the constellations Gemini and Cancer. Here the stars seem to set bodily
towards the neighbouring part of the Milky Way. The general drift
in that direction is too marked, and affects too many stars, to be
regarded as by any possibility referable to accidental coincidence.
It is worthy of note that if the community of star-drift should be
recognized (or I prefer to say, _when_ it is recognized), astronomers
will have the means of determining the relative distances of the
stars of a drifting system. For differences in the apparent direction
and amount of motion can be due but to differences of distance and
position, and the determination of these differences becomes merely a
question of perspective.[18]
Before long it is likely that the theory of star-drift will be
subjected to a crucial test, since spectroscopic analysis affords the
means of determining the stellar motions of recess or approach. The
task is a very difficult one, but astronomers have full confidence that
in the able hands of Mr. Huggins it will be successfully accomplished.
I await the result with full confidence that it will confirm my views.
(See pages 92–94 for the result.)
* * * * *
Turning to the subject of Star-mist, under which head I include all
orders of nebulæ, I propose to deal with but a small proportion of the
evidence I have collected to prove that none of the nebulæ are external
galaxies. That evidence has indeed become exceedingly voluminous.
I shall dwell, therefore, on three points only.
First, as to the distribution of the nebulæ:—They are not spread with
any approach to uniformity over the heavens, but are gathered into
streams and clusters. The one great law which characterizes their
distribution is an avoidance of the Milky Way and its neighbourhood.
This peculiarity has, strangely enough, been regarded by astronomers
as showing that there is no association between the nebulæ and the
sidereal system. They have forgotten that marked contrast is as clear
a sign of association as marked resemblance, and has always been so
regarded by logicians.
Secondly, there are in the southern heavens two well-marked streams of
nebulæ. Each of these streams is associated with an equally well-marked
stream of stars. Each intermixed stream directs its course towards a
Magellanic Cloud, one towards the Nubecula Minor, the other towards the
Nubecula Major. To these great clusters they flow, like rivers towards
some mighty lake. And within these clusters, which are doubtless
roughly spherical in form, there are found intermixed in wonderful
profusion, stars, star-clusters, and all the orders of nebulæ. Can
these coincidences be regarded as accidental? And if not accidental,
is not the lesson they clearly teach us this, that nebulæ form but
portions of the sidereal system, associating themselves with stars on
terms of equality (if one may so speak), even if single stars be not
more important objects in the scale of creation than these nebulous
masses, which have been so long regarded as equalling, if not outvying,
the sidereal system itself in extent?
The third point to which I wish to invite attention is the way in which
in many nebulæ stars of considerable relative brightness, and belonging
obviously to the sidereal system, are so associated with nebulous
masses as to leave no doubt whatever that these masses really cling
around them. The association is in many instances far too marked to be
regarded as the effect of accident.
Among other instances[19] may be cited the nebula round the stars _c_¹
and _c_² in Orion. In this object two remarkable nebulous nodules
centrally surround two double stars. Admitting the association here to
be real (and no other explanation can reasonably be admitted), we are
led to interesting conclusions respecting the whole of that wonderful
nebulous region which surrounds the sword of Orion. We are led to
believe that the other nebulæ in that region are really associated with
the fixed stars there; that it is not a mere coincidence, for instance,
that the middle star in the belt of Orion is involved in nebula, or
that the lowest star of the sword is similarly circumstanced. It is
a legitimate inference from the evidence that all the nebulæ in this
region belong to one great nebulous group, which extends its branches
to these stars. As a mighty hand, this nebulous region seems to gather
the stars here into close association, showing us, in a way there is no
misinterpreting, that these stars form one system.
The nebula around the strange variable star, Eta Argûs, is another
remarkable instance of this sort. More than two years ago I ventured
to make two predictions about this object. The first was a tolerably
safe one. I expressed my belief that the nebula would be found to be
gaseous. After Mr. Huggins’s discovery that the great Orion nebula is
gaseous, it was not difficult to see that the Argo nebula must be
so too. At any rate, this has been established by Captain Herschel’s
spectroscopic researches. The other prediction was more venturesome.
Sir John Herschel, whose opinion on such points one would always
prefer to share, had expressed his belief that the nebula lies far out
in space beyond the stars seen in the same field of view. I ventured
to express the opinion that those stars are involved in the nebula.
Lately there came news from Australia that Mr. Le Sueur, with the
great reflector erected at Melbourne, has found that the nebula has
changed largely in shape since Sir John Herschel observed it. Mr. Le
Sueur accordingly expressed his belief that the nebula lies _nearer_ to
us than the fixed stars seen in the same field of view. More lately,
however, he has found that the star Eta Argûs is shining with the
light of burning hydrogen, and he expresses his belief that the star
has consumed the nebulous matter near it. Without agreeing with this
view, I recognize in it a proof that Mr. Le Sueur now considers the
nebula to be really associated with the stars around it. My belief is
that as the star recovers its brilliancy observation will show that the
nebula in its immediate neighbourhood becomes brighter (_not_ fainter
through being consumed as fuel). In fact, I am disposed to regard the
variations of the nebula as systematic, and due to orbital motions
among its various portions around neighbouring stars.
As indicative of other laws of association bearing on the relations I
have been dealing with, I may mention the circumstance that red stars
and variable stars affect the neighbourhood of the Milky Way or of
well-marked star-streams. The constellation Orion is singularly rich in
objects of this class. It is here that the strange “variable” Betelgeux
lies. At present this star shows no sign of variation, but a few years
ago it exhibited remarkable changes. One is invited to believe that
the star may have been carried by its proper motion into regions where
there is a more uniform distribution of the material whence this orb
recruits its fires. It may be that in the consideration of such causes
of variation affecting our sun in long past ages a more satisfactory
explanation than any yet obtained may be found of the problem
geologists find so perplexing—the former existence of a tropical
climate in places within the temperate zone, or even near the Arctic
regions.[20]
* * * * *
It remains that I should exhibit the general results to which I have
been led. It has seemed to many that my views tend largely to diminish
our estimate of the extent of the sidereal system. The exact reverse
is the case. According to accepted views there lie within the range of
our most powerful telescopes millions of millions of suns. According
to mine the primary suns within the range of our telescopes must be
counted by tens of thousands, or by hundreds of thousands at the
outside. What does this diminution of numbers imply but that the space
separating sun from sun is enormously greater than accepted theories
would permit? And this increase implies an enormous increase in the
estimate we are to form of the vital energies of individual suns. For
the vitality of a sun, if one may be permitted the expression, is
measured not merely by the amount of matter over which it exercises
control, but by the extent of space within which that matter is
distributed. Take an orb a thousand times vaster than our sun, and
spread over its surface an amount of matter exceeding a thousandfold
the combined mass of all the planets of the solar system:—So far as
living force is concerned, the result is—_nil_. But distribute that
matter throughout a vast space all round the orb:—That orb becomes
at once fit to be the centre of a host of dependent worlds. Again,
according to accepted theories, when the astronomer has succeeded in
resolving the milky light of a portion of the galaxy into stars, he
has in that direction, at any rate, reached the limits of the sidereal
system. According to my views, what he has really done has been but to
analyze a definite aggregation of stars, a mere corner of that great
system. Yet once more, according to accepted views, thousands and
thousands of galaxies, external to the sidereal system, can be seen
with powerful telescopes. If I am right, the external star-systems lie
far beyond the reach of the most powerful telescope man has yet been
able to construct, insomuch that perchance the nearest of the outlying
galaxies may lie a million times beyond the range even of the mighty
mirror of the great Rosse telescope.
But this is little. Wonderful as is the extent of the sidereal system
as thus viewed, even more wonderful is its infinite variety. We know
how largely modern discoveries have increased our estimate of the
complexity of the planetary system. Where the ancients recognized
but a few planets, we now see, besides the planets, the families of
satellites; we see the rings of Saturn, in which minute satellites must
be as the sands on the sea-shore for multitude; the wonderful zone
of asteroids; myriads on myriads of comets; millions on millions of
meteor-systems, gathering more and more richly around the sun, until
in his neighbourhood they form the crown of glory which bursts into
view when he is totally eclipsed. But wonderful as is the variety seen
within the planetary system, the variety within the sidereal system is
infinitely more amazing. Besides the single suns, there are groups and
systems and streams of primary suns; there are whole galaxies of minor
orbs; there are clustering stellar aggregations, showing every variety
of richness, of figure, and of distribution; there are all the various
forms of nebulæ, resolvable and irresolvable, circular, elliptical,
and spiral; and lastly, there are irregular masses of luminous gas,
clinging in fantastic convolutions around stars and star-systems. Nor
is it unsafe to assert that other forms and variety of structure will
yet be discovered, or that hundreds more exist which we may never hope
to recognize.
But lastly, even more wonderful than the infinite variety of the
sidereal system, is its amazing vitality. Instead of millions of inert
masses, we see the whole heavens instinct with energy—astir with busy
life. The great masses of luminous vapour, though occupying countless
millions of cubic miles of space, are moved by unknown forces like
clouds before the summer breeze; star-mist is condensing into clusters;
star-clusters are forming into suns; streams and clusters of minor orbs
are swayed by unknown attractive energies; and primary suns singly or
in systems are pursuing their stately path through space, rejoicing as
giants to run their course, extending on all sides the mighty arm of
their attraction, gathering from ever-new regions of space supplies of
motive energy, to be transformed into the various forms of force—light
and heat and electricity—and distributed in lavish abundance to the
worlds which circle round them.
Truly may I say, in conclusion, that whether we regard its vast extent,
its infinite variety, or the amazing vitality which pervades its every
portion, the sidereal system is, of all the subjects man can study,
the most imposing and the most stupendous. It is as a book full of
mighty problems—of problems which are as yet almost untouched by man,
of problems which it might seem hopeless for him to attempt to solve.
But those problems are given to him for solution, and he _will_ solve
them, whenever he dares attempt to decipher aright the records of that
wondrous volume.
_MALLET’S THEORY OF VOLCANOES._
There are few subjects less satisfactorily treated in scientific
treatises than that which Humboldt calls the Reaction of the Earth’s
Interior. We find, not merely in the configuration of the earth’s
crust, but in actual and very remarkable phenomena, evidence of
subterranean forces of great activity; and the problems suggested seem
in no sense impracticable: yet no theory of the earth’s volcanic energy
has yet gained general acceptance. While the astronomer tells us of the
constitution of orbs millions of times further away than our own sun,
the geologist has hitherto been unable to give an account of the forces
which agitate the crust of the orb on which we live.
The theory put forward respecting volcanic energy, however, by the
eminent seismologist Mallet, promises not merely to take the place
of all others, but to gain a degree of acceptance which has not been
accorded to any theory previously enunciated. It is, in principle,
exceedingly simple, though many of the details (into which I do not
propose to enter) involve questions of considerable difficulty.
Let us, in the first place, consider briefly the various explanations
which had been already advanced.
There was first the chemical theory of volcanic energy, the favourite
theory of Sir Humphry Davy. It is possible to produce on a small scale
nearly all the phenomena due to subterranean activity, by simply
bringing together certain substances, and leaving them to undergo
the chemical changes due to their association. As a familiar instance
of explosive action thus occasioned, we need only mention the results
experienced when any one unfamiliar with the methods of treating
lime endeavours over hastily to “slake” or “slack” it with water.
Indeed, one of the strong points of the chemical theory consisted in
the circumstance that volcanoes only occur where water can reach the
subterranean regions—or, as Mallet expresses it, that “without water
there is no volcano.” But the theory is disposed of by the fact, now
generally admitted, that the chemical energies of our earth’s materials
were almost wholly exhausted before the surface was consolidated.
Another inviting theory is that according to which the earth is
regarded as a mere shell of solid matter surrounding a molten nucleus.
There is every reason to believe that the whole interior of the earth
is in a state of intense heat; and if the increase of heat with depth
(as shown in our mines) is supposed to continue uniformly, we find that
at very moderate depths a degree of heat must prevail sufficient to
liquefy any known solids under ordinary conditions. But the conditions
under which matter exists a few miles only below the surface of the
earth are not ordinary. The pressure enormously exceeds any which our
physicists can obtain experimentally. The ordinary distinction between
solids and liquids cannot exist at that enormous pressure. A mass
of cold steel could be as plastic as any of the glutinous liquids,
while the structural change which a solid undergoes in the process
of liquefying could not take place under such pressure even at an
enormously high temperature. It is now generally admitted that if the
earth really has a molten nucleus, the solid crust must, nevertheless,
be far too thick to be in any way disturbed by changes affecting the
liquid matter beneath.
Yet another theory has found advocates. The mathematician Hopkins,
whose analysis of the molten-nucleus theory was mainly effective in
showing that theory to be untenable, suggested that there may be
isolated subterranean lakes of fiery matter, and that these may be the
true seat of volcanic energy. But such lakes could not maintain their
heat for ages, if surrounded (as the theory requires) by cooler solid
matter, especially as the theory also requires that water should have
access to them. It will be observed also that none of the theories just
described affords any direct account of those various features of the
earth’s surface—mountain ranges, table-lands, volcanic regions, and so
on—which are undoubtedly due to the action of subterranean forces. The
theory advanced by Mr. Mallet is open to none of these objections. It
seems, indeed, competent to explain all the facts which have hitherto
appeared most perplexing.
It is recognized by physicists that our earth is gradually parting with
its heat. As it cools it contracts. Now if this process of contraction
took place uniformly, no subterranean action would result. But if the
interior contracts more quickly than the crust, the latter must in some
way or other force its way down to the retreating nucleus. Mr. Mallet
shows that the hotter internal portion must contract faster than the
relatively cool crust; and then he shows that the shrinkage of the
crust is competent to occasion all the known phenomena of volcanic
action. In the distant ages when the earth was still fashioning, the
shrinkage produced the _irregularities of level_ which we recognize in
the elevation of the land and the depression of the ocean-bed. Then
came the period when as the crust shrank it formed _corrugations_, in
other words, when the foldings and elevations of the somewhat thickened
crust gave rise to the mountain-ranges of the earth. Lastly, as the
globe gradually lost its extremely high temperature, the continuance
of the same process of shrinkage led no longer to the formation of
ridges and table-lands, but to local crushing-down and dislocation.
This process is still going on, and Mr. Mallet not only recognizes here
the origin of earthquakes, and of the changes of level now in progress,
but the true cause of volcanic heat. The modern theory of heat as
a form of motion here comes into play. As the solid crust closes in
upon the shrinking nucleus, the work expended in crushing down and
dislocating the parts of the crust is transformed into heat, by which,
at the places where the process goes on with greatest energy, “the
materials of the rock so crushed and of that adjacent to it are heated
even to fusion. The access of water to such points determines volcanic
eruption.”
Now all this is not mere theorising. Mr. Mallet does not come before
the scientific world with an ingenious speculation, which may or may
not be confirmed by observation and experiment. He has measured and
weighed the forces of which he speaks. He is able to tell precisely
what proportion of the actual energy which must be developed as the
earth contracts is necessary for the production of observed volcanic
phenomena. It is probable that nine-tenths of those who have read
these lines would be disposed to think that the contraction of the
earth must be far too slow to produce effects so stupendous as those
which we recognize in the volcano and the earthquake. But Mr. Mallet
is able to show, by calculations which cannot be disputed, that less
than one-fourth of the heat at present annually lost by the earth is
sufficient to account for the total annual volcanic action, according
to the best data at present in our possession.
As I have said, I do not propose to follow out Mr. Mallet’s admirable
theory into all its details. I content myself with pointing out how
excellently it accounts for certain peculiarities of the earth’s
surface configuration. Few that have studied carefully drawn charts
of the chief mountain-ranges can have failed to notice that the
arrangement of these ranges does not accord with the idea of upheaval
through the action of internal forces. But it will be at once
recognized that the aspect of the mountain-ranges accords exactly with
what would be expected to result from such a process of contraction
as Mr. Mallet has indicated. The shrivelled skin of an apple affords
no inapt representation of the corrugated surface of our earth,
and according to the new theory, the shrivelling of such a skin is
precisely analogous to the processes at work upon the earth when
mountain-ranges were being formed. Again, there are few students of
geology who have not found a source of perplexity in the foldings and
overlappings of strata in mountainous regions. No forces of upheaval
seem competent to produce this arrangement. But by the new theory this
feature of the earth’s surface is at once explained; indeed, no other
arrangement could be looked for.
It is worthy of notice that Mr. Mallet’s theory of Volcanic energy
is completely opposed to ordinary ideas respecting earthquakes and
volcanoes. We have been accustomed vaguely to regard these phenomena
as due to the eruptive outbursting power of the earth’s interior; we
shall now have to consider them as due to the subsidence and shrinkage
of the earth’s exterior. Mountains have not been upheaved, but valleys
have sunk down. And in another respect the new theory tends to modify
views which have been generally entertained in recent times. Our
most eminent geologists have taught that the earth’s internal forces
may be as active now as in the epochs when the mountain-ranges were
formed. But Mr. Mallet’s theory tends to show that the volcanic energy
of the earth is a declining force. Its chief action had already been
exerted when mountains began to be formed; what remains now is but
the minutest fraction of the volcanic energy of the mountain-forming
era; and each year, as the earth parts with more and more of its
internal heat, the sources of her subterranean energy are more and more
exhausted. The thought once entertained by astronomers that the earth
might explode like a bomb, her scattered fragments producing a ring of
bodies resembling the zone of asteroids, seems further than ever from
probability; if ever there was any danger of such a catastrophe, the
danger has long since passed away.
_TOWARDS THE NORTH POLE._