examine the figure of the earth, on which so many researches and such
great observations have been made. The earth being, as it appears by the
equality of its diurnal motion and the constancy of the inclination of
its axis, composed of homogeneous parts, which attract each other in
proportion to their quantity of matter, it would necessarily have taken
the figure of a globe perfectly spherical, if the motion of impulsation
had been given it in a perpendicular direction to the surface; but this
stroke having been obliquely given, the earth turned on its axis at the
moment it took its form; and from the combination of this impulsive
force, the attraction of the parts, there has resulted a spheroid
figure, more elevated under the great circle of rotation, and lower at
the two extremities of the axis, and this because the action of the
centrifugal force proceeding from the diurnal rotation must diminish the
action of gravity. Thus, the earth being homogeneous, and having
received a rotative motion, necessarily took a spheroidical figure, the
two axes of which differ a 230th part from each other. This may be
clearly demonstrated, and does not depend on any hypothesis whatever.
The laws of gravity are perfectly known, and we cannot doubt that
bodies attract each other in a direct ratio of their masses, and in an
inverted ratio, at the squares of their distances; so likewise we cannot
doubt, that the general action of any body is not composed of all the
particular actions of its parts. Thus each part of matter mutually
attracts in a direct ratio of its mass and an inverted ratio of its
distance, and from all these attractions there results a sphere when
there is no rotatory motion, and a spheroid when there is one. This
spheroid is longer or shorter at the two extremities of the axis of
rotation, in proportion to the velocity of its diurnal motion, and the
earth has then, by virtue of its rotative velocity, and of the mutual
attraction of all its parts, the figure of a spheroid, the two axes of
which are as 229 to 230 to one another.
Thus, by its original constituent, by its homogeneousness, and
independent of every hypothesis from the direction of gravity, the earth
has taken this figure of a spheroid at its formation, and agreeable to
mechanical laws: its equatorial diameter was raised about 6-1/2 leagues
higher than under the poles.
I shall dwell on this article, because there are still geometricians who
think that the figure of the earth depends upon theory, and this from a
system of philosophy they have embraced, and from a supposed direction
of gravity. The first thing we have to demonstrate is, the mutual
attraction of every part of matter, and the second the homogeneousness
of the terrestrial globe; if we clearly prove, that these two
circumstances are really so, there will no longer be any hypothesis to
be made on the direction of gravity: the earth will necessarily have the
figure Newton decided in favour of, and every other figure given to it
by virtue of vortexes or other hypotheses, will not be able to subsist.
It cannot be doubted, that it is the force of gravity which retains the
planets in their orbits; the satellites of Saturn gravitate towards
Saturn, those of Jupiter towards Jupiter, the Moon gravitates towards
the Earth: and Saturn, Jupiter, Mars, the Earth, Venus, and Mercury,
gravitate towards the Sun: so likewise Saturn and Jupiter gravitate
towards their satellites, the Earth gravitates towards the Moon, and the
Sun towards the whole of the planets. Gravitation is therefore general
and mutual in all the planetary system, for action cannot be exercised
without a re-action; all the planets, therefore, act mutually one on the
other. This mutual attraction serves as a foundation to the laws of
their motion, and is demonstrated to exist by its effects. When Saturn
and Jupiter are in conjunction, they act one on the other, and this
attraction produces an irregularity in their motion round the Sun. It is
the same with the Earth and the Moon, they also mutually attract each
other; but the irregularities of the motion of the Moon, proceeds from
the attraction of the Sun, so that the Earth, the Sun, and the Moon,
mutually act one on the other. Now this mutual attraction of the
planets, when the distances are equal, is proportional to their quantity
of matter, and the same force of gravity which causes heavy matter to
fall on the surface of the Earth, and which extends to the Moon, is also
proportional to the quantity of matter; therefore the total gravity of a
planet is composed of the gravity of each of its parts; from whence all
the parts of the matter, either in the Earth or in the planets, mutually
attract each other and the Earth, by its rotation round its own axis,
has necessarily taken the figure of a spheroid, the axes of which are as
229 to 230. The direction of the weight must be perpendicular to the
Earth's surface; consequently no hypothesis, drawn from the direction of
gravity, can be sustained, unless the general attraction of the parts of
matter be denied; but the existence of this mutual attraction is
demonstrated by observations, and the experiment of pendulums prove,
that its extension is general; therefore we cannot support an hypothesis
on the direction of gravity without going against experience and reason.
Let us now proceed to examine whether the matter of which the
terrestrial globe is composed be homogeneous. I admit, that if it is
supposed the globe is more dense in some parts than in others, the
direction of gravity must be different from what we have just assigned,
and that the figure of the Earth would also differ agreeable to those
suppositions. But what reason have we to make these suppositions? Why,
for example, should we suppose that the parts near the centre are denser
than those which are more remote? Are not all the particles which
compose the globe collected together by their mutual attraction? hence,
each particle is a centre, and there is no reason to believe, that the
parts which surround the centre are denser than those which are about
any other point. Besides, if one considerable part of the globe was
denser than another, the axis of rotation would be found near the dense
parts, and an inequality would ensue in the diurnal revolution; we
should remark an inequality in the apparent motion of the fixed stars;
they would appear to move more quick or slow in the zenith, or horizon,
according as we should be placed on the denser or lighter parts of the
earth; and the axis of the globe no longer passing through the centre of
gravity, would also very sensibly change its position: but nothing like
this ever happens; on the contrary, the diurnal motion of the earth is
equal and uniform. At all parts of the Earth's surface, the stars appear
to move with the same velocity at all heights, and if there be any
rotation in its axis, it is so trifling as to have escaped observation:
it must therefore be concluded, that the globe is homogeneous, or nearly
so in all its parts.
If the earth was a hollow and void globe, and the crust of which, for
example, not more than two or three miles thick; it would produce these
effects. 1. The mountains would be such considerable parts of the whole
thickness of the crust, that great irregularities in the motions of the
Earth would be occasioned by the attraction of the Moon and Sun: for
when the highest parts of the globe, as the Cordeliers, should have the
Moon at noon, the attraction would be much stronger on the whole globe
than when she was in the meridian of the lowest parts. 2. The attraction
of mountains would be much more considerable than it is in comparison
with the attraction of the whole globe, and experiments made at the
mountain of Chimboraco, in Peru, would in this case give more degrees
than they have given seconds for the deviation of the plumb line. 3. The
weight of bodies would be greater on the tops of high mountains than on
the planes; so that we should feel ourselves considerably heavier, and
should walk with more difficulty in high than in low places. These
observations, with many others that might be added, must convince us,
that the inner parts of the globe is not void, but filled with a dense
matter.
On the other hand, if below the depth of two or three miles, the earth
was filled with a matter much more dense than any known, it would
necessarily occur, that every time we descended to moderate depths, we
should weigh much more, and the motion of pendulums would be more
accelerated than in fact they are when carried from an eminence into a
plain: thus, we may presume that the internal part of the Earth is
filled with a matter nearly similar to that which composes its surface.
What may complete our determination in favour of this opinion is, that
in the first formation of the globe, when it took its present
spheroidical figure, the matter which composed it was in fusion, and,
consequently, all its parts were homogeneous, and nearly equally dense.
From that time the matter on the surface, although originally the same
with the interior, has undergone a variety of changes by external
causes, which has produced materials of such different densities; but it
must be remarked, that the densest matters, as gold and metals, are also
those the most seldom to be met with, and consequently the greatest part
of the matter at the surface of the globe has not undergone any very
great changes with relation to its density; the most common materials,
as sand and clay, differ very little, insomuch, that we may conjecture,
with great probability, that the internal part of the earth is composed
of a vitrified matter, the density of which is nearly the same as that
of sand, and that consequently the terrestrial globe in general may be
regarded as homogeneous.
Notwithstanding this, it may be urged, that although the globe was
composed of concentrical strata of different densities, the diurnal
motion might be equally certain, and the uniform inclination of the axis
as constant and undisturbed as it could be, on the supposition of its
being composed of homogeneous matter. I acknowledge it, but I ask at the
same time, if there is any reason to believe that strata of different
densities do exist? If these conclusions be not rather a desire to
adjust the works of Nature to our own ideas? And whether in physics we
ought to admit suppositions which are not founded on observations or
analogy?
It appears, therefore, that the earth, by virtue of the mutual
attraction of its parts and its diurnal motion, assumed the figure of a
spheroid; that it necessarily took that form from being in a state of
fluidity; that, agreeable to the laws of gravity and of a centrifugal
force, it could have no other figure: that in the moment of its
formation as at present, there was a difference between the two
diameters equal to a 230th part, and that, consequently, every
hypothesis in which we find greater or less difference are fictions
which merit no attention.
But it may be said, if this theory is true, and if 229 to 230 is the
just relation of the axis, why did the mathematicians, sent to Lapland
and Peru, agree to the relation of 174 to 175? From whence does this
difference arise between theory and practice? And is it not more
reasonable to give the preference to practice and measures, especially
when we have been taken by the most able mathematicians of
Europe[109:A], and with all necessary apparatus to establish the result.
To this I answer, that I have paid attention to the observations made at
the equator and near the polar circle; that I have no doubt of their
being exact, and that the earth may possibly be elevated an 175th part
more at the equator than at the poles. But, at the same time, I maintain
my theory, and I see clearly how the two conclusions may be reconciled.
This difference is about four leagues in the two axes, so that the parts
at the equator are raised two leagues more than they ought to be,
according to my theory; this height answers exactly to the greatest
inequalities on the surface of the globe, produced by the motion of the
sea, and the action of the fluids. I will explain; it appears that when
the earth was formed, it must necessarily have taken, by virtue of the
mutual attraction of its parts, and the action of the centrifugal force,
a spheroidical figure, the axes of which differ a 230th part: the
original earth must have had this figure, which it took when it was
fluid, or rather liquified by the fire; but after its formation the
vapours which were extended and rarefied, as in the atmosphere and tail
of a comet, became condensed, and fell on the surface in form of air and
water: and when these waters became agitated by the flux and reflux, the
matters were, by degrees, carried from the poles towards the equatorial
parts; so that the poles were lowered about a league, and those of the
equator raised in the same proportion; this was not suddenly done, but
by degrees in succession of time; the earth being also exposed to the
action of the winds, air, and sun; all these irregular causes concurred
with the flux and reflux to furrow its surface, hollow it into valleys,
and raise it into mountains; and producing other inequalities and
irregularities, of which, nevertheless, the greatest thickness does not
exceed one league at the equator; this inequality of two leagues, is,
perhaps, the greatest which can be on the surface of the earth, for the
highest mountains are scarce above one league in height, and there is
much probability of the sea's not being more at its greatest depth. The
theory is therefore true, and practice may be so likewise; the earth at
first could not be raised above 6-1/2 leagues more at the equator than
the poles, but the changes which have happened to its surface might
afterwards raise it still more. Natural History wonderfully confirms
this opinion, for we have proved in the preceding discourse that the
flux and reflux, and other motions of the water, have produced mountains
and all the inequalities on the surface of the globe, that this surface
has undergone considerable changes, and that at the greatest depths, as
well as on the greatest heights, bones, shells and other wrecks of
animals, which inhabit the sea and earth, are met with.
It may be conjectured, from what has been said, that to find ancient
earth, and matters which have never been removed from the spot in which
they were first placed, we must dig near the poles, where the bed of the
earth must be thinner than in the Southern climates.
On the whole, if we strictly examine the measures by which the figure of
the earth is determined, we shall perceive this hypothesis enters into
such determination; for it supposes the earth to have the figure of a
regular curve, whereas from the constant changes the earth is
continually undergoing from a variety and combination of causes, it is
almost impossible that it should have retained any regular figure, and
hence the poles might, originally, only be flattened a 230th part, as
Newton says, and as my theory requires. Besides, although we had exactly
the length of the degree at the polar circle and equator, have we not
also the length of the degree as exactly in France? And the measure of
M. Picard, has it not been verified? Add to this that the augmentation
and diminution in the motion of the pendulum, do not agree with the
result drawn from measurement, and that, on the contrary, they differ
very little from the theory of Newton. This is surely more than is
requisite to convince us that the poles are not flattened more than a
230th part, and that if there is any difference, it can proceed only
from the inequalities, which the water and other external causes have
produced on its surface; but these inequalities being more irregular
than regular, we must not form any hypothesis thereon, nor suppose, that
the meridians are ellipses, or any other regular curves. From whence we
perceive, that if we should successively measure many degrees of the
earth in all directions, we still should not be certain by that alone,
of the exact situation of the poles, nor whether they were depressed
more or less than the 230th part.
May it not also be conjectured, that if the inclination of the axis of
the earth has changed, it can only be produced by the changes which have
happened to the surface, since all the rest of the globe is homogeneous;
that consequently this variation is too little sensible to be perceived
by astronomers, and that if the earth is not encountered with a comet,
or deranged, by any other external cause, its axis will remain
perpetually inclined as it is at present, and as it has always been?
In order not to omit any conjecture which appears reasonable, may it not
be said, that as the mountains and inequalities which are on the
surface of the earth have been formed by the flux and reflux of the sea,
the mountains and inequalities which we remark on the surface of the
moon, have been produced by a similar cause? they certainly are much
higher than those of the earth, but then her tides are also much
stronger, occasioned by the earth's being considerably larger than the
moon, and consequently producing her tides with a superior force; and
this effect would be much greater if the moon had, like the earth, a
rapid rotation; but as the moon presents always the same surface to the
earth, the tides cannot operate but in proportion to the motion arising
from her libration, by which it alternatively discovers to us a segment
of its other hemisphere; this, however, must produce a kind of flux and
reflux, quite different from that of our sea, and the effects of which
will be much less considerable than if the moon had from its course a
revolution round its axis, as quick as the rotation of the terrestrial
globe.
I should furnish a volume as large as that of Burnet or Whiston's, if I
were to enlarge on the ideas which arise in support of the above; by
giving them a geometrical air, in imitation of the last author, I might
add considerably to their weight; but, in my opinion, hypothesis,
however probable, ought not to be treated with such pomposity; it being
a dress which borders so much on quackery.
FOOTNOTES:
[78:A] Vide Newton, 2d edit. page 525.
[79:A] Vid. Newton, page 405.
[109:A] M. de Maupertuis' Figure of the Earth.
ARTICLE II.
FROM THE SYSTEM OF WHISTON[115:A].
This Author commences his treatise by a dissertation on the creation of
the world; he says that the account of it given by Moses in the text of
Genesis has not been rightly understood; that the translators have
confined themselves too much to the letter and superficial views,
without attending to nature, reason, and philosophy. The common notion
of the world being made in six days, he says is absolutely false, and
that the description given by Moses, is not an exact and philosophical
narration of the creation and origin of the universe, but only an
historical representation of the terrestrial globe. The earth,
according to him, existed in the chaos; and, at the time mentioned by
Moses, received the form, situation and consistency necessary to be
inhabited by the human race. I shall not enter into a detail of his
proofs, nor undertake their refutation. The exposition we have just
made, is sufficient to demonstrate the difference of his opinion with
public facts, its contrariety with scripture, and consequently the
insufficiency of his proofs. On the whole, he treats this matter as a
theological controvertist, rather than as an enlightened philosopher.
Leaving these erroneous principles, he flies to ingenious suppositions,
which, although extraordinary, yet have a degree of probability to those
who, like him, incline to the enthusiasm of system. He says, that the
ancient chaos, the origin of our earth, was the atmosphere of a comet:
that the annual motion of the earth began at the time it took its new
form, but that its diurnal motion began only when the first man fell.
That the ecliptic cut the tropic of cancer, opposite to the terrestrial
paradise, which was situated on the north-west side of the frontiers of
Assyria: that before the deluge, the year began at the autumnal
equinox: that the orbits of the planets, and the earth were then
perfect circles. That the deluge began the 18th of November, 2365 of the
Julian period, or 2349 years before Christ. That the solar and lunar
year were then the same, and that they exactly contained 360 days. That
a comet descending in the plane of the ecliptic towards its perihelion,
passed near the globe of the earth the same day as the deluge began:
that there is a great heat in the internal part of the terrestrial
globe, which constantly diffuses itself from the centre to the
circumference; that the form of the earth is like that of an egg, the
ancient emblem of the globe; that mountains are the lightest part of the
earth, &c. He afterwards attributes all the alterations and changes
which have happened to the earth, to the universal deluge; then blindly
adopts the theory of Woodward, and indiscriminately makes use of all the
observations of that author on the present state of the globe; but
assumes originality when he speaks of its future state: according to him
it will be consumed by fire, and its destruction will be preceded by
terrible earthquakes, thunder, and frightful meteors; the Sun and Moon
will have an hideous aspect, the heavens will appear to fall, and the
flames will be general over all the earth; but when the fire shall have
devoured all the impurities it contains; when it shall be vitrified and
rendered transparent as crystal, the saints and the blessed spirits will
return and take possession of it, and there remain till the day of
judgment.
These hypotheses, at the first glance, appear to be rash and extravagant
assertions; nevertheless the author has managed them with such address,
and treated them with such strength, that they cease to appear
absolutely chimerical. He supports his subjects with much science, and
it is surprising that, from a mixture of ideas so very absurd, a system
could be formed with an air of probability. It has not affected vulgar
minds so much as it has dazzled the eyes of the learned, because they
are more easily deceived by the glare of erudition, and the power of
novel ideas. Mr. Whiston was a celebrated astronomer, in the constant
habit of considering the heavens, observing the stars, and contemplating
the wonderful course of nature; he could never persuade himself that
this small grain of sand, this Earth which we inhabit, occupied more the
attention of the Creator than the universe, the vast extent of which
contains millions of other Suns and Earths. He pretends, that Moses has
not given us the history of the first creation of this globe, but only a
detail of the new form that it took when the Almighty turned it from the
mass of a comet into a planet, and formed it into a proper habitation
for men. Comets are, in fact, subjected to terrible vicissitudes by
reason of the eccentricity of their orbits. Sometimes, like that in
1680, it is a thousand times hotter there than red-hot iron; and
sometimes a thousand times colder than ice; if they are, therefore,
inhabited it must be by strange creatures, of which we can have no
conception.
The planets, on the contrary, are places of rest, where the distance of
the sun not varying much, the temperature remains nearly the same, and
permits different kinds of plants and animals to grow and multiply.
In the beginning God created the world; but, observes our author, the
earth was then an uninhabitable comet, suffering alternatively the
excess of heat and cold, its liquifying and freezing by turns formed a
chaos, or an abyss, surrounded with thick darkness: "and darkness
covered the face of the deep," _& tenebræ erant superfaciam abissi_.
This chaos was the atmosphere of the comet, a body composed of
heterogeneous matters, the centre occupied by spherical, solid, and hot
substances, of about two thousand leagues in diameter, round which a
very great surface of a thick fluid extended, mixed with an unshapen and
confused matter, like the chaos of the ancient _rudis & indigestaque
moles_.