the diagram below it illustrates the distance apart.
]
[Sidenote: Laplace’s Theory Abandoned]
A fundamental difficulty is the extreme tenuity of the gas which is
assumed to have formed the planetary rings. A second difficulty, which
has been emphasised by Professors Chamberlin and Moulton, is to be
found in the comparatively small amount of rotational energy which
the system at present possesses, for this is less than 1/200 of that
which, on the most favourable assumption, must have been contained
within the original nebula. Less fundamental, but equally fatal, is the
fact that one of the satellites of Saturn revolves round its primary
in a direction opposed to that of the rotation of the planet itself.
[Recently Mr. Stratton, following out a suggestion of Professor W. H.
Pickering, has shown that this is quite consistent, and, indeed, is a
natural deduction from Laplace’s hypothesis.] Hence for these and other
reasons we are reluctantly compelled to abandon an hypothesis which
for over a century has exercised an influence on our conception of the
cosmos not less profound, penetrating, and far-reaching than that of
the famous Darwinian doctrine of natural selection, now on its trial.
[Sidenote: What are the Nebulæ?]
At present, unanimity of opinion, even on questions of the most primary
kind, is far to seek. Philosophers are not even agreed as to the
constitution of the nebulæ. It is questioned whether even those least
resolvable and most diffused forms which give bright line spectra
really consist of masses of incandescent gas. Many observers, among
them Sir Norman Lockyer, now maintain that they are formed of swarms of
meteorites, which, moving with prodigious velocity, meet in frequent
collision, and by their impact evolve sufficient heat to become
self-luminous. Others, again, like the distinguished investigator
Arrhenius, while admitting the gaseous nature of these nebulæ, deny
that they are incandescent, and assert that their temperature is not
much above that of surrounding space. Their exterior parts consist of
the lighter gases in a highly rarefied state, and minute particles of
negative electricity, which are always careering through space, on
penetrating these gases produce a luminous discharge. A nebula composed
of swarms of meteorites would, as Sir George Darwin has shown, behave
very much in the same way as one composed of gas, and if in rotation
would rotate as a solid mass. The meteorites would stand in the same
relation to the nebula as molecules to a gas, and thus the question of
the constitution of the nebula, although of great interest in itself,
becomes of subsidiary importance in tracing its subsequent history.
[Sidenote: Shaping of the Planets]
One of the latest attempts to frame a nebular hypothesis is that of
Professor J. H. Jeans. His reasoning is of a highly mathematical
character, and his conclusions are expressed in the most general terms.
Starting with a spherical nebula of gas or meteorites endowed with a
small amount of rotation, he shows that as it cools or loses energy
the temperature of the interior will not fall continuously in precise
correspondence with the cooling of the outer parts, and this “lag” of
the interior temperature will bring about a tendency to instability.
The contraction of the nebula due to cooling will increase the velocity
of rotation, and this again will tend to instability. As a result of
the instability so produced the nebula will change its form, and become
more or less pear-shaped. The narrow end of the pear will then separate
from the body and assume an independent existence as a primitive
planet. This process will recur again and again till the nebula is
resolved into a sun with its attendant planets. The planets, existing
at first as gaseous masses or quasi-gaseous masses, will be liable
to the same kind of transformation, and may thus bud off moons or
satellites.
If the nebula were not in rapid rotation, a slight disturbing cause,
acting at the critical moment when a planet was being ejected, might
determine the inclination of the planet’s orbit, which might thus be
very oblique to the equatorial plane of the nebula. Thus the hypothesis
is not open to one of the objections which have been urged against
that of Laplace--namely, that the orbits of some of the planets in the
solar system are inclined at a large angle with the plane of the sun’s
equator.
[Illustration:
This illustrates Laplace’s theory, which conceived of a vast nebula
filling the whole space of the solar system and rotating around a
central axis. The outer and thinner part had much greater movement
than the denser central mass, finally being thrown off as a ring,
which in turn rolled up into a ball, still following the same
course as the ring had followed. Thus the earth broke off from the
sun and the moon from the earth. The theory is, however, no longer
credited by scientists.
The pear-shaped nebula is the theory of a young English
mathematician, Professor J. H. Jeans. Starting with a spherical
nebula, he argues that in cooling it will assume the form
illustrated above, and that the smaller part will separate and form
a satellite rotating independently but within a distance influenced
by the parent mass.
The spiral nebula in Canes Venatici, a revolving mass of gas or
meteorites, supplies, according to the nebular hypothesis of
Messrs. Chamberlin and Moulton, an excellent example of how the
earth and moon were formed. We may reasonably imagine the smaller
spiral to represent the moon in the act of being thrown off by the
earth.
THREE FAMOUS THEORIES OF THE BEGINNING OF THE EARTH]
[Sidenote: Heavenly Bodies in Collision]
Jeans mentions two disturbing causes in particular which might easily
arise--one the penetration of the nebula by a wandering meteorite,
which might precipitate an event already on the verge of happening,
and simultaneously determine both the birth of a planet and the
obliquity of its orbit; the second, the presence of some distant
mass, such as a star, which, by raising a quasi-tide in the nebula,
would give the final touch required to overturn its equilibrium. The
influence of a distant body, such as a passing star, has been invoked
by Moulton in another version of the nebular hypothesis. In conjunction
with Chamberlin, he calls special attention to the spiral nebulæ, which
are by far the commonest kind, as presenting the closest approach to
the conditions which obtain when planets are actually in course of
formation. Chamberlin and Moulton enter on a detailed account of the
manner in which they suppose the planets to have grown by the gradual
accretion of meteoric masses as these encountered each other while
moving in various elliptical orbits.
At present it would seem impossible to speak with certainty as to
the precise history of the solar system. Meanwhile, we may console
ourselves with the closing words of Professor Jeans’ paper, to the
effect that “no difficulty need be experienced in referring existing
planetary systems to a nebulous or meteoric origin on the ground
that the configurations of these systems are not such as could have
originated out of a rotating mass of liquid.”
An investigation by Sir George Darwin, which has furnished inspiration
to such hypotheses as that of Jeans, brings us nearer the immediate
subject of this essay, since it treats of one of the last acts in the
great drama of planetary existence, and attempts to derive the earth
and moon from a common origin in a single rotating sphere.
[Sidenote: Why the Day is Growing Longer]
It is well known that, owing to the frictional effects produced by the
tides, the earth is being gradually slowed down as it rotates upon
its axis. Thus the day is constantly getting longer, so that in a few
millions of years it will have increased in length from twenty-four
to twenty-five hours. On the other hand, in past time it must have
been shorter than at present: a few millions of years ago it was only
twenty-three hours in length, and many millions of years earlier it
was still less, only some five hours or so. At that time the earth
was hotter than it is now, less rigid, more yielding, and, owing to
its rapid rotation, less stable. The action on the moon of the tides
produced in it by the earth is similar, and the rotation of the moon
has been so far diminished by them that its day has become as long as
the month--_i.e._, our satellite only turns once round on its axis in
the time that it takes to revolve once round the earth; it is for this
reason that our satellite keeps always the same face turned towards us.
[Sidenote: The Moon Was Part of Our Sphere]
The retardation of the earth in its rotation has, however, a very
remarkable effect on the revolution of the moon; it involves--by the
principle of the conservation of moment of momentum--a corresponding
acceleration of the moon in its orbit, and, as a consequence of this,
an enlargement of this orbit--that is, the moon is pushed away from us,
as it were, and thus becomes more remote. But if so, the moon must have
been nearer to us in times past. It is possible to trace the approach
of the moon to the earth as we go backwards in time till the distance
between them was only two and a half terrestrial radii instead of the
sixty radii which now separate them. Mathematics do not take us farther
back than this. But it is difficult to resist the suggestion that in
the immediately preceding stage of development the earth and moon
formed together a single sphere.
If we may adopt this view, then we must regard the sphere as subject to
the tidal influence of the sun. It was much hotter, and therefore more
yielding, than the present earth; it was also rotating much faster,
probably once in about four or five hours. It would be contracting as a
consequence of cooling, and the contraction would lead to instability
(gravitational instability); its rapid rotation would also tend toward
instability (rotational instability). It is difficult to say which
of these two, gravitational or rotational instability, would be the
most effective; but the combined result would be to give a pear-shaped
form to the rotating mass, and eventually to deepen the constriction
between the narrow and the broad end, till the smaller protuberance
became completely dissevered from the larger mass, and so entered on
an independent existence as the moon. This final step in the process
would probably depend on the tide-producing power of the sun; the
larger mass remained behind as the earth, whose individual existence
may be said to date from this event.
[Sidenote: How the Moon Broke Away]
The young earth would be subject to very much the same conditions after
as before the ejection of the moon, and might very possibly again pass
into a pear-shaped form, but without proceeding further through those
subsequent changes, which would have led to the formation of another
satellite; and while possessing some such form as this, she might
very well have consolidated. With advancing years she would lose, as
we have seen, the activity of her youth, the drag of the tides would
cause her to spin ever more slowly on her axis, till the day would
become prolonged to the twenty-four hours of the present. With this
diminished rate of spin, the earth, if free to yield, would lose the
pear-shaped form and become an oblate spheroid, and the oblateness of
this spheroid would continually diminish, so that it would continually
approach towards a true sphere. Suppose, however, that the earth as it
cooled lost its power of readily yielding--and at present it is more
rigid than a globe of steel--then it would pass from form to form,
not by a flowing movement, but by a series of ruptures, and its form
at any moment might be a little in arrear of that which it would have
possessed if it had been in the fluid state.
Thus it might indeed be possible still to discover some trace of an
old-fashioned form in the existing planet; and a careful examination
of the distribution of land and sea as represented on a terrestrial
globe does, in fact, reveal a remarkable symmetry, in which we seem
to recognise a surviving vestige of its early state. The great
continent of Africa projects like the narrow end of a pear; around it
are oceans--the Atlantic, the Indian Ocean, and the Mediterranean Sea,
which was once of far greater extent; then comes a great dismembered
ring of land, the two Americas, the Antarctic continent, Australia,
Asia, and Europe. Within these, on the side opposite to Africa, is the
great Pacific Ocean, which covers over the broad end of the pear.
[Sidenote: Earth’s Unknown Changes]
[Illustration: THE SHAPING OF THE FACE OF THE EARTH
Soon after the earth had cooled down, so that the oceans were
formed, the shaping of the great continents began. The action of
moving water in the making of new land is well illustrated by the
vast delta of the Mississippi, where an area larger than Wales has
been formed by debris deposited by the river.
]
A line drawn from somewhere in Central Africa to its antipodes in the
Pacific, through the centre of the earth, would correspond to the long
axis of the pear; a second, at right angles to this, would correspond
to its breadth; and a third, at right angles to both, would correspond
to the axis on which it rotates. A diameter of the earth taken through
the equator is almost 8,000 miles in length, the Polar diameter is
about sixteen miles shorter, and this slight difference measures the
oblateness of the spheroid, or the departure of the form of the earth
from a true sphere. Further, it would appear that the diameter drawn
through Africa is about half a mile longer than the equatorial diameter
taken at right angles to it, and this insignificant quantity measures
the departure of the form of the earth from that of an oblate spheroid
to that of a pear, so nearly complete is the adjustment of its form to
existing conditions. Before this nice adjustment was reached, the earth
must have suffered many changes, passed through many times of stress
and storm, and witnessed many geological revolutions.
[Sidenote: An Age of Red-hot Rain!]
If, at the beginning of her career, the earth was molten, or at a
very high temperature, she must have been surrounded by a very deep
and dense atmosphere, for all the waters which now rest on her
surface--oceans, lakes, and rivers--would have contributed to it in the
state of steam; and not till the temperature of the ground had fallen
to 380 deg. C. could liquid water have begun to accumulate. Then a
steady downpour of almost red-hot rain would have set in, filling up
the neck of the pear and extending far and wide over its broad end.
The temperature would now fall somewhat rapidly, and in a short space
of time the surface of the earth would have become as cool as it is at
the present day. Directly the waters of the firmament had collected
into the oceans, leaving behind an atmosphere like that which now
exists, geological agencies of the kind we are now familiar with would
begin their sway. Air and rain would exert their insidious power upon
the rocks, sapping their strength, converting the hardest granite into
soft sand and clay, which would be washed away by the rain through
brooks and rivulets into the channels of many rivers, all hastening
with their burden of sediment, to deposit it finally in the sea. Here
it would accumulate, layer after layer, building up those mighty masses
of strata which now form the greater part of the visible land. While
this general action was everywhere in progress, wearing down continents
and islands towards the level of the sea, more specialised activities
were assisting to the same end.
[Illustration: TWO STAGES IN THE LIFE OF THE EARTH
This illustrates in striking manner, based on the calculations
of the best authorities, the comparative sizes of the earth,
first as a gaseous mass, and, second, after it had cooled down
and solidified into the planet on which we live. The small dot
represents 8,000 miles, the earth’s diameter.
]
The waves which fall upon our coasts are now constantly undermining
the cliffs and extending the margin of the sea at the expense of the
land, and rivers not only serve to transport sediment, but cut down
their channels deep into the rock, and so carve out the most varied
landscapes of hill and valley from monotonous tableland.
[Sidenote: Action of Winds and Tides]
When we enter into calculations we are astonished at the rapidity
with which these agents perform their work even at the present day;
but as we proceed farther back into the past, when the earth was full
of youthful energy, their power must have been greatly enhanced. We
might almost take the measure of the day as the measure of their
work, for they probably accomplished as much during the eight hours’
day which once existed as they do now in twenty-four hours. A little
consideration will make this clear. It is the winds which, blowing
over the surface of the ocean, produce the sea waves, and it is these
falling on our coasts that perform the work of marine denudation. But
the winds are due in the first place to the heat of the sun, and the
difference of temperature established at the equator and the poles;
and, in the next place, to the rotation of the earth. Thus, with the
increased rapidity of rotation which we know to have existed, and
with increased radiation from the sun, a very probable contingency,
the winds would increase in strength and more powerfully erode our
coasts. Again, with the moon in greater proximity, and with a more
rapid rotation of the earth, the tides would be much higher and more
frequent, and these, raising and lowering the cutting edge of the sea,
greatly assist it in its work of destruction. The winds and the tides
produce various marine currents, and these help to distribute the
sediment which the rivers deliver into the sea, so that when stronger
currents flowed as a result of more powerful tides and more violent
winds, the sediments would be strewn over wider areas; hence, the more
ancient strata of our planet are far more widely distributed than are
those of later time.
[Illustration: THREE VIEWS OF THE GLOBE SHOWING HOW THE GREAT MOUNTAIN
RANGES WERE FORMED
In the days when the earth’s crust had formed but was still
unstable, the process of cooling not having gone far enough,
there would not be the mountains which now characterise it. These
came when the earth contracted and crumpled up along certain
well defined lines, which are now represented by the three great
mountain chains of the world.
]
[Sidenote: Building Up the Earth]
Finally, a heavier rainfall would result from a more active atmospheric
circulation, creating larger rivers, and thus, at the beginning, all
those denuding agents which are engaged in wearing the land down into
the sea would be working at a more rapid pace. Correspondingly, all
the agents which are occupied in building up deposits of sediments
would have extended their operations over a wider area, laying down a
foundation broad and deep.
On the other hand, the contraction of the earth, due to the loss of
its energy of rotation as well as of its internal heat, would also
have proceeded more rapidly, new land would have emerged from the sea,
old lands would have been submerged beneath it far less slowly than at
the present day; ruptures of the crust, accompanied by earthquakes and
volcanic action, would have been more frequent and thus, by the more
rapid loss of its intrinsic energy, the renovation of the earth would
have kept pace with its accelerated destruction.
One effect of the contraction of the earth which has manifested itself
in even late geological times is the crumpling up of the terrestrial
crust into the sharp folds of mountain chains; but at the beginning
this crumpling must have been far more universal and energetic. In this
connection it is interesting to observe that the most ancient rocks
known to us--the Archæan--never present themselves under any other form
than as intensely plicated masses. They originally consisted of lava
flows and volcanic ashes, of ancient sediments and limestones, into
which subterranean masses of granite and other molten, deep-seated
rocks have been injected; but under the intense pressures to which
they were subjected after their formation they and the invading
granite have entirely lost their original character, and have been
metamorphosed into gneisses, schists, and marble, all sharply and
closely folded together. In any given district the direction of their
folding is maintained with wonderful constancy over great distances.
There is no succeeding system of rocks that has been so completely
transformed, so universally plicated, as this ancient Archæan complex.
In later times we can pass from stratum to stratum of the sedimentary
series and read their history almost as we turn over the pages of a
book; in the Archæan all are kneaded together into a state of such
desperate entanglement as to defy the powers of human ingenuity to
unravel them. Thus the line of demarcation between the Archæan and
subsequent sedimentary systems is the sharpest and most absolute that
is known to us in the history of the earth. It marks the close of our
planet’s infancy, the several events of which have passed into oblivion
as profound as that of our own forgetfulness of our earliest days.
Later events, on the other hand, are recorded in the stratified series
with a faithfulness which increases as we approach existing times.
[Sidenote: How We Know These Wonders]
[Sidenote: The Ocean 100 million Years old!]
[Sidenote: The Part Radium may play]