TEMPORARY STARS, METEORS, AND COMETS.
What, then, is the probable cause of these terrific conflagrations,
as they appear to us? Take an ordinary electric induction machine,--a
Holtz or a Wimshurst,--and, if the surrounding air is moist, as we
operate it we will find that the results are poor, the sparks short
and relatively few; but let us take the machine into another room
in which the atmosphere is dry and crisp. A wondrous change will
occur, and instead of a current which could scarcely flash across
a few inches of space, we will now have so great an increase of
energy that its tension will even cause the spark to perforate and
destroy the glass walls of the heavy Leyden jars in which it is
condensed. The vast realms of space, with their attenuated vapors,
are the field in which the planetary electric generators operate,
and into which, likewise, myriads of suns constantly pour their
light and heat. We may consider this space, according to the popular
view, to be uniform in constitution and density throughout all its
parts,--that it is, in fact, like a vast, silent, and motionless dead
sea. But this cannot possibly be true, any more than throughout the
vast compass of our own atmosphere; for while some parts of space are
peopled by millions of solar systems, others, as we can plainly see,
so far as telescopic vision extends, are comparatively vacant. Far
more electricity is being abstracted (so to speak) in some parts of
space than in others, and far more heat and light are being poured
back to restore the equilibrium in some than in others. We have
already seen that the temperature at the exterior surface of the
terrestrial atmosphere is estimated to be more than two hundred
degrees higher than in the realms of open interplanetary space;
hence there must be currents,--currents of rotation like cyclones,
vortical currents like whirlwinds, currents of transmission like our
land- and sea-breezes and the trade-winds,--and, in fact, all space
must be in a state of constant displacement and replacement, and, if
visible, we should see it like a vast room filled with smoke, in which
currents of every shape and direction and of all velocities would be
manifest. Such currents could throw nebulæ during their condensation
into rotation which could never rotate of their own motion, or gather
to centers of aggregation vast whirling clouds of spatial matter,
and in the spiral nebulæ we may see many such movements of rotation in
apparent active progress. Of these we read in Appleton's Cyclopædia,
"They have the appearance of a maelstrom of stellar matter, and are
among the most interesting objects in the heavens." In Professor
Nichol's splendid work ("The Architecture of the Heavens," 1850)
we may see magnificent engravings of these wonderful phenomena,
from the drawings by Lord Rosse, and no one can study these figures
without realizing the presence of vast currents in space.
In the great spiral nebula in the constellation Canes Venatici (see
illustration in Chapter XII.) we perceive that the tail of the smaller
nebula has been drawn into the outer convolution of the great spiral,
against the radial repulsion of the latter nebula, as we can see by
its curvature. This can only be due to a tremendous inflowing current
in space. Were the deflection due to gravity the trend would be to the
center and not to the outer convolution of the larger nebula. Professor
Nichol says, "The spiral figure is characteristic of an extensive
class of galaxies." Not only in the spiral, but in other forms of
nebulæ we may observe these currents of space, so that we cannot fail
to perceive that they exist, and we should even conclude, a priori,
that these must exist.
In the elongated linear nebula in Sobieski's Crown, illustrated
above, its length is deflected into irregular curves apparently due
to counter-currents of space. These gaseous nebulæ, Flammarion says,
"appear like immense vaporous clouds tossed about by some rough winds,
pierced with deep rents, and broken in jagged portions." It may be
said generally that every sun, as it drifts through space, must leave
a wake of increased electric potential among the molecules which
line its pathway. Beyond the limits of every vortex extend radial
or tangential, polar or equatorial, streams of space, and these
must extend without limit until deflected or neutralized by other
conditions. Throughout all space, just as in our own atmosphere,
but vastly more slowly, there must be an infinitude of movements in
every direction,--movements in lines, circles, vortices, ellipses and
irregular curvatures, and of all possible varieties of mass and volume.
Suppose, now, a sailing vessel lighted with incandescent lamps,
the electrical currents for the support of which are derived from
the chemical action of sea-water on multiple pairs of suitable
metallic plates arranged to extend downward as a galvanic battery
into the ocean as the ship sails along, and that these plates,
by the chemical action of the sea-water at ordinary, temperatures,
should furnish a sufficient current to properly light the vessel. If
the constancy of such current depended on the average temperature
of the sea-water, at, say, sixty degrees Fahrenheit, we should find
that, on suddenly crossing into the Gulf Stream, with a temperature
twenty degrees higher, the energy of the battery would be rapidly
increased and the lights would glow with increased brilliancy until,
on emerging from the Gulf Stream at its opposite side, the original
status would be gradually restored. If these distant solar systems,
in their drift through space, should encounter a corresponding stream
under an increased molecular tension, more highly heated, for example,
or charged with electrical potential by the surrounding solar systems,
or otherwise, we should expect a similar result to ensue,--that the
currents would be increased suddenly, both in quantity and intensity,
and all the phenomena of "blazing" stars be revealed in the precise
order in which we see them. Professor Proctor seems to have had
some such idea of space vaguely in his mind when he says, in his
"Familiar Essays," "One is invited to believe that the star may have
been carried by its proper motions 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 found
so perplexing,--the former existence of a tropical climate in places
within the temperate zone, or even near the arctic regions. Sir John
Herschel long since pointed to the variation of the sun as a possible
cause of such changes of climate." In confirmation of the view that
such changes may be due to the passage of a solar system into or
through such a "Gulf Stream" of space, we quote the following from
Professor Proctor's "Suns in Flames:" "It is noteworthy that all the
stars which have blazed out suddenly, except one, have appeared in
a particular region of the heavens,--the zone of the Milky Way (all,
too, in one-half of that zone). The single exception is the star in
the Northern Crown, and that star appeared in a region which I have
found to be connected with the Milky Way by a well-marked stream
of stars; not a stream of a few stars scattered here and there, but
a stream where thousands of stars are closely aggregated together,
though not quite so closely as to form a visible extension of the Milky
Way.... Now, the Milky Way and the outlying streams of stars connected
with it seem to form a region of the stellar universe where fashioning
processes are still at work." In just such regions of potential energy
should we look for such currents in space, as, on our own earth,
the Gulf Stream and the trade-winds, as well as cyclones and other
atmospheric movements, find their origin under precisely parallel
circumstances,--to wit, the outpour upon and direct precipitation of
increased quantities of heat at the tropics or other local centers
of such development. The effects of such an increase of quantity and
potential in an electrical current are clearly illustrated in the
device previously referred to, in which electrolytic decomposition
was effected in a pail of water; we find it also in the burning out
of the brushes and commutators in dynamo-electric machines and in
telegraphic apparatus during thunder-storms and the like. Allowing a
solar system a drift through space only equal to that of our own, which
has a relatively slow movement, it would traverse such a "Gulf Stream"
of space seven hundred thousand miles wide in a single day. But it may
not even have passed through; it may merely have grazed the margin of
such a current; for the motions of solar systems are not controlled by
the same forces as those upon which their electrical energies depend.
Professor Ball, in his chapter on the great heat-wave of 1892, says,
"Towards the end of July an extraordinarily high temperature, even
for that period of the year, prevailed over a very large part of the
North American continent. The so-called heat-wave then seems to have
travelled eastward and crossed the Atlantic Ocean; ... a fortnight
after the occurrence of unusually great heat in the New World there
was a similar experience in the Old World.... This discussion will at
all events enable us to make some reply to the question which has often
been asked, as to what was the cause of the great heat-wave.... It is,
however, quite possible that certain changes in progress on the sun
may act in a specific manner on our climate.... It cannot be denied
that local, if not general, changes in the sun's temperature must be
the accompaniment of the violent disturbances by which our luminary
is now and then agitated. It is, indeed, well known that there are
occasional outbreaks of solar activity, and that these recur in a
periodic manner; it is accordingly not without interest to notice that
the present year has been one of the periods of this activity. We
are certainly not going so far as to say that any connection has
been definitely established between a season of exuberant sun-spots
and a season remarkable for excessive warmth; but, as we know that
there is a connection between the magnetic condition of the earth
and the state of solar activity, it is by no means impossible that
climate and sun-spots may also stand in some relationship to each
other." These local deviations are doubtless due to planetary positions
with reference to the sun, but more general variations must depend upon
the constitution of such parts of space as the solar system may occupy;
but even then they will be but temporary, since the sun's volume will
rapidly expand or contract so as finally to restore the normal emission
of solar heat, as will be further explained later on in this work.
There are other causes also, readily conceivable, for such increased
electrical action; for instance, in that thickly-peopled region of
space, two solar systems adjacent might easily have their exterior
planets so related to each other as suddenly, at their points of
nearest approach, to cause one or more to direct an abnormally large
electrical current into the sun of the adjacent system; this would
correspond in electric energy, in fact, to a violent "perturbation"
in its orbit by the action of gravity produced by a neighboring
planet or system. No reversal of polarity could take place between
these planets under these circumstances, any more than between the
earth and the moon. In some portions of the Milky Way, doubtless,
suns blaze by dozens across the sky at night, and by day as well,
to which, in our more solitary skies, we are strangers. Revolving
in perfect harmony, perturbations must nevertheless be frequent, and
to what limits they may there be confined we shall never know until
we realize the extent of these galaxies and the relative contiguity
of their solar systems to each other. It is enough to show how such
variations may occur; in what particular way they do occur does not
affect the question of their origin. Even if such increased energy
were to continue by permanently increased planetary action, it is not
necessary to suppose that a corresponding permanent increase of light
and heat would result on the part of the sun, for its density is such
(only one-fourth that of the earth) that, under the tremendous force
of its gravity (twenty-seven and one-tenth times that of the earth),
its constituents cannot be maintained in solid form, but must be,
as before stated, either liquid or gaseous, and perhaps in part
both. Now, as it has been computed that the sun, by contraction to
its present density, would have evolved its present light and heat
for a period of millions of years, it is obvious that any increase in
its present volume, without increase of mass, would produce precisely
opposite and compensated results, so that the sun could receive from
outside sources as much heat as would expand its present volume
to that at the initial point of such assumed condensation without
increased emission of light and heat. The sun is thus, in effect,
a self-compensating machine, and its passage through a region of
increased electrical generation would first manifest itself in
a vast increase of brilliancy, due to higher incandescence of its
hydrogen envelope; this, in turn, would be communicated to the deeper
structures of the sun, producing increased volatilization and dark
absorption bands, and finally to the whole solar mass, expanding
its volume in proportion to the heat absorbed. Hence we should see
precisely the phenomena that we do see in flaming stars or so-called
new stars. We find such compensations all through nature, and it is
simply in accordance with her universal laws that they occur. It is a
singular circumstance that the catastrophe which is foretold in the
biblical record as the termination of all human life on earth, for
the present cycle, at least, should be almost literally in accordance
with the phenomena characteristic of such an increase of solar energy,
and one produced in some such manner. If the temperature of the solar
atmosphere were rapidly raised by increased planetary action to a
point which would reverse the lines of hydrogen from dark to bright,
say to a brightness eight hundred times that of the normal, as in
the case of the temporary star cited, though the heat would not, of
course, be increased in any such proportion, yet the heavens would be
indeed rolled up as a scroll, and all life would be extinguished in a
very brief period. But the planets would continue to roll along their
orbits, the integrity of the earth's mass would still be intact, and
after a few days or weeks the sun would begin to decline in brightness,
the volatilized vapors would slowly recede within the solar atmosphere,
and the temperature would gradually fall again to its normal, leaving,
however, a lifeless world to roll on its way henceforth, but as bright
and cheerful in all its possibilities, when the former conditions had
gradually become restored, as before. Perhaps some distant astronomer
in the neighborhood of Sirius--if we shall have travelled so far away
by that time--might send a note to the morning papers to announce
that the temporary star near Alpha Centauri had again receded to the
tenth magnitude. In due time--perhaps a thousand years--all would be
ready for a new development of life, and the cycle would continue as
before. Perchance, too, in some deep abyss, or buried far beneath
the surface, some germs of life might still continue to exist;
and from these, like the seeds resurrected from buried mummies,
a new life might again begin, guided along once more through vast
ages in a progressive ascent from development to development until,
in some new and strange forms, the higher types of life might again
appear. To these there would indeed be revealed a new heaven and a
new earth. Who knows how many such cycles of life may have come and
gone on earth, in which, like the dwellers of Jerusalem, new peoples
have built new cities, one above another, upon the unknown graves of
the past? In the words of Tennyson,--
"A wondrous eft was of old the Lord and Master of earth,
For him did his high sun flame, and his river billowing ran,
And he felt himself in his force to be Nature's crowning race.
As nine months go to the shaping an infant ripe for his birth,
So many a million of ages have gone to the making man:
He now is first, but is he the last?"
Whatever the coming, the progress, or the going of life on earth,
the course of our solar system will go on the same, the processes of
creation unchanged and her mechanism unimpaired. It is obvious that
no such conditions could prevail in the return to unorganizable chaos
which must be the consequence of any possible planetary collisions
in space. No conceivable process of creation could return a system
disrupted into meteorites to an operative solar system again. Even
the nebular hypothesis contemplates nothing of that sort as, by the
wildest conjecture, ever possible. But with us the danger is far
distant. Professor Proctor says, in his article "Suns in Flames,"
"As Sir William Herschel long since pointed out, we can recognize in
various parts of the heavens various stages of development, and chief
among the regions where as yet nature's work seems incomplete is the
Galactic zone,--especially that half of it where the Milky Way consists
of irregular streams and clouds of stellar light. As there is no reason
for believing that our sun belongs to this part of the galaxy, but,
on the contrary, good ground for considering that he belongs to the
class of insulated stars, few of which have shown signs of irregular
variation, while none have ever blazed suddenly out with many hundred
times their former lustre, we may fairly infer a very high degree of
probability in favor of the belief that, for many ages still to come,
the sun will continue steadily to discharge his duties as fire, light,
and life of the solar system." The passage of our system through
gradually changing regions of space, as contrasted with streams
or vortices, could not affect our sun's light even temporarily, as
the contraction and expansion of its volume would fully compensate
for any such gradual or partial variation, and, by position, he is
far from likely to pass into any of those whirlpools or torrents of
space which seem to mark at irregular intervals the region of the
irregularly variable stars.
Allied in appearance to such stars which suddenly flame out in space,
but totally different in reality, are comets. These strangers to our
own system have excited the wonder and astonishment of mankind from
the earliest ages. They seem to defy all rules and all explanation;
but, when properly examined, they will fall inevitably into the
general scheme of the source and mode of solar energy which we
have endeavored to present. These bodies enter our solar system
from without. Appleton's Cyclopædia says, "Schiaparelli, to whom
the discovery is in part due, considers the meteors to be dispersed
portions of the comet's original substance,--that is, of the substance
with which the comet entered the solar domain." Professor Proctor,
"Meteoric Astronomy," says, "A word or two may be permitted on the
question of the condition of comets freshly arriving on the scene of
the solar system. It is assumed sometimes that the train of meteors
already exists when the comet first comes within the solar domain." In
the "Romance of Astronomy" (R. Kalley Miller, M.A.) it is said,
"In a sort of debatable territory between our own solar system and
the infinite stellar universe around we come upon these erratic
and anomalous bodies--the comets; some of which have accidentally
become permanent attendants upon our sun; others have only paid it a
single casual visit in the course of their wanderings through space,
and are not likely again to come within the range of its attracting
influence; while countless millions are doubtless scattered throughout
the realms of the infinite, whose existence will never be revealed
to human ken at all." Professor Helmholtz, in fact (see addendum
to his lecture on the origin of the planetary system), advanced the
idea in a speculative way, that our terrestrial life might have had
its origin in one of these meteoric bodies by the "transmission of
organisms through space." In Professor Proctor's article on comets
("Mysteries of Time and Space") he says, "The paths followed by
comets show no resemblance either to the planetary orbits or to each
other. Here we see a comet travelling in a path of moderate extent
and not very eccentric; then another which rushes from a distance
of two or three thousand millions of miles, approaches the sun with
ever-increasing velocity until nearer to him than parts of his own
corona (as seen in eclipses), sweeps around him with inconceivable
rapidity, and makes off again to where the aphelion of its orbit lies
far out in space beyond the most distant known planet,--Neptune. Some
comets travel in a direct, some in a retrograde path; a few near the
plane of the earth's orbit, many in planes showing every variety of
inclination. Some comets regularly return after intervals of a few
years; some after hundreds of years; others are only seen once or
twice, and then unaccountably vanish; and not a few show by the paths
they follow that they have come from interstellar space to pay our
system but a single visit, passing out again to traverse we know not
what other systems or regions.... When we have said that these objects
obey the law of gravity, we have mentioned the only circumstance--as
it would appear--in which they conform to the relations observed in
terrestrial and planetary arrangements. And even this law--the widest
yet revealed to man--they seem to obey half unwillingly. We see the
head of a comet tracing out systematically enough its proper orbit,
while the comet's tail is all unruly and disobedient.... The fact,
then, is demonstrated that two of the meteor streams encountered by the
earth are so far associated with two comets as to travel on the same
orbits. We may not unsafely infer that all the meteor systems are in
like manner associated with other comets. Nor is it very rash to assume
that all comets are in like manner associated with meteor systems."
Concerning the influence of gravitation of the planets, the same
author says ("Meteoric Astronomy"), "Now, the circumstances under
which a comet approaching the sun on a parabolic or hyperbolic orbit
can be thus affected must be regarded as exceptional. The planet's
influence must, in the first place, be very energetically exercised;
in other words, the arriving comet must pass very close to the planet,
for under any other circumstances the sun's influence so enormously
outvies the planet's that the figure of the cometic orbit would be
very little affected. Moreover, the planet's attraction must produce an
important balance of retardation. The planet will inevitably accelerate
the comet up to a certain point, and afterwards will retard it; the
latter influence must greatly exceed the former. To show how greatly
the comet must be retarded, it is only necessary to mention that the
actual velocity of the November meteors when they cross the orbit
of Uranus is less than one-third of the velocity with which Uranus
himself travels, but their velocity at the same distance from the
sun, when they were approaching him from some distant stellar domain,
exceeded the velocity of Uranus in his orbit in the proportion of about
seven to five.... It follows, not merely as a probable inference, but,
I think, as a demonstrated conclusion, that if the November meteors
came originally into our system as a comet travelling sunward from
infinity, then either that comet was very compact or else Uranus
captured only a small portion of the comet, the remaining portions
moving thenceforth on orbits wholly different from the path of the
November meteors.... No other planet than Uranus can have brought
about the subjection of this comet to solar rule." In his article on
comets he says, "It may be well here to consider a case in which some
active force (other than gravity) exerted by the sun seems to have
brought the destruction of a comet, or at least to have broken up
the comet into unrecognizable fragments." He refers to Biela's comet,
with an orbital period of six and two-thirds years, and a path which
was found to approach very near to the path of the earth. In 1832 the
comet crossed the earth's track several weeks before the arrival of
the earth at the same point without appreciable interference. On its
second return, in 1845-46, it was found to be divided into two comets
travelling side by side; in 1852 they reappeared, still divided,
and gradually diverging from each other. Since then they have never
reappeared, though diligently sought for at every period. Professor
Proctor adds, "It has been seen again, though not as a comet; nay, the
occasion on which it was seen in the way referred to was predicted,
and the prediction fulfilled, even in details. For a full account of
its reappearance--as a meteor stream--I refer the reader to my essay
on Biela's comet in 'Familiar Science Studies.'"
In Miller's "Romance of Astronomy" we read, "Encke's comet, which
possesses the smallest orbit of any connected with our system, is
sensibly drawing nearer and nearer to the sun at every revolution." In
Professor Proctor's "Cometic Mysteries," the author says, "We hear
it stated that the nucleus of a comet is made up of meteoric stones
(Professor P. G. Tait says--for unknown reasons--that they resemble
'paving stones or even bricks') as confidently as though the earth had
at some time passed through the nucleus of a comet, and some of our
streets were now paved with stones which had fallen to the earth on
such an occasion. As a matter of fact, all that has yet been proved
is that meteoric bodies follow in the track (which is very different
from the tail) of some known comets, and that probably all comets
are followed by trains of meteors. These may have come out of the
head or nucleus in some way as yet unexplained; but it is by no means
certain that they have done so, and it is by many astronomers regarded
as more than doubtful. The most important point to be noticed in the
behavior of large comets as they approach the sun is, that usually the
side of the coma which lies towards the sun is the scene of intense
disturbance. Streams of luminous matter seem to rise continually
towards the sun, attaining a certain distance from the head, when,
assuming a cloud-like appearance, they seem to form an envelope
around the nucleus. This envelope gradually increases its distance
from the sun, growing fainter and larger, while within it the process
is repeated and a new envelope is formed. This, in turn, ascends
from the nucleus, expanding as it does so, while within it a new
envelope is formed. Meanwhile the first one formed has grown fainter,
perhaps has disappeared. But sometimes the process goes on so rapidly
(a day or two sufficing for the formation of a complete new envelope)
that several envelopes will be seen at the same time,--the outermost
faintest, the innermost most irregular in shape and most varied in
brightness, while the envelope or envelopes between are the best
developed and most regular. The matter raised up in these envelopes
seems to have undergone a certain change of character, causing it no
longer to obey the sun's attractive influence, but to experience a
strong repulsive action from him, whereby it is apparently swept away
with great rapidity to form the tail. 'It flows past the nucleus,'
says Dr. Huggins, 'on all sides, still ever expanding and shooting
backward until a tail is formed in the direction opposite to the
sun. This tail is usually curved, though sometimes rays or extra
tails sensibly straight are also seen.'"
In "The Sun as a Perpetual Machine," Professor Proctor says,
"Take, again, the phenomena of comets, which still remain among the
greatest of nature's mysteries. We have reason to believe ... that
the nucleus of a comet consists of an aggregation of stones similar
to meteorites. Adopting this view, and assuming that these stones
have absorbed somewhere gases to the amount of six times their volume
(taken at atmospheric pressure), we may ask, What will be the effect
of such a mass of stones advancing towards the sun at a velocity
reaching in perihelion the prodigious rate of three hundred and
sixty-six miles per second (as observed in the comet of 1843), being
twenty-three times our orbital rate of motion?" Professor Ball says,
"One of the most important results of the great shower of 1866 was the
demonstration that the swarm of little bodies to which that shower
owed its origin was connected with a comet. The swarm was found, in
fact, to follow the exact track which the comet pursued around the
sun.... Of this connection between the cometary orbits and revolving
swarms of meteors many other instances could be cited. I may refer to
the remarkable lists published by the British Association, in which,
beside the name of the comet or the designation which astronomers had
affixed to it, the meteoric swarm with which the comet is associated
is also given.... On these grounds it appears to be perfectly certain
that the origin of the shooting stars which appear in swarms cannot
be disassociated from the origin of the comets by which those swarms
are accompanied." The author makes a distinction between such ordinary
shooting stars and meteorites, and attributes the appearance of the
latter on earth to masses thrown forth from some volcano somewhere, but
this has nothing to do with the special phenomena to be interpreted. It
may be said, however, that the presence of olefiant gas as one of
the occluded gases in a meteorite (four and fifty-five-hundredths per
cent., as stated by Professor Proctor, in his article "The Sun as a
Perpetual Machine"), and the remarkable fact, stated in the article
"Spectrum Analysis" in Appleton's Cyclopædia, that, in Winnecke's
comet of 1868, "the bands agree in position with those obtained as
the spectrum of carbon, by passing the electric spark through olefiant
gas, "would lead one to consider a cometic origin, for this particular
meteorite at least, to be highly probable. Professor Ball further says,
"There have been several instances in which a comet has approached
so close to a planet that the attraction between the two bodies must
have had significant influence on the planet, if the cometary mass
had been at all comparable with that of the more robust body. The
most celebrated instance is presented in the case of Lexell's comet,
which happened to cross the track of Jupiter. The effect upon this
body was so overwhelming that it was wrenched from its original path
and started afresh along a wholly different track." The same writer,
speaking of the tails of comets, says, "I have no intention to discuss
here the vexed question of the tails of comets. I do not now inquire
whether the repulsion by which the tail is produced be due to the
intense radiation from the sun, or to electricity, or to some other
agent. It is sufficient for our present purpose to note that, even if
the tails of comets do gravitate towards the sun, the attraction is
obscured by a more powerful repulsive force.... Nor do the directions
in which the comets move exhibit any conformity; some move round the
sun in one direction, some move in the opposite direction. Even the
planes which contain the orbits of the comets are totally different
from each other. Instead of being inclined at only a very few degrees
to their mean position, the planes of the comets hardly follow any
common law; they are inclined at all sorts of directions. In no respect
do the comets obey those principles which are necessary to prevent
constitutional disorder in the planetary system.... Now, all we have
hitherto seen with regard to comets tends to show that the masses of
comets are extremely small. Attempts have been made to measure them,
but have always failed, because the scales in which we have attempted
to weigh them have been too coarse to weigh anything of the almost
spiritual texture of a comet. It is unnecessary to go as far as some
have done, and to say that the weight of a large comet may be only
a few pounds or a few ounces. It might be more reasonable to suppose
that the weight of a large comet was thousands of tons, though even
thousands of tons would be far too small a weight to admit of being
measured by the very coarse balance which is at our disposal." In
the chapter "Visitors from the Sky," the same author says, "As such
a comet in its progress across the heavens passes between us and
the stars, those stars are often seen twinkling brilliantly right
through the many thousand miles of cometary matter which their rays
have to traverse. The lightest haze in our atmosphere would suffice
to extinguish the faint gleam of these small stars; indeed, a few
feet of mist would have more power of obstructing the stellar light
than cometary material scores of thousands of miles thick. It is
true that the central portions of many of these comets often exhibit
much greater density than is found in the exterior regions; still,
in the great majority of such objects there is no opacity, even in
the densest part, sufficient to put out a star. In the case of the
more splendid bodies of this description, it may be supposed that the
matter is somewhat more densely aggregated as well as more voluminous;
still, however, it will be remembered that the great comet of 1858
passed over Arcturus, and that the star was seen shining brilliantly,
notwithstanding the interposition of a cometary curtain millions of
miles in thickness. So far as I know, no case is known in which the
nucleus of a really bright and great comet has been witnessed in the
act of passage over a considerable star. It would indeed be extremely
interesting to ascertain whether in such case the star experienced
any considerable diminution in its lustre."