appears to require a very powerful comet to separate even that much
from the body of the sun; but if we reflect on the prodigious velocity
of comets in their perihelion, a velocity so much the greater as they
approach nearer the sun; if, besides, we pay attention to the density
and solidity of the matter of which they must be composed, to suffer,
without being destroyed, the inconceivable heat they endure; and
consider the bright and solid light which shines through their dark and
immense atmospheres, which surround, and must obscure them, it cannot be
doubted that the comets are composed of extremely solid and dense
matters, and that they contain a greater quantity of matter in a small
compass; that consequently a comet of no extraordinary bulk may have
sufficient weight and velocity to displace the sun, and give a
projectile motion to a quantity of matter, equal to the 650th part of
the mass of this luminary. This perfectly agrees with what is known
concerning the density of planets, which always decreases as their
distance from the sun is increased, they having less heat to support; so
that Saturn is less dense than Jupiter, and Jupiter much less than the
earth; therefore if the density of the planets be, as Newton asserts,
proportionable to the quantity of heat which they have to support,
Mercury will be seven times more dense than the earth, and twenty-eight
times denser than the sun; and the comet of 1680 would be 28,000 times
denser than the earth, or 112,000 times denser than the sun, and by
supposing it as large as the earth, it would contain nearly an equal
quantity of matter to the ninth part of the sun, or by giving it only
the 100th part of the size of the earth, its mass would still be equal
to the 900th part of the sun. From whence it is easy to conclude, that
such a body, though it would be but a small comet, might separate and
drive off from the sun a 900th or a 650th part, particularly if we
attend to the immense velocity with which comets move when they pass in
the vicinity of the sun.
Besides this, the conformity between the density of the matter of the
planets, that of the sun deserves some attention. It is well known,
that, both on and near the surface of the earth, there are some matters
14 or 1500 times denser than others. The densities of gold and air are
nearly in this relation. But the internal parts of the earth and planets
are composed of a more uniform matter, whose comparative density varies
much less; and the conformity in the density of the planets and that of
the sun is such, that of 650 parts which compose the whole of the matter
of the planets, there are more than 640 of the same density as the
matter of the sun, and only ten parts out of these 650 which are of a
greater density, for Saturn and Jupiter are nearly of the same density
as the sun, and the quantity of matter which these planets contain, is
at least 64 times greater than that of the four inferior planets, Mars,
the Earth, Venus, and Mercury. We must therefore admit, that the matter
of which the planets are generally composed is nearly the same as that
of the sun, and that consequently the one may have been separated from
the other.
But it may be said, if the comet, by falling obliquely on the sun, drove
off the matter which compose the planets, they, instead of describing
circles of which the sun is the centre, would, on the contrary, at each
revolution, have returned to the same point from whence they departed,
as every projectile would which might be thrown off with sufficient
force from the surface of the earth, to oblige it to turn perpetually:
for it is easy to demonstrate that such, in that instance, would be the
case, and therefore that the projection of the planets from the sun
cannot be attributed to the impulsion of a comet.
To this I reply, that the matter which composes the planets did not come
from the sun, in ready formed globes, but in the form of torrents, the
motion of the anterior parts of which were accelerated by that of the
posterior; and that the attraction of the anterior parts also
accelerated the motion of the posterior, and that this acceleration
produced by one or other of these causes, or perhaps by both, might be
so great as to change the original direction of the motion occasioned by
the impulse of the comet, from which cause a motion has resulted, such
as we at present observe in the planets; especially when it is
considered the sun is displaced from its station by the shock of the
comet. An example will render this more reasonable; let us suppose, that
from the top of a mountain a musket ball is discharged, and that the
strength of the powder was sufficient to send it beyond the
semi-diameter of the earth, it is certain that this ball would pass
round the earth, and at each revolution return to the spot from whence
it had been discharged: but, if instead of a musket-ball, we suppose a
rocket had been discharged, wherein the action of the fire being
durable, would greatly accelerate the motion of impulsion; this rocket,
or rather the cartouch which contained it, would not return to the same
place like the musket-ball, but would describe an orbit, whose perigee
would be much farther distant from the earth, as the force of
acceleration would be greater, and have changed the first direction.
Thus, provided there had been any acceleration in the motion of
impulsion communicated to the torrent of matter by the fall of the
comet, it is probable that the planets formed in this torrent, acquired
the motion which we know they have in the circles and ellipsis of which
the sun is the centre and focus.
The manner in which the great eruptions of volcanos are made, may afford
us an idea of this acceleration of motion. It has been remarked that
when Vesuvius begins to roar and eject the inflamed matter it contains,
the first cloud has but a small degree of velocity, but which is soon
accelerated by the impulse of the second; the second by the action of a
third, and so on, until the heavy mass of bitumen, sulphur, cinders,
melted metal, and huge stones, appear like massive clouds, and although
they succeed each other nearly in the same directions, yet they greatly
change that of the first, and drive it far beyond what it would have
reached of itself.
In answer to this objection, it may be further observed, that the sun
having been struck by the comet, received a degree of motion by the
impulse, which displaced it from its former situation; and that although
this motion of the sun is at present too little sensible for the notice
of astronomers, nevertheless it may still exist, and the sun describe a
curve round the centre of gravity of the whole system and if this is so,
as I presume it is, we see perfectly that the planets, instead of
returning near the sun at each revolution, will, on the contrary, have
described orbits, the points of the perihelion of which will be as far
distant from the sun, as it is itself from the place it originally
occupied.
It may also be said, that if this acceleration of motion is made in the
same direction, no change in the perihelion will be produced: but can it
be thought that in a torrent, the particles of which succeed each other,
there has been no change of direction; it is, on the contrary, very
probable that a considerable change did take place, sufficient to cause
the planets to move in the course they at present occupy.
It may be further urged, that if the sun had been displaced by the shock
of a comet, it would move uniformly, and that hence this motion being
common to the whole system, no alteration was necessary; but might not
the sun before the shock have had a motion round the centre of the
cometry system, to which primitive motion the stroke of the comet may
have added or diminished? and would not that fully account for the
actual motion of the planets?
If these suppositions are not admitted, may it not be presumed, that in
the stroke of the comet against the sun, there was an elastic force
which raised the torrent above the surface of the sun, instead of
directly impelling it? which alone would be sufficient to remove the
perihelion, and give the planets the motion they have retained. This
supposition is not without probability, for the matter of the sun may
possibly be very elastic, since light, the only part of it we are
acquainted with, seems, by its effects, to be perfectly so. I own that
I cannot say whether it is by the one or the other of these reasons,
that the direction of the first motion of the impulse of the planets has
changed, but they suffice to shew that such an alteration is not only
possible but even probable, and that is sufficient for my purpose.
But, without dwelling any longer on the objections which might be made,
I shall pursue the subject, and draw the fair conclusions on the proofs
which analogies might furnish in favour of my hypothesis: let us,
therefore, first see what might happen when these planets, and
particularly the earth, received their impulsive motion, and in what
state they were after having been separated from the sun. The comet
having, by a single stroke, communicated a projectile motion to a
quantity of matter equal to the 650th part of the sun's mass, the light
particles would of course separate from the dense, and form, by their
mutual attraction, globes of different densities: Saturn being composed
of the most gross and light parts, would be the most remote from the
sun: Jupiter being more dense than Saturn, would be less distant, and so
on. The larger and least solid planets are the most remote, because
they received an impulsive motion stronger than the smallest, and more
dense: for, the force of impulsion communicating itself according to the
surface, the same stroke would have moved the grosser and lighter parts
of the matter of the sun with more velocity than the smallest and more
weighty; a separation therefore will be made of the dense parts of
different degrees, so that the density of the sun being equal to 100,
that of Saturn will be equal to 67, that of Jupiter to 94-1/2, that of
Mars to 200, that of the Earth to 400, that of Venus to 800, and that of
Mercury to 2800. But the force of attraction not communicating like that
of impulsion, according to the surface, but acting on the contrary on
all parts of the mass, it will have checked the densest portions of
matter; and it is for this reason that the densest planets are the
nighest the sun, and turn round that planet with greater rapidity than
the less dense planets, which are also the most remote.
Jupiter and Saturn, which are the largest and principal planets of the
solar system, have retained the relation between their density and
impulsive motions, in the most exact proportions; the density of Saturn
is to that of Jupiter as 67 to 94-1/2 and their velocities are nearly
as 88-2/3 to 120-1/72, or as 67 to 90-11/16; it is seldom that pure
conjectures can draw such exact relations. It is true, that by following
this relation between the velocity and density of planets, the density
of the earth ought to be only as 206-7/18, and not 400, which is its
real density; from hence it may be conceived, that our globe was
formerly less dense than it is at present. With respect to the other
planets, Mars, Venus, and Mercury, as their densities are known only by
conjecture, we cannot be certain whether this circumstance will destroy
or confirm our hypothesis. The opinion of Newton is, that density is so
much the greater, as the heat to which the planet is exposed is the
stronger; and it is on this idea that we have just said that Mars is one
time less dense than the Earth, Venus one time, Mercury seven times, and
the comet in 1680, 28,000 times denser than the earth: but this
proportion between the density of the planets and the heat which they
sustain, seems not well founded, when we consider Saturn and Jupiter,
which are the principal objects; for, according to this relation between
the density and heat, the density of Saturn would be about 4-7/18, and
that of Jupiter as 14-17/22, instead of 67 and 94-1/2, a difference too
great to be admitted, and must destroy the principles upon which it was
founded. Thus, notwithstanding the confidence which the conjectures of
Newton merit, I can but think that the density of the planets has more
relation with their velocity than with the degree of heat to which they
are exposed. This is only a final cause, and the other a physical
relation, the preciseness of which is remarkable in Jupiter and Saturn:
it is nevertheless true, that the density of the earth, instead of being
206-7/8, is found to be 400, and that consequently the terrestrial globe
must be condensed in this ratio of 206-7/8 to 400.
But have not the condensations of the planets some relation with the
quantity of the heat of the sun which they sustain? If so, Saturn, which
is the most distant from that luminary, will have suffered little or no
condensation; and Jupiter will be condensed from 90-11/16 to 94-1/2. Now
the heat of the sun in Jupiter being to that of the sun upon the earth
as 14-17/22 are to 400, the condensations ought to be in the same
proportion. For instance, if Jupiter be condensed, as 90-11/16 to
94-1/2, and the earth had been placed in his orbit, it would have been
condensed from 206-7/8 to 215-990/1451, but the earth being nearer the
sun, and receiving a heat, whose relation to that which Jupiter receives
is from 400 to 14-17/22, the quantity of condensation it would have
experienced on the orbit of Jupiter by the proportion of 400 to
14-17/22, which gives nearly 234-1/3 for the quantity which the earth
would be condensed. Its density was 206-7/8, by adding the quantity of
its acquired condensation, we find 400-7/8 for its actual density, which
nearly approaches the real density 400, determined to be so by the
parallax of the moon. As to other planets, I do not here pretend to give
exact proportions, but only approximations, to point out that their
densities have a strong relation to their velocity in their respective
orbits.
The comet, therefore, by its oblique fall upon the surface of the sun,
having driven therefrom a quantity of matter equal to the 650th part of
its whole mass; this matter, which must be considered in a liquid state,
will at first have formed a torrent, the grosser and less dense parts of
which will have been driven the farthest, and the smaller and more
dense, having received only the like impulsion, will remain nearest its
source; the force of the sun's attraction would inevitably act upon all
the parts detached from him, and constrain them to circulate around his
body, and at the same time the mutual attraction of the particles of
matter would form themselves into globes at different distances from the
sun, the nearest of which necessarily moving with greater rapidity in
their orbits than those at a distance.
But another objection may be started, and it may be said, if the matter
which composes the planets had been separated from the sun, they, like
him, would have been burning and luminous bodies, not cold and opaque,
for nothing resembles a globe of fire less than a globe of earth and
water; and by comparison, the matter of the earth and planets is
perfectly different from that of the sun?
To this it may be answered, that in the separation the matter changed
its form, and the light or fire was extinguished by the stroke which
caused this motion of impulsion. Besides, may it not be supposed that if
the sun, or a burning star, moved with such velocity as the planet, that
the fire would soon be extinguished; and that is the reason why all
luminous stars are fixed, and that those stars which are called new, and
which have probably changed places, are frequently extinguished and
lost? This remark is somewhat confirmed by what has been observed in
comets; they must burn to the centre when they pass to their perihelium:
nevertheless they do not become luminous themselves, they only exhale
burning vapours, of which they leave a considerable part behind them in
their course.
I own, that in a medium where there is very little or no resistance,
fire may subsist and suffer a very great motion without being
extinguished: I also own, that what I have just said extends only to the
stars which totally disappear, and not to those which have periodical
returns, and appear and disappear alternately without changing place in
the heavens. The phenomena of these stars has been explained in a very
satisfactory manner by M. de Maupertuis, in his discourse on the figures
of the planets. But the stars which appear and afterwards disappear
entirely, must certainly have been extinguished, either by the velocity
of their motion, or some other cause. We have not a single example of
one luminous star revolving round another; and among the number of
planets which compose our system, and which move round the sun with
more or less rapidity, there is not one luminous of itself.
It may also be added, that fire cannot subsist so long in the small as
in large masses, and that the planets must have burnt for some time
after they were separated from the sun, but were at length extinguished
for want of combustible matter, as probably would be the sun itself, and
for the same reason; but in a length of time as far beyond that which
extinguished the planets, as it exceeds in quantity of matter. Be this
as it may, the matter of which the planets are formed being separated
from the sun, by the stroke of a comet, that appears a sufficient reason
for the extinction of their fires.
The earth and planets at the time of their quitting the sun, were in a
state of total liquid fire; in this state they remained only as long as
the violence of the heat which had produced it; and which heat
necessarily underwent a gradual decay: it was in this state of fluidity
that they took their circular forms, and that their regular motions
raised the parts of their equators, and lowered their poles. This
figure, which agrees so perfectly with the laws of hydrostatics, I am of
opinion with Leibnitz, necessarily supposes that the earth and planets
have been in a state of fluidity, caused by fire, and that the internal
part of the earth must be a vitrifiable matter, of which sand, granite,
&c. are the fragments and scoria.
It may, therefore, with some probability, be thought that the planets
appertained to the sun, that they were separated by a single stroke,
which gave to them a motion of impulsion, and that their position at
different distances from the sun proceeds only from their different
densities. It now only remains, to complete this theory, to explain the
diurnal motion of the planets, and the formation or the satellites; but
this, far from adding difficulties to my hypothesis, seems, on the
contrary, to confirm it.
For the diurnal motion, or rotation, depends solely on the obliquity of
the stroke, an oblique impulse therefore on the surface of a body will
necessarily give it a rotative motion; this motion will be equal and
always the same, if the body which receives it is homogeneous, and it
will be unequal if the body is composed of heterogeneous parts, or of
different densities; hence we may conclude that in all the planets the
matter is homogeneous, since their diurnal motions are equal, and
regularly performed in the same period of time. Another proof that the
separation of the dense or less dense parts were originally from the
sun.
But the obliquity of the stroke might be such, as to separate from the
body of the principal planet a small part of matter, which would of
course continue to move in the same direction; these parts would be
united, according to their densities, at different distances from the
planet, by the force of their mutual attraction, and at the same time
follow its course round the sun, by revolving about the body of the
planet, nearly in the plane of its orbit. It is plain, that those small
parts so separated are the satellites: thus the formation, position, and
direction of the motions of the satellites perfectly agree with our
theory; for they have all the same motion in concentrical circles round
their principal planet; their motion is in the same direction, and that
nearly in the plane of their orbits. All these effects, which are common
to them, and which depend on an impulsive force, can proceed only from
one common cause, which is, impulsive motion, communicated to them by
one and the same oblique stroke.
What we have just said on the cause of the motion and formation of the
satellites, will acquire more probability, if we consider all the
circumstances of the phenomena. The planets which turn the swiftest on
their axis, are those which have satellites. The earth turns quicker
than Mars in the relation of about 24 to 15; the earth has a satellite,
but Mars has none. Jupiter, whose rapidity round its axis is five to six
hundred times greater than that of the earth, has four satellites, and
there is a great appearance that Saturn, which has five, and a ring,
turns still more quickly than Jupiter.
It may even be conjectured with some foundation, that the ring of Saturn
is parallel to the equator of the planet, so that the plane of the
equator of the ring, and that of Saturn, are nearly the same; for by
supposing, according to the preceding theory, that the obliquity of the
stroke by which Saturn has been set in motion was very great, the
velocity around the axis will, at first, have been in proportion as the
centrifugal force exceeds that of gravity, and there will be detached
from its equator and neighbouring parts, a considerable quantity of
matter, which will necessarily have taken the figure of a ring, whose
plane must be nearly the same as that of the equator of the planet; and
this quantity of matter having been detached from the vicinity of the
equator of Saturn, must have lowered the equator of that planet, which
causes that, notwithstanding its rapidity, the diameters of Saturn
cannot be so unequal as those of Jupiter, which differ from each other
more than an eleventh part.
However great the probability of what I have advanced on the formation
of the planets and their satellites may appear to me, yet, every man has
his particular measurement, to estimate probabilities of this nature;
and as this measurement depends on the strength of the understanding to
combine more or less distant relations, I do not pretend to convince the
incredulous. I have not only thought it my duty to offer these ideas,
because they appear to me reasonable, and calculated to clear up a
subject, on which, however important, nothing has hitherto been written,
but because the impulsive motion in the planets enter at least as one
half of the composition of the universe, which gravity alone cannot
unfold. I shall only add the following questions to those who are
inclined to deny the possibility of my system.
1. Is it not natural to imagine, that a body in motion has received that
motion by the stroke of another body?
2. Is it not very probable, that when many bodies move in the same
direction, that they have received this direction by one single stroke,
or by many strokes directed in the same manner?
3. Is it not more probable that when many bodies have the same direction
in their motion, and are placed in the same plane, that they received
this direction and this position by one and the same stroke, rather than
by a number?
4. At the time a body is put in motion by the force of impulsion, is it
not probable that it receives it obliquely, and, consequently, is
obliged to turn on its axis so much the quicker, as the obliquity of the
stroke will have been greater? If these questions should not appear
unreasonable, the theory, of which we have presented the outlines, will
cease to appear an absurdity.