Nathaniel Hillyer. Egleston.

The North American review (Volume 139) online

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fact, both processes are but different forms of the same kind of
work. Precisely as we say that the sun can communicate a
velocity of three hundred and eighty-two miles per second to a



THE ORIGIN OF COMETS. 115

body approaching him from interstellar distances, and that
therefore the sun can withdraw such velocity from a body
leaving his surface at that rate, and eventually bring such a
body to rest out yonder in interstellar space, so can we make a
corresponding statement for any planet, Jupiter or Saturn,
the Earth, our Moon, and even for the least of all, the asteroidal
family (supposing only the mass and size known). In the case
of Jupiter, for instance, we find that the utmost velocity he can
impart to a body reaching him from external space is about
thirty-six miles per second. Thatj at least, is the velocity with
which such a body would reach the visible surface of the planet.
What the velocity might be with which the real surface, far
down below the visible envelope of clouds, would be reached,
we do not know, not knowing where that surface lies. In the
case of our own earth, the velocity with which a body would
reach the surface, if brought thither solely by the earth's action
from interstellar space, would be a little over seven miles per
second, or more than twenty-seven times greater than the veloc
ity of the swiftest cannon-ball.

But while Jupiter to keep for the moment to our giant
planet has thus, theoretically, the power of giving or taking
away a velocity of thirty-six miles per second, he is not practi
cally able to do anything of the sort. He is not left to draw
matter to himself, or to act on the recession of matter from
himself, alone. The bodies which come near to him from outer
space have been drawn by solar might within that distance from
the sun, and almost the whole velocity they there possess is sun-
imparted. We have seen what it is, some eleven miles per
second. Now, manifestly, this greatly affects Jupiter's power
of imparting or withdrawing velocity. Both processes require
time, and it is clearly impossible for Jupiter to produce any
thing like the same effect on a body rushing past him with a
cun-imparted velocity of eleven miles per second as he would
produce on a body left undisturbed to his own attraction.
Jupiter's action at any moment is the same whether the body is
moving or at rest ; but the number of moments is very much
reduced owing to the swift rush of the body past the planet.
To use the old-fashioned expression of the first students of
gravitation (an expression which has always seemed to me
amusingly quaint) the solicitations of Jupiter's attractive force
are as urgent on a swiftly rushing body as on one at rest ; but



116 THE NORTH AMERICAN REVIEW.

if a body will not stay to hearken to them much less effect
must be produced. In all this part of my reasoning, I may
remark, I am not pleading a cause, but indicating what every
student of celestial dynamics knows.

"We may fairly regard twenty-five miles per second as the
utmost velocity that Jupiter can impart or take from any body
coming out of interplanetary space past him, as close as such a
body can pass without being actually captured. Moreover, in
every possible case, Jupiter can only abstract or add a small por
tion of this amount ; for this reason, simply, that in every pos
sible case there will be first an action of one kind (abstraction or
addition of velocity), and afterward an action of the opposite
kind (addition or abstraction respectively). It will be but the
difference between these effects, in most cases very nearly
equal, which will actually tell on the body's future period of revo
lution around the sun.* This makes an enormous reduction on
Jupiter's potency to modify cometic revolution. Certainly ten
miles per second is a very full estimate of the velocity he can
abstract or add in the case of a body passing quite close to his
apparent surface.

But even this may seem ample. Seeing that a loss of three
miles or so per second would cause a body which had reached
Jupiter's distance from the sun, after a journey from out of
interplanetary space, to travel in the same period around the
sun as Jupiter himself, and since we seem to recognize a power
in Jupiter to abstract ten miles per second, it would seem as
though Jupiter's capturing power were in fact demonstrated.

But while, to begin with, the close approach required for
this capturing power to exist is something very different from
that approach within a million miles which I before considered,
there is a much more important difficulty to be considered, in
the circumstance that we have thus far dealt with Jupiter's
capturing power on one body, not on a flight of bodies, such as a
comet approaching from interstellar space is held to be, accord
ing to the theory I am discussing. Let us take the former point,
though the least important, first.

At Jupiter's apparent surface the actual maximum velocity
which the planet could give to a body approaching from a

* As distinguished from the orbit. The orbit might be largely affected
even in a ease where the velocity at Jupiter's distance remained absolutely
unchanged ; but in this case the period of revolution would remain the same.



THE ORIGIN OF COMETS. 117

practically infinite distance would be about thirty-six miles per
second, and we reduced the actual maximum effect on a body
passing Jupiter very close, under such conditions as actually
prevail in the solar system, to ten miles per second. Let us see
what would be the corresponding numbers in the case of a body
passing within a million miles of him, remembering that even
that would carry such a body right through Jupiter's system of
satellites, the span of that system being about four and a half
millions of miles. Since a distance of one million miles exceeds
the distance of Jupiter's surface from his center nearly twenty-
five times, it follows (I need not explain why, mathematicians
will know, and for non-mathematicians the explanations would
be tedious and difficult) that the velocities which Jupiter can
give or abstract at the greater distance would all be reduced to
little more than one-fifth those determined for Jupiter's surface.
So, instead of ten miles per second, we should get but two miles
per second, as the greatest Jupiter could abstract from a body
approaching him within a million miles. And this would not
be sufficient reduction to make such a body travel thenceforth
in Jupiter's period, still less in one of the much shorter periods
observed throughout what has been called Jupiter's comet-
family.

But the other difficulty is altogether more serious. A comet
approaches Jupiter, on the theory we are dealing with, and
indeed the same may be assumed on any theory, as a flight of
scattered bodies. Either this flight is so close as to be in effect,
because of mutual attractions, a single body, or it is not. If it
is, the flight will not be broken up by Jupiter's action j and, if
not so broken up, will remain forever after a united family.
But if, as is more in accordance with observed facts, the cometic
flight is so large that the attraction of the flight, as a whole, on
the separate members, can be overcome by Jupiter's action, then
not only will the flight be broken up, but the orbits given to
different members of it by Jupiter's disturbing action will be
widely different. Suppose, for example, the extent of the flight
to be such that the parts coming nearest to Jupiter approach
his center within fifty thousand miles (a very close approach,
indeed, to his surface), while those parts which are remotest
from him at the time when the flight, as a whole, is nearest,
came only within sixty thousand miles from his center. Then,
in round figures, the reduction of velocity of the nearer members
VOL. cxxxix. NO. 333. 9



118 THE NORTH AMERICAN REVIEW.

of the flight will be greater than the reduction for the farther
members, as six exceeds five. Supposing, for argument's sake,
the former reduction to be three miles per second, as it must be
to make those members of the flight travel thenceforth in
Jupiter's period round the sun, then the reduction for the
outermost members would be but three and a half miles per
second j or thenceforth one set of meteors formerly belonging
to the comet would have at Jupiter's distance a velocity of eight
miles per second (eleven less three), while another set would
have a velocity of eight and a half miles per second (eleven less
two and a half) at that distance. This means that thenceforth
the mean distance of the latter set from the sun would exceed
the mean distance of the former set about as nine exceeds eight.*
Since the former set would thenceforth be traveling at Jupiter's
distance, or about 5.2 times the earth's, the latter set would be
traveling at a mean distance greater by one-eighth of this, or .65
of the earth's distance, say some sixty millions of miles. The
latter set would be at their nearest to the sun when at Jupiter's
distance, would pass sixty millions of miles farther away to their
mean distance, and as much farther away still at their greatest
distance. Practically, then, even in this case, as favorable for
capture as can be well imagined, the capture, though effected,
would result in spreading out the comet, which had arrived as
a compact flight of meteors ten thousand miles only in span,
over a region one hundred and twenty millions of miles broad.
It is hardly necessary to say that nothing like this is observed
in the case of any member of Jupiter's comet-family. We know
that along their track meteors are strewn to distances which, in
some cases, may well exceed even the enormous distance just
named ; but they lie along the track, not ranging more than a
few hundred thousand miles on either side from the path of the
comet's head. This means that the orbit of every single meteor
of such a system has, practically, the same mean distance from
the sun.

The difficulty last considered is simply fatal to the theory
that the comets forming what have been called the comet-
families of the giant planets were captured by those orbs in the

* The simple law is, that for two bodies having different velocities at the
same distance from the sun, the mean distances from him differ as the
square of those velocities. Now, the square of eight and a half is seventy-
two and a quarter ; that of eight is sixty-four.



THE ORIGIN OF COMETS. 119

way imagined by Heis, Schiaparelli, and others. "We must seek
for a different explanation, if we are to account for the peculiar
relations of these comet-families at all. It may be that the
peculiarity, like many others presented by comets, may not
admit of being explained. The considerations I am about to
advance may to many appear not altogether convincing ; never
theless, as they involve the study and discussion of known facts,
they are worth investigating, quite apart from all questions of
the validity of the theory with which I associate them.

Observing that the giant planets have each their comet-
family, we may safely infer that the sun also has his special
family of comets ; that is, a family the dominion of which he
does not in any sense share with the giant families. The comets
which we should thus regard as specially solar are those whose
paths approach exceptionally near to his globe. Among num
bers of comets which come from out of interstellar space toward
the sun, and, sweeping around him, pass away again into the
depths from which they came, many have paths passing so far
from his globe that we cannot regard them as in any special
way associated with him. Bodies coming casually, so to speak,
from outside regions would have just such paths. So that of
many comets, not belonging to the comet-families of the giant
planets, we may say that neither do they belong to the comet-
family of the sun. Yet even these teach something. Whatever
theory we adopt as to the origin of comets, it must give an
account of these comets, as well as of those which, passing very
near to the globe of the sun, may be regarded as belonging
specially to him, and those others which we assign as the special
dependents of the giant planets.

Now, taking the two last-named classes, we recognize in the
movements of the members of each class evidence of the intro
duction of these comets into the solar system, through the
intervention, in some way, (1) of the giant planets in the case of
one class, and (2) of the sun in the case of the other class. "We
have seen that the giant planets could not have introduced their
comet-families from out of interstellar space by perturbing
influences. We may infer with almost equal probability, or
almost with certainty, that neither did the sun introduce his
comet-family by drawing them from out of interstellar space.

Since, then, the sun and the giant planets did not introduce
their special comet-families from interstellar space, yet did most



120 THE NORTH AMERICAN REVIEW.

manifestly introduce them in some way, where else can these
comets have come from but from within the orbs of the sun and
of the giant planets respectively ?

At first sight this theory seems so strange and fanciful that
we are almost deterred from examining it further by its apparent
grotesqueness. We seek about for a way of escape from so wild
a theory. We look back to a remote period when, in accordance
with the ideas of Laplace, the sun's mass extended far beyond
the present orb of the sun, and the giant planets also had orbs
extending even as far as the orbits of their outermost satellites.
Undoubtedly, if a flight of meteors in that far distant period
rushed through the outer vaporous surroundings either of sun
or of giant planets, the effects imagined by Schiaparelli and by
Heis might have been produced. The diminution of the veloci
ties of the meteors forming such a flight might well be far
more effective than in the case we have hitherto considered,
of free space around a planet's globe.

But we may regard this theory respecting the introduction
of comets into the solar system as one which may wait its turn
until the other, of ejection, strange and fanciful though it may
seem, has been examined. For there is nothing in the capture
theory, considered in itself, to invite us specially to its adoption.
It gives no account whatever of the actual origin of comets. It
only suggests how, having somehow come into existence in
interstellar space, comets would be drawn sunward, and might
be captured by the sun or by planets. If to this inherent diffi
culty in Schiaparelli's theory we are to add all the difficulties
involved in the supposition that the sun and the giant planets
were once much larger than they now are, and that being thus
large they were able to capture comets by actual interruption of
their movements, we may at least consider that before discuss
ing such views, before attempting to carry back our thoughts
over the practically interminable time intervals involved in
such a process, it may be well to examine a theory which, though
startling at a first view, promises to explain something more, if
confirmed, than the scarcely less startling theory of comet cap
ture by expanded sun and by expanded planets.

Suppose that instead of looking into remote regions of space,
and toward far off periods of time, we examine meteoric masses,
and inquire of them whence they came. We cannot expect each
meteorite to have a story to tell j but after a goodly number



THE ORIGIN OF COMETS. 121

have been examined, we may light upon one speaking with toler
able clearness respecting its origin. Our first studies shall be
with the microscope.

Now, passing over a number of microscopic studies of
meteorites which are suggestive enough, but not decisive, we
come on the strange fact that certain meteorites show under the
microscope the clearest evidence of having once been in the
form of tiny globules of molten metal, numbers of which have
become agglomerated together. The eminent microscopist and
mineralogist Sorby, of Sheffield (England), asks respecting these
particular meteors, where else could they possibly have existed
in the form of metallic globules (liquid) except in the interior of
a body like the sun ? In the interstellar spaces intense cold pre
vails. In rushing close past the sun a meteoric mass might be
molten, but would scarcely be vaporized, even though the orbit
of the flight passed very near the sun's surface. But the
meteorites which have visited our earth have not been associated
with comets passing near to the sun. Manifestly the chances
are very small that any meteorite following in the train of a
comet like Newton's or the comet of 1843 that is, a comet trav
eling close past the sun would ever reach the earth. But
Sorby found microscopic evidence such as I have described in
quite a large number of meteorites which he examined.

At any rate, the assumption for the moment, that such
meteorites had their origin within the interior of a body like our
sun, accords well with the theory we have had suggested to us,
that comets and meteor flights (kindred bodies) came from within
the orbs with which we still find them associated.

Turn now to the chemical analysis of meteorites. Here the
evidence is perhaps even more suggestive. Masses of meteoric
iron being placed under the air-pump, hydrogen which had been
present in their substance occluded in the iron, as it is technic
ally expressed has come out in such quantities that Professor
Graham (of London) considers the amount fully six times as great
as could be occluded in the substance of iron by any process
known to chemists or physicists. This Lenarto meteor, he says,
has brought to us across the interstellar spaces the hydrogen of
the fixed stars. In other words, Professor Graham could see no
other interpretation of the presence of so much hydrogen within
the substance of this mass of meteoric iron than that the
hydrogen had been forced into the iron while yet within the



122 THE NORTH AMERICAN REVIEW.

interior of a star. We know that beneath the visible surface of
our sun there must be both the vapor of iron and hydrogen at
enormous pressure. Under such conditions alone could masses
such as the Lenarto meteorite be formed. Professor Graham,
therefore, assumed confidently that the Lenarto meteorite and
others of the same sort were formed in the interior of a body
like our sun. He rejected, rightly, the idea that it was in our
sun himself that the meteorites of that class were formed. For
the chance of any meteorite ejected from the sun reaching our
earth is but about as one in twenty-two hundred millions. The
greater number of the sun-ejected meteorites he saw must have
beenejectedfrom the interior of the other suns which people space.
There are hundreds of millions of such suns even within the
range of telescopic vision ; millions of millions doubtless exist ;
so that if we once admit the possibility of the ejection of meteoric
masses from within a sun or star, we recognize the probability,
or rather the certainty, that there must be billions of billions of
such masses traveling amid the interstellar spaces.

All this was reasoned out thus before it had been shown that
suns ever do eject masses with sufficient energy to carry them
beyond the attractive influences of their parent orbs ; nay, Sorby
and Graham expressed their views respecting the origin of some
meteorites when it seemed utterly unlikely that we ever should
get evidence of stellar eruptive powers which that theory re
quires.

But such evidence has now been obtained. Professor Young,
of Princeton, N. J. (then of Dartmouth, N. H.), was the first, in
1872, to obtain evidence of the actual ejection of matter from
the sun's interior with velocities sufficing to carry such matter
forever away from him ; but the evidence was decisive, and since
then kindred observations have been frequently made. What
Young saw, indeed, was apparently the ascent of filaments of
hydrogen, at an average rate of nearly two hundred miles per
second j but it was easy to see that the irregular streaks of
hydrogen were not themselves the ejected matter. If a thin
gas like hydrogen could rush through the region immediately
above the sun's visible surface at the rate of two hundred miles
an hour, which I reject as incredible, the shape of such
hydrogen missiles would be such as to indicate very clearly the
resistance they were encountering. They would be pear-shaped,
the rounded part of the pear in front, like fire-balls in our air.



THE ORIGIN OF COMETS. 123

But these were irregular streaks, like the luminous tracks of
meteors, and such doubtless they were. A flight of masses of
considerable density must have been shot out on that occasion,
and on other occasions when similar phenomena have been
observed, and rushing through the hydrogen in the sun's neigh
borhood, caused the gas to glow along their track, just as fire
balls in our air leave behind them long luminous trails. The
rate at which these missiles advanced could be inferred from the
rate at which the luminous trails followed them. Calculation,
in which the sun's retarding action was taken duly into account,
showed that the matter thus expelled from the sun left his sur
face at a rate of not less, probably, than five hundred miles per
second. The ejected matter left the sun, then, never to return,
and in the form of precisely such a flight of meteoric missiles as
microscopic and chemical researches had shown to be traveling
through the interstellar spaces.

When we consider the three lines of evidence, and note how
independent they are of each other, we see that the theory of the
ejection of masses akin to meteors from the suns which people
space is rendered all but certain independently of any line of
d, priori reasoning which had led us to look for evidence of such
processes. Certain meteors have shown under microscopic study
that they were certainly once in a condition such as could hardly
exist except in the interior of a body like the sun ; others have
shown under chemical analysis that they must have been ejected
from the interior of a sun ; and now we have evidence showing that
from our sun, and therefore presumably from his fellow-suns, the
stars, flights of missiles akin to meteoric bodies are ejected from
time to time with velocities sufficient to carry them into interstellar
space. It seems reasonable to infer that here we have the solu
tion of our difficulty 5 we see that the sun, at any rate, has
power to eject at times from his interior flights of meteoric
masses, such as we recognize in the streams of meteors which
exist within the solar system, and that the velocity of outrush
is in some cases so enormous that the masses thus ejected can
never return to the sun, but pass away through interstellar space.
We find also that meteoric streams, which we are thus led to
associate with the solar eruptions, are also associated with
comets, every known meteoric stream traveling, probably (as
many certainly do), in the track of a comet. Now, knowing the
small masses of many comets, it is no very wild thought to sug-



124 THE NORTH AMERICAN REVIEW.

gest that those comets whose present orbits carry them close to
the sun were originally expelled from his own interior.
Assuredly the flights of missiles which we know to be at times
driven from his interior are in all respects akin to what we
know many comets actually to be, akin in structure, akin in
mass, and akin probably in condition. For in whatever respects
the coma and tail of a comet may seem unlike mere meteoric
masses, we know that such peculiarities of condition are due to
solar action, and that a flight of meteoric masses ejected from
the sun himself would as certainly present these peculiarities
under subsequent solar influences as any other flight of meteoric
masses not ejected originally from the sun.

May not this reasoning be extended to the giant planets, either
in their present demonstrably somewhat sunlike state, or in those
past stages of their career when they were veritable suns, though
small ones ? In the great red spot of Jupiter, however, we have
had evidence of even a present intensity of eruptive action by
which meteoric and come tic matter might well have been ejected
in such sort as to pass forever beyond the control of the giant
planet. At any rate, the great disturbance suggests, by parity
of reasoning, that within comparatively recent times Jupiter
and Saturn have possessed the necessary expulsive power. It
must be remembered that thus to eject matter with velocities



Online LibraryNathaniel Hillyer. EglestonThe North American review (Volume 139) → online text (page 12 of 60)