"The Lord cast down great stones from heaven upon them unto Azekah,
and they died."--_Joshua_ x. II.
[Page 118]
[Illustration: A SWARM OF METEORS MEETING THE EARTH.
Their orbits are all parallel. Those coming in direct line to the
eye appear as stars, having no motion. Those on one side of this
line are seen in foreshortened perspective. Those furthest from
the centre, other things being equal, appear longest. The centre,
called the radiant point, of these November meteors is situated
in Leo; that of the August meteors in Perseus. Over fifty such
radiant points have been discovered. Over 30,000 meteors have been
visible in an hour.]
[Page 119]
VII.
_SHOOTING-STARS, METEORS, AND COMETS._
Before particularly considering the larger aggregations of matter
called planets or worlds as individuals, it is best to investigate
a part of the solar system consisting of smaller collections of
matter scattered everywhere through space. They are of various
densities, from a cloudlet of rarest gas to solid rock; of various
sizes, from a grain's weight to little worlds; of various relations
to each other, from independent individuality to related streams
millions of miles long. When they become visible they are called
shooting-stars, which are evanescent star-points darting through
the upper air, leaving for an instant a brilliant train; meteors,
sudden lights, having a discernible diameter, passing over a large
extent of country, often exploding with violence (Fig. 48), and
throwing down upon the earth aerolites; and comets, vast extents
of ghostly light, that come we know not whence and go we know not
whither. All these forms of matter are governed by the same laws
as the worlds, and are an integral part of the solar system--a
part of the unity of the universe.
[Illustration: Fig. 48.--Explosion of a Bolide.]
Everyone has seen the so-called shooting-stars. They break out
with a sudden brilliancy, shoot a few degrees with quiet speed,
and are gone before we can say, "See there!" The cause of their
appearance, the [Page 120] conversion of force into heat by their
contact with our atmosphere, has been already explained. Other facts
remain to be studied. They are found to appear about seventy-three
miles above the earth, and to disappear about twenty miles nearer
the surface. Their average velocity, thirty-five, sometimes rises to
one hundred miles a second. They exhibit different colors, according
to their different chemical substances, which are consumed. The
number of them to be seen on different nights is exceedingly
variable; sometimes not more [Page 121] than five or six an hour,
and sometimes so many that a man cannot count those appearing in a
small section of sky. This variability is found to be periodic.
There are everywhere in space little meteoric masses of matter, from
the weight of a grain to a ton, and from the density of gas to rock.
The earth meets 7,500,000 little bodies every day--there is
collision--the little meteoroid gives out its lightning sign of
extinction, and, consumed in fervent heat, drops to the earth as gas
or dust. If we add the number light enough to be seen by a
telescope, they cannot be less than 400,000,000 a day. Everywhere we
go, in a space as large as that occupied by the earth and its
atmosphere, there must be at least 13,000 bodies--one in 20,000,000
cubic miles--large enough to make a light visible to the naked eye,
and forty times that number capable of revealing themselves to
telescopic vision. Professor Peirce is about to publish, as the
startling result of his investigations, "that the heat which the
earth receives directly from meteors is the same in amount which it
receives from the sun by radiation, and that the sun receives
five-sixths of its heat from the meteors that fall upon it."
[Illustration: Fig. 49.--Bolides.]
[Page 121]
In 1783 Dr. Schmidt was fortunate enough to have a telescopic view
of a system of bodies which had turned into meteors. These were two
larger bodies followed by several smaller ones, going in parallel
lines till they were extinguished. They probably had been revolving
about each other as worlds and satellites before entering our
atmosphere. It is more than probable that the earth has many such
bodies, too small to be visible, revolving around it as moons.
[Illustration: Fig. 50.--Santa Rosa Aerolite.]
_Aerolites._
Sometimes the bodies are large enough to bear the heat, and the
unconsumed centre comes to the earth. [Page 123] Their velocity has
been lessened by the resisting air, and the excessive heat
diminished. Still, if found soon after their descent, they are too
hot to be handled. These are called aerolites or air-stones. There
was a fall in Iowa, in February, 1875, from which fragments
amounting to five hundred pounds weight were secured. On the evening
of December 21st, 1876, a meteor of unusual size and brilliancy
passed over the states of Kansas, Missouri, Illinois, Indiana, and
Ohio. It was first seen in the western part of Kansas, at an
altitude of about sixty miles. In crossing the State of Missouri it
began to explode, and this breaking up continued while passing
Illinois, Indiana, and Ohio, till it consisted of a large flock of
brilliant balls chasing each other across the sky, the number being
variously estimated at from twenty to one hundred. It was
accompanied by terrific explosions, and was seen along a path of not
less than a thousand miles. When first seen in Kansas, it is said to
have appeared as large as the full moon, and with a train from
twenty-five to one hundred feet long. Another, very similar in
appearance and behavior, passed over a part of the same course in
February, 1879. At Laigle, France, on April 26th, 1803, about one
o'clock in the day, from two to three thousand fell. The largest did
not exceed seventeen pounds weight. One fell in Weston, Connecticut,
in 1807, weighing two hundred pounds. A very destructive shower is
mentioned in the book of Joshua, chap. x. ver. 11.
These bodies are not evenly distributed through space. In some
places they are gathered into systems which circle round the sun
in orbits as certain as those of the [Page 124] planets. The chain
of asteroids is an illustration of meteoric bodies on a large scale.
They are hundreds in number--meteors are millions. They have their
region of travel, and the sun holds them and the giant Jupiter by
the same power. The Power that cares for a world cares for a
sparrow. If their orbit so lies that a planet passes through it, and
the planet and the meteors are at the point of intersection at the
same time, there must be collisions, and the lightning signs of
extinction proportioned to the number of little bodies in a given
space.
It is demonstrated that the earth encounters more than one hundred
such systems of meteoric bodies in a single year. It passes through
one on the 10th of August, another on the 11th of November. In
a certain part of the first there is an agglomeration of bodies
sufficient to become visible as it approaches the sun, and this is
known as the comet of 1862; in the second is a similar agglomeration,
known as Temple's comet. It is repeating the same thing to say that
meteoroids follow in the train of the comets. The probable orbit
of the November meteors and the comet of 1866 is an exceedingly
elongated ellipse, embracing the orbit of the earth at one end and
a portion of the orbit of Uranus at the other (Fig. 51). That of
the August meteors and the comet of 1862 embraces the orbit of
the earth at one end, and thirty per cent. of the other end is
beyond the orbit of Neptune.
[Illustration: Fig. 51.--Orbit of the November Meteors and the Comet
or 1866.]
In January, 1846, Biela's comet was observed to be divided. At
its next return, in 1852, the parts were 1,500,000 miles apart.
They could not be found on their periodic returns in 1859, 1865,
and 1872; but it [Page 125] should have crossed the earth's orbit
early in September, 1872. The earth itself would arrive at the point
of crossing two or three months later. If the law of revolution
held, we might still expect to find some of the trailing meteoroids
of the comet not gone by on our arrival. It was shown that the point
of the earth that would strike them would be toward a certain place
in the constellation of Andromeda, if the remains of the diluted
comet were still there. The prediction was verified in every
respect. At the appointed time, place, [Page 126] and direction, the
streaming lights were in our sky. That these little bodies belonged
to the original comet none can doubt. By the perturbations of
planetary attraction, or by different original velocities, a comet
may be lengthened into an invisible stream, or an invisible stream
agglomerated till it is visible as a comet.
_Comets._
Comets will be most easily understood by the foregoing considerations.
They are often treated as if they were no part of the solar system;
but they are under the control of the same laws, and owe their
existence, motion, and continuance to the same causes as Jupiter and
the rest of the planets. They are really planets of wider wandering,
greater ellipticity, and less density. They have periodic times
less than the earth, and fifty times as great as Neptune. They
are little clouds of gas or meteoric matter, or both, darting into
the solar system from every side, at every angle with the plane
of the ecliptic, becoming luminous with reflected light, passing
the sun, and returning again to outer darkness. Sometimes they
have no tail, having a nucleus surrounded by nebulosity like a
dim sun with zodiacal light; sometimes one tail, sometimes half a
dozen. These follow the comet to perihelion, and precede it afterward
(Fig. 52). The orbits of some comets are enormously elongated; one
end may lie inside the earth's orbit, and the other end be as far
beyond Neptune as that is from the sun. Of course only a small
part of such a curve can be studied by us: the comet is visible
only when near the sun. The same curve around the sun may be an
orbit that will bring it back again, [Page 127] or one that will
carry it off into infinite space, never to return. One rate of speed
on the curve indicates an elliptical orbit that returns; a greater
rate of speed indicates that it will take a parabolic orbit, which
never returns. The exact rate of speed is exceedingly difficult to
determine; hence it cannot be confidently asserted that any comet
ever visible will not return. They may all belong to the solar
system; but some will certainly be gone thousands of years before
their fiery forms will greet the watchful eyes of dwellers on the
earth. A comet that has an elliptic orbit may have it changed to
[Page 128] parabolic by the accelerations of its speed, by
attracting planets; or a parabolic comet may become elliptic, and so
permanently attracted to the system by the retardations of
attracting bodies. A comet of long period may be changed to one of
short period by such attraction, or _vice versa_.
[Illustration: Fig. 52.--Aspects of Remarkable Comets.]
The number of comets, like that of meteor streams, is exceedingly
large. Five hundred have been visible to the naked eye since the
Christian era. Two hundred have been seen by telescopes invented
since their invention. Some authorities estimate the number belonging
to our solar system by millions; Professor Peirce says more than
five thousand millions.
_Famous Comets._
The comet of 1680 is perhaps the one that appeared in A.D. 44, soon
after the death of Julius Cæsar, also in the reign of Justinian,
A.D. 531, and in 1106. This is not determined by any recognizable
resemblance. It had a tail 70° long; it was not all arisen when
its head reached the meridian. It is possible, from the shape of
its orbit, that it has a periodic time of nine thousand years, or
that it may have a parabolic orbit, and never return. Observations
taken two hundred years ago have not the exactness necessary to
determine so delicate a point.
On August 19th, 1682, Halley discovered a comet which he soon declared
to be one seen by Kepler in 1607. Looking back still farther, he
found that a comet was seen in 1531 having the same orbit. Still
farther, by the same exact period of seventy-five years, he found
that it was the same comet that had disturbed [Page 129] the
equanimity of Pope Calixtus in 1456. Calculations were undertaken as
to the result of all the accelerations and retardations by the
attractions of all the planets for the next seventy-five years.
There was not time to finish all the work; but a retardation of six
hundred and eighteen days was determined, with a possible error of
thirty days. The comet actually came to time within thirty-three
days, on March 12th, 1759. Again its return was calculated with more
laborious care. It came to time and passed the sun within three days
of the predicted time, on the 16th of November, 1835. It passed from
sight of the most powerful telescopes the following May, and has
never since been seen by human eye. But the eye of science sees it
as having passed its aphelion beyond the orbit of Neptune in 1873,
and is already hastening back to the warmth and light of the sun. It
will be looked for in 1911; and there is good hope of predicting,
long before it is seen, the time of its perihelion within a day.
_Biela's lost Comet._--This was a comet with a periodic time of
six years and eight months. It was observed in January, 1846, to
have separated into two parts of unequal brightness. The lesser
part grew for a month until it equalled the other, then became
smaller and disappeared, while the other was visible a month longer.
At disappearance the parts were 200,000 miles asunder. On its next
return, in 1852, the parts were 1,500,000 miles apart; sometimes
one was brighter and sometimes the other; which was the fragment
and which was the main body could not be recognized. They vanished
in September, 1852, and have never been seen since. Three revolutions
have been made since that time, but no [Page 130] trace of it could
be discovered. Probably the same influence that separated it into
parts, separated the particles till too thin and tenuous to be seen.
There is ground for believing that the earth passed through a part
of it, as before stated under the head of meteors.
_The Great Comet of_ 1843 passed nearer the sun than any known
body. It almost grazed the sun. If it ever returns, it will be in
A.D. 2373.
_Donati's Comet of_ 1858.--This was one of the most magnificent
of modern times. During the first three months it showed no tail,
but from August to October it had developed one forty degrees in
length. Its period is about two thousand years. Every reader remembers
the comet of the summer of 1875.
_Encke's Comet._--This comet has become famous for its supposed
confirmation of the theory that space was filled with a substance
infinitely tenuous, which resisted the passage of this gaseous
body in an appreciable degree, and in long ages would so retard
the motion of all the planets that gravitation would draw them
all one by one into the sun. We must not be misled by the term
retardation to suppose it means behind time, for a retarded body
is before time. If its velocity is diminished, the attraction of
the sun causes it to take a smaller orbit, and smaller orbits mean
increased speed--hence the supposed retardation would shorten its
periodic time. This comet was thought to be retarded two and a
half hours at each revolution. If it was, it would not prove the
existence of the resisting medium. Other causes, unknown to us,
might account for it. Subsequent and more exact calculations fail
to find any retardations in at least two revolutions between 1865
and [Page 131] 1871. Indications point to a retardation of one and a
half hours both before and since. But such discrepancy of result
proves nothing concerning a resisting medium, but rather is an
argument against its existence. Besides, Faye's comet, in four
revolutions of seven years each, shows no sign of retardation.
The truth may be this, that a kind of atmosphere exists around the
sun, perhaps revealed by the zodiacal light, that reaches beyond
where Encke's comet dips inside the orbit of Mercury, and thus
retards this body, but does not reach beyond the orbit of Mars,
where Faye's comet wheels and withdraws.
_Of what do Comets consist?_
The unsolved problems pertaining to comets are very numerous and
exceedingly delicate. Whence come they? Why did they not contract to
centres of nebulæ? Are there regions where attractions are balanced,
and matter is left to contract on itself, till the movements of
suns and planets adds or diminishes attractive force on one side,
and so allows them to be drawn slowly toward one planet, and its
sun, or another? There is ground for thinking that the comet of
1866 and its train of meteors, visible to us in November, was thus
drawn into our system by the planet Uranus. Indeed, Leverrier has
conjecturally fixed upon the date of A.D. 128 as the time when it
occurred; but another and closer observation of its next return,
in 1899, will be needed to give confirmation to the opinion. Our
sun's authority extends at least half-way to the nearest fixed star,
one hundred thousand times farther than the orbit of the earth.
Meteoric and cometary matter lying [Page 132] there, in a spherical
shell about the solar system, balanced between the attraction of
different suns, finally feels the power that determines its destiny
toward our sun. It would take 167,000,000 years to come thence to
our system.
The conditions of matter with which we are acquainted do not cover
all the ground presented by these mysterious visitors. We know
a gas sixteen times as light as air, but hydrogen is vastly too
heavy and dense; for we see the faintest star through thousands of
miles of cometary matter; we know that water may become cloudy vapor,
but a little of it obscures the vision. Into what more ethereal,
and we might almost say spiritual, forms matter may be changed we
cannot tell. But if we conceive comets to be only gas, it would
expand indefinitely in the realms of space, where there is no force
of compression but its own. We might say that comets are composed
of small separate masses of matter, hundreds of miles apart; and,
looking through thousands of miles of them, we see light enough
reflected from them all to seem continuous. Doubtless that is sometimes
the case. But the spectroscope shows another state of things: it
reveals in some of these comets an incandescent gas--usually some
of the combinations of carbon. The conclusion, then, naturally is
that there are both gas and small masses of matter, each with an
orbit of its own nearly parallel to those of all the others, and
that they afford some attraction to hold the mass of intermingled and
confluent gas together. Our best judgment, then, is that the nucleus
is composed of separate bodies, or matter in a liquid condition,
capable of being vaporized by the heat of the sun, and driven off,
[Page 133] as steam from a locomotive, into a tail. Indications of
this are found in the fact that tails grow smaller at successive
returns, as the matter capable of such vaporization becomes
condensed. In some instances, as in that of the comet of 1843, the
head was diminished by the manufacture of a tail. On the other hand,
Professor Peirce showed that the nucleus of the comets of 1680,
1843, and 1858 must have had a tenacity equal to steel, to prevent
being pulled apart by the tidal forces caused by its terrible
perihelion sweep around the sun.
It is likely that there are great varieties of condition in different
comets, and in the same comet at times. We see them but a few days
out of the possible millions of their periodic time; we see them
only close to the sun, under the spur of its tremendous attraction
and terrible heat. This gives us ample knowledge of the path of
their orbit and time of their revolution, but little ground for
judgment of their condition, when they slowly round the uttermost
cape of their far-voyaging, in the terrible cold and darkness,
to commence their homeward flight. The unsolved problems are not
all in the distant sun and more distant stars, but one of them
is carried by us, sometimes near, sometimes far off; but our
acquaintance with the possible forms and conditions of matter is
too limited to enable us to master the difficulties.
_Will Comets strike the Earth?_
Very likely, since one or two have done so within a recent period.
What will be the effect? That depends on circumstances. There is
good reason to suppose we passed through the tail of a comet in
1861, and the only [Page 134] observable effect was a peculiar
phosphorescent mist. If the comet were composed of small meteoric
masses a brilliant shower would be the result. But if we fairly
encountered a nucleus of any considerable mass and solidity, the
result would be far more serious. The mass of Donati's comet has
been estimated by M. Faye to be 1/20000 of that of the earth. If
this amount of matter were dense as water, it would make a globe
five hundred miles in diameter; and if as dense as Professor Peirce
proved the nucleus of this comet to be, its impact with the earth
would develop heat enough to melt and vaporize the hardest rocks.
Happily there is little fear of this: as Professor Newcomb says, "So
small is the earth in comparison with celestial space, that if one
were to shut his eyes and fire at random in the air, the chance of
bringing down a bird would be better than that of a comet of any
kind striking the earth." Besides, we are not living under a
government of chance, but under that of an Almighty Father, who
upholdeth all things by the word of his power; and no world can come
to ruin till he sees that it is best.
[Page 135]
VIII.