Rotation of the Earth invariable—Decrease in the Earth’s mean
Temperature—Earth originally in a state of Fusion—Length of Day
constant—Decrease of Temperature ascribed by Sir John Herschel to the
variation in the Excentricity of the Terrestrial Orbit—Difference in
the Temperature of the two Hemispheres erroneously ascribed to the
Excess in the Length of Spring and Summer in the Southern Hemisphere;
attributed by Sir Charles Lyell to the Operation of existing
Causes—Three principal Axes of Rotation—Position of the Axis of
Rotation on the Surface of the Earth invariable—Ocean not sufficient
to restore the Equilibrium of the Earth if deranged—Its Density and
mean Depth—Internal Structure of the Earth.
The rotation of the earth, which determines the length of the day, may
be regarded as one of the most important elements in the system of the
world. It serves as a measure of time, and forms the standard of
comparison for the revolutions of the celestial bodies, which, by their
proportional increase or decrease, would soon disclose any changes it
might sustain. Theory and observation concur in proving that, among the
innumerable vicissitudes which prevail throughout creation, the period
of the earth’s diurnal rotation is immutable. The water of rivers,
falling from a higher to a lower level, carries with it the velocity due
to its revolution with the earth at a greater distance from the centre;
it will therefore accelerate, although to an almost infinitesimal
extent, the earth’s daily rotation. The sum of all these increments of
velocity, arising from the descent of all the rivers on the earth’s
surface, would in time become perceptible, did not nature, by the
process of evaporation, raise the waters back to their sources, and
thus, by again removing matter to a greater distance from the centre,
destroy the velocity generated by its previous approach; so that the
descent of rivers does not affect the earth’s rotation. Enormous masses
projected by volcanoes from the equator to the poles, and the contrary,
would indeed affect it, but there is no evidence of such convulsions.
The disturbing action of the moon and planets, which has so powerful an
effect on the revolution of the earth, in no way influences its
rotation. The constant friction of the trade winds on the mountains and
continents between the tropics does not impede its velocity, which
theory even proves to be the same as if the sea, together with the
earth, formed one solid mass. But, although these circumstances be
insufficient, a variation in the mean temperature would certainly
occasion a corresponding change in the velocity of rotation. In the
science of dynamics it is a principle in a system of bodies or of
particles revolving about a fixed centre, that the momentum or sum of
the products of the mass of each into its angular velocity and distance
from the centre is a constant quantity, if the system be not deranged by
a foreign cause. Now, since the number of particles in the system is the
same whatever its temperature may be, when their distances from the
centre are diminished, their angular velocity must be increased, in
order that the preceding quantity may still remain constant. It follows,
then, that, as the primitive momentum of rotation with which the earth
was projected into space must necessarily remain the same, the smallest
decrease in heat, by contracting the terrestrial spheroid, would
accelerate its rotation, and consequently diminish the length of the
day. Notwithstanding the constant accession of heat from the sun’s rays,
geologists have been induced to believe, from the fossil remains, that
the mean temperature of the globe is decreasing.
The high temperature of mines, hot springs, and above all the internal
fires which have produced, and do still occasion, such devastation on
our planet, indicate an augmentation of heat towards its centre. The
increase of density corresponding to the depth and the form of the
spheroid, being what theory assigns to a fluid mass in rotation, concurs
to induce the idea that the temperature of the earth was originally so
high as to reduce all the substances of which it is composed to a state
of fusion or of vapour, and that in the course of ages it has cooled
down to its present state; that it is still becoming colder; and that it
will continue to do so till the whole mass arrives at the temperature of
the medium in which it is placed, or rather at a state of equilibrium
between this temperature, the cooling power of its own radiation, and
the heating effect of the sun’s rays.
Previous to the formation of ice at the poles, the ancient lands of
northern latitudes might, no doubt, have been capable of producing those
tropical plants preserved in the coal-measures, if indeed such plants
could flourish without the intense light of a tropical sun. But, even if
the decreasing temperature of the earth be sufficient to produce the
observed effects, it must be extremely slow in its operation; for, in
consequence of the rotation of the earth being a measure of the periods
of the celestial motions, it has been proved that, if the length of the
day had decreased by the three-thousandth part of a second since the
observations of Hipparchus two thousand years ago, it would have
diminished the secular equation of the moon by 44ʺ·4. It is, therefore,
beyond a doubt that the mean temperature of the earth cannot have
sensibly varied during that time. If, then, the appearances exhibited by
the strata are really owing to a decrease of internal temperature, it
either shows the immense periods requisite to produce geological
changes, to which two thousand years are as nothing, or that the mean
temperature of the earth had arrived at a state of equilibrium before
these observations.
However strong the indications of the primitive fluidity of the earth,
as there is no direct proof of it, the hypothesis can only be regarded
as very probable. But one of the most profound philosophers and elegant
writers of modern times has found in the secular variation of the
excentricity of the terrestrial orbit an evident cause of decreasing
temperature. That accomplished author, in pointing out the mutual
dependencies of phenomena, says, “It is evident that the mean
temperature of the whole surface of the globe, in so far as it is
maintained by the action of the sun at a higher degree than it would
have were the sun extinguished, must depend on the mean quantity of the
sun’s rays which it receives, or—which comes to the same thing—on the
total quantity received in a given invariable time; and, the length of
the year being unchangeable in all the fluctuations of the planetary
system, it follows that the total amount of solar radiation will
determine, _cæteris paribus_, the general climate of the earth. Now, it
is not difficult to show that this amount is inversely proportional to
the minor axis of the ellipse described by the earth about the sun
(N. 143), regarded as slowly variable; and that, therefore, the major
axis remaining, as we know it to be, constant, and the orbit being
actually in a state of approach to a circle, and consequently the minor
axis being on the increase, the mean annual amount of solar radiation
received by the whole earth must be actually on the decrease. We have,
therefore, an evident real cause to account for the phenomenon.” The
limits of the variation in the excentricity of the earth’s orbit are
unknown. But, if its ellipticity has ever been as great as that of the
orbit of Mercury or Pallas, the mean temperature of the earth must have
been sensibly higher than it is at present. Whether it was great enough
to render our northern climates fit for the production of tropical
plants, and for the residence of the elephant and other animals now
inhabitants of the torrid zone, it is impossible to say.
Of the decrease in temperature of the northern hemisphere there is
abundant evidence in the fossil plants discovered in very high
latitudes, which could only have existed in a tropical climate, and
which must have grown near the spot where they are found, from the
delicacy of their structure and the perfect state of their preservation.
This change of temperature has been erroneously ascribed to an excess in
the duration of spring and summer in the northern hemisphere, in
consequence of the excentricity of the solar ellipse. The length of the
seasons varies with the position of the perihelion (N. 64) of the
earth’s orbit for two reasons. On account of the excentricity, small as
it is, any line passing through the centre of the sun divides the
terrestrial ellipse into two unequal parts, and by the laws of
elliptical motion the earth moves through these two portions with
unequal velocities. The perihelion always lies in the smaller portion,
and there the earth’s motion is the most rapid. In the present position
of the perihelion, spring and summer north of the equator exceed by
about eight days the duration of the same seasons south of it. And
10,492 years ago the southern hemisphere enjoyed the advantage we now
possess from the secular variation of the perihelion. Yet Sir John
Herschel has shown that by this alternation neither hemisphere acquires
any excess of light or heat above the other; for, although the earth is
nearer to the sun while moving through that part of its orbit in which
the perihelion lies than in the other part, and consequently receives a
greater quantity of light and heat, yet as it moves faster it is exposed
to the heat for a shorter time. In the other part of the orbit, on the
contrary, the earth, being farther from the sun, receives fewer of his
rays; but because its motion is slower, it is exposed to them for a
longer time; and, as in both cases the quantity of heat and the angular
velocity vary exactly in the same proportion, a perfect compensation
takes place (N. 144). So that the excentricity of the earth’s orbit has
little or no effect on the temperature corresponding to the difference
of the seasons.
Sir Charles Lyell, in his excellent works on Geology, refers the
increased cold of the northern hemisphere to the operation of existing
causes with more probability than most theories that have been advanced
in solution of this difficult subject. The loftiest mountains would be
represented by a grain of sand on a globe six feet in diameter, and the
depth of the ocean by a scratch on its surface. Consequently the gradual
elevation of a continent or chain of mountains above the surface of the
ocean, or their depression below it, is no very great event compared
with the magnitude of the earth, and the energy of its subterranean
fires, if the same periods of time be admitted in the progress of
geological as in astronomical phenomena, which the successive and
various races of extinct beings show to have been immense. Climate is
always more intense in the interior of continents than in islands or
sea-coasts. An increase of land within the tropics would therefore
augment the general heat, and an increase in the temperate and frigid
zones would render the cold more severe. Now it appears that most of the
European, North Asiatic, and North American continents and islands were
raised from the deep after the coal-measures were formed in which the
fossil tropical plants are found; and a variety of geological facts
indicate the existence of an ancient and extensive archipelago
throughout the greater part of the northern hemisphere. Sir Charles
Lyell is therefore of opinion that the climate of these islands must
have been sufficiently mild, in consequence of the surrounding ocean, to
clothe them with tropical plants, and render them a fit abode for the
huge animals whose fossil remains are so often found; that the
arborescent ferns and the palms of these regions, carried by streams to
the bottom of the ocean, were imbedded in the strata which were by
degrees heaved up by the subterranean fires during a long succession of
ages, till the greater part of the northern hemisphere became dry land
as it now is, and that the consequence has been a continual decrease of
temperature.
It is evident, from the marine shells found on the tops of the highest
mountains and in almost every part of the globe, that immense continents
have been elevated above the ocean which must have engulfed others. Such
a catastrophe would be occasioned by a variation in the position of the
axis of rotation on the surface of the earth; for the seas tending to a
new equator would leave some portions of the globe and overwhelm others.
Now, it is found by the laws of mechanics that in every body, be its
form or density what it may, there are at least three axes at right
angles to each other, round any one of which, if the solid begins to
rotate, it will continue to revolve for ever, provided it be not
disturbed by a foreign cause, but that the rotation about any other axis
will only be for an instant, and consequently the poles or extremities
of the instantaneous axis of rotation would perpetually change their
position on the surface of the body. In an ellipsoid of revolution the
polar diameter and every diameter in the plane of the equator are the
only permanent axes of rotation (N. 145). Hence, if the ellipsoid were
to begin to revolve about any diameter between the pole and the equator,
the motion would be so unstable that the axis of rotation and the
position of the poles would change every instant. Therefore, as the
earth does not differ much from this figure, if it did not turn round
one of its principal axes, the position of the poles would change daily;
the equator, which is 90° distant, would undergo corresponding
variations; and the geographical latitudes of all places, being
estimated from the equator, assumed to be fixed, would be perpetually
changing. A displacement in the position of the poles of only two
hundred miles would be sufficient to produce these effects, and would
immediately be detected. But, as the latitudes are found to be
invariable, it may be concluded that the terrestrial spheroid must have
revolved about the same axis for ages. The earth and planets differ so
little from ellipsoids of revolution, that in all probability any
libration from one axis to another, produced by the primitive impulse
which put them in motion, must have ceased soon after their creation
from the friction of the fluids at their surface.
Theory also proves that neither nutation, precession, nor any of the
disturbing forces that affect the system, have the smallest influence on
the axis of rotation, which maintains a permanent position on the
surface, if the earth be not disturbed in its rotation by a foreign
cause, as the collision of a comet, which might have happened in the
immensity of time. But, had that been the case, its effects would still
have been perceptible in the variations of the geographical latitudes.
If we suppose that such an event had taken place, and that the
disturbance had been very great, equilibrium could then only have been
restored with regard to a new axis of rotation by the rushing of the
seas to the new equator, which they must have continued to do till the
surface was everywhere perpendicular to the direction of gravity. But it
is probable that such an accumulation of the waters would not be
sufficient to restore equilibrium if the derangement had been great, for
the mean density of the sea is only about a fifth part of the mean
density of the earth, and the mean depth of the Pacific Ocean is
supposed not to be more than four or five miles, whereas the equatorial
diameter of the earth exceeds the polar diameter by about 26-1/2 miles.
Consequently the influence of the sea on the direction of gravity is
very small. And, as it thus appears that a great change in the position
of the axis is incompatible with the law of equilibrium, the geological
phenomena in question must be ascribed to an internal cause. Indeed it
is now demonstrated that the strata containing marine diluvia, which are
in lofty situations, must have been formed at the bottom of the ocean,
and afterwards upheaved by the action of subterraneous fires. Besides,
it is clear, from the mensuration of the arcs of the meridian and the
length of the seconds’ pendulum, as well as from the lunar theory, that
the internal strata and also the external outline of the globe are
elliptical, their centres being coincident and their axes identical with
that of the surface—a state of things which, according to the
distinguished author lately quoted, is incompatible with a subsequent
accommodation of the surface to a new and different state of rotation
from that which determined the original distribution of the component
matter. Thus, amidst the mighty revolutions which have swept innumerable
races of organized beings from the earth, which have elevated plains and
buried mountains in the ocean, the rotation of the earth and the
position of the axes on its surface have undergone but slight
variations.
The strata of the terrestrial spheroid are not only concentric and
elliptical, but the lunar inequalities show that they increase in
density from the surface of the earth to its centre. This would
certainly have happened if the earth had originally been fluid, for the
denser parts must have subsided towards the centre as it approached a
state of equilibrium. But the enormous pressure of the superincumbent
mass is a sufficient cause for the phenomenon. Professor Leslie observes
that air compressed into the fiftieth part of its volume has its
elasticity fifty times augmented. If it continues to contract at that
rate, it would, from its own incumbent weight, acquire the density of
water at the depth of thirty-four miles. But water itself would have its
density doubled at the depth of ninety-three miles, and would even
attain the density of quicksilver at a depth of 362 miles. Descending
therefore towards the centre through nearly 4000 miles, the condensation
of ordinary substances would surpass the utmost powers of conception.
Dr. Young says that steel would be compressed into one-fourth and stone
into one-eighth of its bulk at the earth’s centre. However, we are yet
ignorant of the laws of compression of solid bodies beyond a certain
limit; from the experiments of Mr. Perkins they appear to be capable of
a greater degree of compression than has generally been imagined.
But a density so extreme is not borne out by astronomical observation.
It might seem to follow therefore that our planet must have a widely
cavernous structure, and that we tread on a crust or shell whose
thickness bears a very small proportion to the diameter of its sphere.
Possibly, too, this great condensation at the central regions may be
counterbalanced by the increased elasticity due to a very elevated
temperature.