does not affect the ſenſes
fo diſagreeably, if the atmoſphere be in a calm ftagnant
ftate, as at other times, when the mercury is ten or even
twenty degrees higher, (and conſequently the weather
warmer) with a briſk wind, as has been frequently
obferved by thoſe who attend to their thermometers.
Now when a Comet is at its greateſt diſtance from the
Sun, its atmoſphere, being uniformly condenſed round
its globe, might ſettle into a dead calm, for
any
diſtur-
bances it could receive from without: For whatever
influences baneful or ſalutary, the heavenly bodies may
reciprocally communicate while in the neighbourhood
of each other, the Comets in their aphelia, are removed
to ſuch inconceivable diſtances from all the other
globes of the Syſtem, that their mutual effects, phy-
fically conſidered muſt vaniſh. And in ſo calm an
atmoſphere, of ſo great a denſity, illuminated by the
Sun's rays, the inhabitants of the Comets may, even
when moſt remote from the Sun, be as warm, or at
leaſt as comfortable as the inhabitants of the Earth,
or of any other Planet.
This ſubject cannot be better cloſed than in the
words of Doctor Williamſon, viz.*
“ One of the primary ideas we form of the
ſupreme Being is, that he is the ſource of life, intelli-
66
gence and happineſs, and delights to communicate
" them; the Earth we tread, the water we drink, and
very air in which we breathe, ſwarm with living
66 the
* See Tranſactions of the American Philoſophical Society of Philadelphia.---
Appendix page 30.
creatures,

7 84
“ creatures, all fitted to their ſeveral habitations,
“ Are we to ſuppoſe that this little globe is the only
“ animated part of the creation, while the Comets,
“ many of which are larger worlds, and run a nobler
courſe, are an idle chaos, formed for the fole
poſe of being frozen and burnt in turns ? We
cannot admit the thought; the Comets are doubt.
66 Jefs inhabited,"
66
pur
66
APPENDIX.

Fig. 1
N
e
f
A B
O
Я g
Fig 2
Fig 6
O
6
O
O
A
B
С
D
B
С
Fig. 3
А A
O
a
Fig. 4
Во
7
*
2
• X
Fig. 56
C
S


( 85 )
I
A P P E N D I X.
F we ſuppoſe all the ſolid globes of the ſolar Syſtem
to be annihilated, their atmoſpheres remaining :
the power of gravity, which had theretofore con-
denſed them round their reſpective orbs, ceaſing, they
would immediately expand themſelves quaquaverfum,
in conſequence of the mutual repellency of their
par-
ticles, till the whole ſpace in which the bodies of the
Syſtem had revolved, was equally filled with the fluid,
and when its denſity became equal in every part, the
whole would be at reſt.
So, vice verſa, if we ſuppoſe ſuch a fluid, to have
been the firſt material ſubſtance in the order of crea-
tion, equally diffuſed, in that rarefied ſtate, through-
out the mundane ſpace ; next after it, the ſeveral
maffes of the Sun, Planets and Comets, nearly about
the ſame time with each other : This fluid, though its
particles are in a ſtate of mutual repulſion, yet as they
are in the common ſtate of gravitation to the other
bodies of the Syſtem, would be attracted by the feve-
ral globes, every particle moving towards that globe,
which ſhould have thebalance of attraction inits favour.
Mercury's atmoſphere, by reaſon of its neighbour-
hood to the Sun, and the ſmallneſs of his globe would
be leaſt of all and very inconſiderable : For ſuppoſe
right line AB.
$-
drawn from the center of the Sun to the center of
Mercury, to be fo divided in D, that the part next
the Sun may be to the part next Mercury in the ſub-
duplicate ratio of the quantity of matter in the
Sun
A
D
a
B
-O

( 86 )
Sun to the quantity of matter in Mercury ; every
particle between D and B would deſcend to, and
be condenſed round the Sun, and thoſe only be-
tween D and A to Mercury ; and it appears by
computation that DB: DA:: 3693 : I nearly, and in
all oblique directions the particles, whoſe diſtances from
the Sun and Mercury were proportionably greater or
ſmaller, would deſcend to, and condenſe round one or
the other ; unleſs drawn aſide by ſome other Planet :
What is here ſaid of Mercury, is equally applicable to
the other Planets ; and as the attraction of the Sun is
vaſtly greater than the united atcractions of all the
Planets togerher ; fo, in every part of the Syſtem,
where the attractive power of the Sun might be greater
than that of any neighbouring Planet'; the fluid oc-
part
of the ſpace, would deſcend and join
the Sun's atmoſphere ; while the reſt would be con-
tinually condenſing round the Planets, till at length
the ſeveral globes would be accommodated with their
proper atmoſpheres, when the Heavens would be left
a perfect vacuum for the various bodies to revolve in,
without the leaſt reſiſtance.
If the Comets be ſuppoſed to have been created and
projected in their ſeveral orbits,ar their aphelia, or at
their greateſt diſtances from the Sun, it may be eaſy
upon this hypotheſis to account for their having at-
moſpheres ſo much exceeding thoſe of the Planets in
their dimenſions, for providence has ſo ordered it, that
the angles of the inclinations of their orbits to the
eclyptic and to each other are generally very great,
and their motions are directed to all parts of the
Heavens indiſcriminately, whereby their diſtances
from the Planets and from each other at their aphelia,
cupying that
ar

( 87 )
are great beyond human conception ; confequently
they were at liberty to ſhare amongſt themſelves, with-
out any moleftation from the Planets, all that part of
the fluid, which filled the vaſt ſpaces of the Syſtem,
without the planetary regions ; therefore if the hypo-
theſis be granted, they muſt neceſſarily have ſuch at-
moſpheres, as, in fact, we find they have, and which,
in their deſcent through the planetary ſpheres, are, by
the (ſuppoſed) repulſion of the Sun's atmoſphere,
driven to ſuch aſtoniſhing diſtances behind them, as
occaſion may require. Thoſe whoſe aphelion dif-
tances were greateſt, being more ſolitary, would con-
denfe round them the greateſt atmoſpheres, and ſuch,
their greater diſtances from the Sun would require,
upon the foregoing principles, to make them comfor-
table habitations. As the lengths of their tails would
probably, at equal diſtances from the Sun, be propor-
tional to the quantities of this repellent matter contain-
ed in their atmoſpheres reſpectively, it may not be
impoſſible to form a rational conjecture of their real
aphelion diſtances, by obſerving the apparent lengths
of their tails, when at equal diſtances from the Sun, in
their deſcent, and thence computing their real lengths,
d comparing thoſe whoſe aphelion diſtances are
unknown with thoſe which are already determined ;
and as nearly as we can by this method come at their
greateſt diſtances from the Sun, ſo nearly may we
(by comparing their computed trajectories, with thoſe
diſtances,) determine their mean diſtances and periodic
revolutions. But this is humbly ſubmitted to better
ages, and is deſigned only as a hint for future inquiries.
F
I N I S.

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