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Thus most persons 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 scarcely
bear any proportion one to the other. But the aper-
ture of the pupil ic; much greater in the former than
in the latter, and more rays in proportion are conse-
quently admitted. Moreover, there are some animals
with us which retire to their holes and caves at the ap-
proach of day, whose purposes are as well answered
by the glimmering 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 offence. Now if we only suppose that the
inhabitants of Comets in general have, in the original
formation of their optic organs, the power of contract-
ing and dilating their pupils, according to the strength
or weakness of the light which is transmitted through



142 ON COMETS.

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 apheiia their pupils might be
dilated to rheir 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 apheiia is indeed much greater than we should
be apt to imagine ; for let us consider the Comet of
1680, whose period is tlie longest and its aphelion dis-
tance the greatest of any one known, being (according
to Dr. Hal/ ey) to the mean distance of the earth from
the sun nearly as 138 to 1 ; but from the reasoning
of Dr. Sinith, in his optics it appears, that the propor-
tion 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 1 to 19,000, 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 4|-, and consequently that the light of the sun en«
joyed by the inhabitants of that Comet at its aphelion
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 diffus-
ed upon all terrestrial objects, and renders them visi-
ble ; depends upon the reflection of tl^e sun's rays,

• Smith's Opticks, Vol. I, Page 29.



ON COMETS. 143

from the atmosphere, together with the heterogene-
ous corpuscles floating- in it ; withoui v: ich all such
objects would be as obscure as at midnight, even with
the sun shining in fuil splendor above the horizon ;
excepting those upon which the direct rays of the sun
might fail, or such as mighi be faintly iiiuminated by
the reflection of those rays from ricighbouring ob-
jects : 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 observe 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 hetero-
geneous 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.

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 atten-
tion was turned that way, may recollect; during the
former the air w^as very clear, and the sky cloathed



144 ON COMETS.

in a fine blue, excepting, here and there, where a few
summer 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 lat-
ter, (though a much greater eclipse) the air was full
of vapors, the reflection of the sun's rays from v/hich
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 advantage
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 reflections of the
sun's rays from those atmospheres, must be favoured
with perpetual twilight if not day light; foracometic
atmosphere is enlightened by the sun by night as well
as by day, excepting only a column, which ^^ nearly
cylindrical at the aphelion, whose base is a great
circle of the Comet, and whose altitude is equal to
the height of the atmosphere ; which, (considering the
great extent of the latter,) bears but a small propor-
tion to the whole atmospheric hemisphere ; this co-
lumn 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



i



©ff COMETS. 145

of the globe,* does not contain -^gth part of the whole
visible hemisphere ; and is still less upon account of
the refraction of the sun's rays, which shortens and
contracts the cone of the shadow. Whence it is pro-
bable 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 judg-
ment of the reader,

2. It may further be objected ; that if the atmos-
pheres of Comets undergo such amazing rarefactions
and condensations, as they necessarily must, from the
alternate projections and retractions of iheir tails, it
is difficult to conceive that they can at all times 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 philosopher Doctor Edmund Halley ; who, if he
was not the original inventor of the diving' bell, yet^ by
a sagacity peculiar to himself, improved it to a degree
of perfection, which might, before, have rather been
wished for, than expected. f In this beil persons may
be let down with safety to the bottom of the sea; but
the included air differs in density at every depth be-
low the surface of the watef . At the depth of thirty-
two or thirty-three feet, as the air may occasionally
change, or less, in proportion as salt water is heavier
than fresh, the density of the air within the bell is

* See Page 79.

f See Jones's Abridgment of Phllos. Trans.. Vol. IV. P|.g(?
188, and onward.

13 -



06 OS COMETS.

double the density of the external air ; at double tliat^
depth the density is triple ; at three times, fourfold,
and so on. Now if the bell be let down without pro-
per precaution, the too sudden condensation of the
air within, would give the adventurers extreme pain,
as they sometimes found by experience; and should
the bell sink suddenly to the bottom of the sea, the
consequence might be fatal to them ; for the same
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 increasing pressure of the condens-
ing air without it, be crushed inwards, and broken in
pieces, in the same manner as if the air in the bottle
had been exhausted by an air-pump above the surface
of the water ; which effect would be prevented by the
smallest hole in the cork, provided the bell were let
down leisurely, so that the air as it condensed with-
out, might gradually insinuate itself through the hole
into the bottle : On the contrary were the bottle
corked at the bottom of the sea, 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 pre-
vented by a similar precaution. And it is doubtless
from the latter cause that some persons who have as-
cended to the tops of high mountains, have been seiz-
ed with Teachings, vomitings and other inconvenien-
ces related by travellers ; the external air of the at-



OS COMETS. 147

mosphere at such heights being too rare to counteract
by its pressure, the expansive force of the denser air
which is interspersed throughout the various vessels
and organs of animal bodies.

But Doctor Halley testifies from his own experience,
that " if the diving-bell be let down (or drawn up)
" gradually, about twelve feet at a time with an inter-
" val of but a few minutes between, no inconvenience
" 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
»ir 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 era
dozen such as Dq7i de Ulha breathed in, when upon
the tops of the mountains of QuitOy without any in-
convenience. Therefore, if time sufficient 6e allow-
ed for the air, included in the several vessels of the^
human body, gradually to contract, expand or other-
wise accommodate itself, to the increasing or de-
creasing density of the exterior air ; no bad conse-
quences or even inconveniences are to be appre-
hended, although the difference of density be ex-
ceeding great.



148 ON COMETS.

But by the gradual and regular approach of any
Gomet to, or recess from its perihelion, (at and near
■which its velocity is greatest and the consequent
changes in its atmosphere are most sudden of all)
the increase or decrease of the density of its atmos-
phere, 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 rare-
faction, or condensation, is effected in a few minutes,
which might not take place in the cometary atmos-
pheres under some days.

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 un-
dergo, in the various parts of their orbits ; the follow-
ing additional observations are submitted to his con-
sideration; which may tend to lessen those difficul'-
ties, 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 more immediately exposed to his rays, will feel
the effects of his neighbourhood sooner than the op-
posite hemisphere, and consequently will be warmed,
rarefied, and thrown off behind the Comet by the re-
pulsion 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



ON COltfETS. 149

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 succession of the cooler
air from thence ; whereby the inhabitants on the he-
misphere next the sun may be 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 Comet's atmos-*
phere ; besides, as the velocity of the current in-
creased, the density of the fluid 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 grateful than injurious to the Coinetarians : and
how unfit soever the air in such a rarefied state might
be for their use, if stagnant, yet, when thus put in
motion, it may be rendered sufficiently active to an-
swer all the purposes of respiration. This reasoning
is confirmed by daily experience : for it is not an un-
common thing for people of tender frames to faint in
a close hot and rarefied air ; and as the fan is gene-
rally near at hand, it is as common for the bye-stanr



150 ©N COMtTS.

tiers 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
riTotion of the air would also remove or prevent the
disagreeable sensations of heat which the cometary
inhabitants might otherwise suffer from an exposure
to the sun's rays at their perihelia: For, if a person
sit 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 com-
fortable, 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 rota-
tion round an axis of its own, the inhabitants may en-
joy grateful vicissitudes, from the alternate absence
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 by day, in
that neighbourhood, would be of shorter duration,
upon any one part of the Comet than when it is in the
remoter parts of its orbit ; and his heat might be ren-
dered still 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 pla-
nets, so far as our observations can reach, are disco-



©N COMETS. 151

vered to have such motions, we may well be allowed
to suppose that the Comets are not without them ;
especially now we are endeavouring to prove their ha-
bitability, to which this motion is perhaps as necessary
as to the habitabiiity of the primary planets. These
rotations have been already discovered and determined
in Venus ^ the Earthy Mars and Jiifiiter. 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 been discovered in either, by our best in-
struments, but is justly inferred by analogy ; which
method 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 supposition, 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 observed,) 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 parabolical ; aH
which seem wisely to be ordered, that a multitude of
worlds may exist at the same time, and be enlighten-
ed, warmed, and rendered prolific, by the rays of the



15^ ON COTWEtS.

same sun, without interfering in their motions, or dis^
lurbing the harmony of the system.

To illustrate the reasoning in pages 148, 149, 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 Jigure^ but as drops
of the bucket,* Let C represent a Comet with its at-
mosphere and tail, the dark curve line c kd g h on
one side, and c i a e f 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 rafefaction and repul-
sion ; as that part of the atmosphere next the suUj
viz. that in or near the line S b which connects the
centres of the sun and the Comet, is rarefied, the
denser air from behind at c must necessarily flow in
to preserve as near as possible an equilibrium, and
continue so to do as long as the rarefaction continues
to increase. The air next the sun being thus rarefi-
ed, that at c would take a turn round the nucleus
through k and £, but before these separate parcels
came to d or a the rarefaction would so increase, that
they would begin to ascend, and as they ascended, at a
and d they would repel each other ; they would still
keep rising by their increasing rarefaction, through

• In the figure the speck at X and the dots round 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 atmos-
phere, AAA, &c. the atmosphere of the sun-.



ON eOMETS. 153

S b as through a funnel, and increase in their mutual
repellency as they receded from the centre of the
Comet, till at length at / and 771 the repellency of the
sun's atmosphere would compel them to retire through
g h or ef whence they would proceed to the extre-
mity of the tail, the remaining 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 observation more, for the consid-
eration of the reader before we close the subject, viz»
When the air of our atmosphere appears by the
thermometer to be extremely coId> it does not effect
the senses so disagreeably, if the atmosphere be in a
calm stagnant state, as at other times, when the mer-
cury is ten or even twenty degrees higher, (and con-
sequently the weather warmer) with a brisk wind, as
has been frequently observed 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 disturbances it could receive
from without : For whatever influences baneful or
salutary, the heavenly bodies may reciprocally com-
municate while in the neighbourhood of each other,
the Comets in their aphelia, are removed to such in-



154 ON COMETS.

conceivable distances from all the other pjlobes of the
system, that their mutual effects, physically consider-
ed 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 re-
mote from the sun, be as warm, or at least as com-
fortable 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, inteKigence
" and happiness, and delights to communicate them ;
" the earth we tread, the water we drink, and the very
"air in which we breathe, swarm with living crea-
" tures, 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 solfe purpose of

*^ being frozen and burnt in turns? We cannot

« admit the thought ; the Comets are doubtless in-
« habited."

* See Transactions of the American Philosophical Society"
of Philadelphia. — Appendix page 30.



APPENDIX.



If we suppose all the solid globes of the solar sys-
tem to be annihilated^ their atmospheres remaining ;
the power of gravity, which had theretofore condensed
them round their respective orbs, ceasing, they would
immediately expand themselves quaqua versum^ in con-
sequence of the mutual repellency of their particles,
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 diff'used, in that rarefied state, through-
out the mundane space ; next after it, the several
masses 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
several globes, every particle moving towards that



156 APPENDIX,

globe, which should have the balance of attraction in
its 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

AD B

a right line AB. x i ,

drawn from the centre of the sun to the centre of
Mercury, to be so divided in D, that the part next the
sun may be to the part next Mercury in the sub-du-
plicate ratio of the quantity of matter in the 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 between D and A to Mercury ;
and it appears by computation that DB : DA : : 3693 :
1 nearly, and in all oblique directions the particles,
whose distances from the sun and Mercury were pro-
portionably 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
attractions of all the planets together ; so, in every
part of the system, where the attractive power of the
sun might be greater than that of any neighbouring
planet ; the fluid occupying that part of the space,
would descend and join the sun's atmosphere ; while
the rest would be continually condensing round the
planets, till at length the several globes would be ac-
commodated with their proper atmospheres, when



APPENDIX. - 157

ttie heavens would be left a perfect vacuum for the
various bodies to revolve in, v^dihout the least resis-
tance.

If the Comets be supposed to have been created
and projected in their several orbits, at their aphelia,
or at their greatest distances from the sun, it may be
easy upon this hypothesis lo account for their having
atmospheres 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
ecliptic, and to each other, are generally very great,
and their motions are directed to all parts of the hea-
vens indisciiminately, whereby their distances from
the planets and from each other, at their aphelia, are


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Online LibraryJohn WinthropTwo lectures on comets → online text (page 11 of 14)