SHOOTING STARS.
Small Bodies of our System--Their Numbers--How they are
Observed--The Shooting Star--The Theory of Heat--A Great Shooting
Star--The November Meteors--Their Ancient History--The Route
followed by the Shoal--Diagram of the Shoal of Meteors--How the
Shoal becomes Spread out along its Path--Absorption of Meteors by
the Earth--The Discovery of the Relation between Meteors and
Comets--The Remarkable Investigations concerning the November
Meteors--Two Showers in Successive Years--No Particles have ever
been Identified from the Great Shooting Star Showers--Meteoric
Stones--Chladni's Researches--Early Cases of Stone-falls--The
Meteorite at Ensisheim--Collections of Meteorites--The Rowton
Siderite--Relative Frequency of Iron and Stony
Meteorites--Fragmentary Character of Meteorites--Tschermak's
Hypothesis--Effects of Gravitation on a Missile ejected from a
Volcano--Can they have come from the Moon?--The Claims of the Minor
Planets to the Parentage of Meteorites--Possible Terrestrial
Origin--The Ovifak Iron.
In the preceding chapters we have dealt with the gigantic bodies which
form the chief objects in what we know as the solar system. We have
studied mighty planets measuring thousands of miles in diameter, and we
have followed the movements of comets whose dimensions are often to be
told by millions of miles. Once, indeed, in a previous chapter we have
made a descent to objects much lower in the scale of magnitude, and we
have examined that numerous class of small bodies which we call the
minor planets. It is now, however, our duty to make a still further, and
this time a very long step, downwards in the scale of magnitude. Even
the minor planets must be regarded as colossal objects when compared
with those little bodies whose presence is revealed to us in an
interesting and sometimes in a striking manner.
These small bodies compensate in some degree for their minute size by
the profusion in which they exist. No attempt, indeed, could be made to
tell in figures the myriads in which they swarm throughout space. They
are probably of very varied dimensions, some of them being many pounds
or perhaps tons in weight, while others seem to be not larger than
pebbles, or even than grains of sand. Yet, insignificant as these bodies
may seem, the sun does not disdain to undertake their control. Each
particle, whether it be as small as the mote in a sunbeam or as mighty
as the planet Jupiter, must perforce trace out its path around the sun
in conformity with the laws of Kepler.
Who does not know that beautiful occurrence which we call a shooting
star, or which, in its more splendid forms, is sometimes called a meteor
or fireball? It is to objects of this class that we are now to direct
our attention.
A small body is moving round the sun. Just as a mighty planet revolves
in an ellipse, so even a small object will be guided round and round in
an ellipse with the sun in the focus. There are, at the present moment,
inconceivable myriads of such meteors moving in this manner. They are
too small and too distant for our telescopes, and we never see them
except under extraordinary circumstances.
When the meteor flashes into view it is moving with such enormous
velocity that it often traverses more than twenty miles in a second of
time. Such a velocity is almost impossible near the earth's surface: the
resistance of the air would prevent it. Aloft, in the emptiness of
space, there is no air to impede its flight. It may have been moving
round and round the sun for thousands, perhaps for millions of years,
without suffering any interference; but the supreme moment arrives, and
the meteor perishes in a streak of splendour.
In the course of its wanderings the body comes near the earth, and
within a few hundred miles of its surface begins to encounter the upper
surface of the atmosphere with which the earth is enclosed. To a body
moving with the appalling velocity of a meteor, a plunge into the
atmosphere is usually fatal. Even though the upper layers of air are
excessively attenuated, yet they suddenly check the velocity almost as
a rifle bullet would be checked when fired into water. As the meteor
rushes through the atmosphere the friction of the air warms its surface;
gradually it becomes red-hot, then white-hot, and is finally driven off
into vapour with a brilliant light, while we on the earth, one or two
hundred miles below, exclaim: "Oh, look, there is a shooting star!"
We have here an experiment illustrating the mechanical theory of heat.
It may seem incredible that mere friction should be sufficient to
generate heat enough to produce so brilliant a display, but we must
recollect two facts: first, that the velocity of the meteor is, perhaps,
one hundred times that of a rifle bullet; and, second, that the
efficiency of friction in developing heat is proportional to the square
of the velocity. The meteor in passing through the air may therefore
develop by the friction of the air about ten thousand times as much heat
as the rifle bullet. We do not make an exaggerated estimate in supposing
that the latter missile becomes heated ten degrees by friction; yet if
this be admitted, we must grant that there is such an enormous
development of heat attending the flight of the meteor that even a
fraction of it would be sufficient to drive the object into vapour.
Let us first consider the circumstances in which these external bodies
are manifested to us, and, for the sake of illustration, we may take a
remarkable fireball which occurred on November 6th, 1869. This body was
seen from many different places in England; and by combining and
comparing these observations, we obtain accurate information as to the
height of the object and the velocity with which it travelled.
It appears that this meteor commenced to be visible at a point ninety
miles above Frome, in Somersetshire, and that it vanished twenty-seven
miles over the sea, near St. Ives, in Cornwall. The path of the body,
and the principal localities from which it was observed, are shown in
the map (Fig. 75). The whole length of its visible course was about 170
miles, which was performed in a period of five seconds, thus giving an
average velocity of thirty-four miles per second. A remarkable feature
in the appearance which this fireball presented was the long persistent
streak of luminous cloud, about fifty miles long and four miles wide,
which remained in sight for fully fifty minutes. We have in this example
an illustration of the chief features of the phenomena of a shooting
star presented on a very grand scale. It is, however, to be observed
that the persistent luminous streak is not a universal, nor, indeed, a
very common characteristic of a shooting star.
[Illustration: Fig. 75.--The Path of the Fireball of November 6th,
1869.]
The small objects which occasionally flash across the field of the
telescope show us that there are innumerable telescopic shooting stars,
too small and too faint to be visible to the unaided eye. These objects
are all dissipated in the way we have described; it is, in fact, only at
the moment, and during the process of their dissolution, that we become
aware of their existence. Small as these missiles probably are, their
velocity is so prodigious that they would render the earth uninhabitable
were they permitted to rain down unimpeded on its surface. We must,
therefore, among the other good qualities of our atmosphere, not forget
that it constitutes a kindly screen, which shields us from a tempest of
missiles, the velocity of which no artillery could equal. It is, in
fact, the very fury of these missiles which is the cause of their utter
destruction. Their anxiety to strike us is so great, that friction
dissolves them into harmless vapour.
Next to a grand meteor such as that we have just described, the most
striking display in connection with shooting stars is what is known as a
shower. These phenomena have attracted a great deal of attention within
the last century, and they have abundantly rewarded the labour devoted
to them by affording some of the most interesting astronomical
discoveries of modern times.
The showers of shooting stars do not occur very frequently. No doubt the
quickened perception of those who especially attend to meteors will
detect a shower when others see only a few straggling shooting stars;
but, speaking generally, we may say that the present generation can
hardly have witnessed more than two or three such occurrences. I have
myself seen two great showers, one of which, in November, 1866, has
impressed itself on my memory as a glorious spectacle.
To commence the history of the November meteors it is necessary to look
back for nearly a thousand years. On the 12th of October, in the year
902, occurred the death of a Moorish king, and in connection with this
event an old chronicler relates how "that night there were seen, as it
were lances, an infinite number of stars, which scattered themselves
like rain to right and left, and that year was called the Year of the
Stars."
No one now believes that the heavens intended to commemorate the death
of the king by that display. The record is, however, of considerable
importance, for it indicates the year 902 as one in which a great shower
of shooting stars occurred. It was with the greatest interest
astronomers perceived that this was the first recorded instance of that
periodical shower, the last of whose regular returns were seen in 1799,
1833, and 1866. Further diligent literary research has revealed here and
there records of startling appearances in the heavens, which fit in with
successive returns of the November meteors. From the first instance, in
902, to the present day there have been twenty-nine visits of the
shower; and it is not unlikely that these may have all been seen in some
parts of the earth. Sometimes they may have been witnessed by savages,
who had neither the inclination nor the means to place on record an
apparition which to them was a source of terror. Sometimes, however,
these showers were observed by civilised communities. Their nature was
not understood, but the records were made; and in some cases, at all
events, these records have withstood the corrosion of time, and have now
been brought together to illustrate this curious subject. We have
altogether historical notices of twelve of these showers, collected
mainly by the industry of Professor H.A. Newton whose labours have
contributed so much to the advancement of our knowledge of shooting
stars.
Let us imagine a swarm of small objects roaming through space. Think of
a shoal of herrings in the ocean, extending over many square miles, and
containing countless myriads of individuals; or think of those enormous
flocks of wild pigeons in the United States of which Audubon has told
us. The shoal of shooting stars is perhaps much more numerous than the
herrings or the pigeons. The shooting stars are, however, not very close
together; they are, on an average, probably some few miles apart. The
actual bulk of the shoal is therefore prodigious; and its dimensions are
to be measured by hundreds of thousands of miles.
[Illustration: Fig. 76.--The Orbit of a Shoal of Meteors.]
The meteors cannot choose their own track, like the shoal of herrings,
for they are compelled to follow the route which is prescribed to them
by the sun. Each one pursues its own ellipse in complete independence of
its neighbours, and accomplishes its journey, thousands of millions of
miles in length, every thirty-three years. We cannot observe the meteors
during the greater part of their flight. There are countless myriads of
these bodies at this very moment coursing round their path. We never see
them till the earth catches them. Every thirty-three years the earth
makes a haul of these meteors just as successfully as the fisherman
among the herrings, and in much the same way, for while the fisherman
spreads his net in which the fishes meet their doom, so the earth has an
atmosphere wherein the meteors perish. We are told that there is no fear
of the herrings becoming exhausted, for those the fishermen catch are as
nothing compared to the profusion in which they abound in ocean. We may
say the same with regard to the meteors. They exist in such myriads,
that though the earth swallows up millions every thirty-three years,
plenty are left for future showers. The diagram (Fig. 76) will explain
the way in which the earth makes her captures. We there see the orbit
in which our globe moves around the sun, as well as the elliptic path of
the meteors, though it should be remarked that it is not convenient to
draw the figure exactly to scale, so that the path of the meteors is
relatively much larger than here represented. Once each year the earth
completes its revolution, and between the 13th and the 16th of November
crosses the track in which the meteors move. It will usually happen that
the great shoal is not at this point when the earth is passing. There
are, however, some stragglers all along the path, and the earth
generally catches a few of these at this date. They dart into our
atmosphere as shooting stars, and form what we usually speak of as the
November meteors.
It will occasionally happen that when the earth is in the act of
crossing the track it encounters the bulk of the meteors. Through the
shoal our globe then plunges, enveloped, of course, with the surrounding
coat of air. Into this net the meteors dash in countless myriads, never
again to emerge. In a few hours' time, the earth, moving at the rate of
eighteen miles a second, has crossed the track and emerges on the other
side, bearing with it the spoils of the encounter. Some few meteors,
which have only narrowly escaped capture, will henceforth bear evidence
of the fray by moving in slightly different orbits, but the remaining
meteors of the shoal continue their journey without interruption;
perhaps millions have been taken, but probably hundreds of millions have
been left.
Such was the occurrence which astonished the world on the night between
November 13th and 14th, 1866. We then plunged into the middle of the
shoal. The night was fine; the moon was absent. The meteors were
distinguished not only by their enormous multitude, but by their
intrinsic magnificence. I shall never forget that night. On the
memorable evening I was engaged in my usual duty at that time of
observing nebulae with Lord Rosse's great reflecting telescope. I was of
course aware that a shower of meteors had been predicted, but nothing
that I had heard prepared me for the splendid spectacle so soon to be
unfolded. It was about ten o'clock at night when an exclamation from an
attendant by my side made me look up from the telescope, just in time to
see a fine meteor dash across the sky. It was presently followed by
another, and then again by more in twos and in threes, which showed that
the prediction of a great shower was likely to be verified. At this time
the Earl of Rosse (then Lord Oxmantown) joined me at the telescope, and,
after a brief interval, we decided to cease our observations of the
nebulae and ascend to the top of the wall of the great telescope (Fig. 7,
p. 18), whence a clear view of the whole hemisphere of the heavens could
be obtained. There, for the next two or three hours, we witnessed a
spectacle which can never fade from my memory. The shooting stars
gradually increased in number until sometimes several were seen at once.
Sometimes they swept over our heads, sometimes to the right, sometimes
to the left, but they all diverged from the east. As the night wore on,
the constellation Leo ascended above the horizon, and then the
remarkable character of the shower was disclosed. All the tracks of the
meteors radiated from Leo. (_See_ Fig. 74, p. 368.) Sometimes a meteor
appeared to come almost directly towards us, and then its path was so
foreshortened that it had hardly any appreciable length, and looked like
an ordinary fixed star swelling into brilliancy and then as rapidly
vanishing. Occasionally luminous trains would linger on for many minutes
after the meteor had flashed across, but the great majority of the
trains in this shower were evanescent. It would be impossible to say how
many thousands of meteors were seen, each one of which was bright enough
to have elicited a note of admiration on any ordinary night.
The adjoining figure (Fig. 77) shows the remarkable manner in which the
shooting stars of this shower diverged from a point. It is not to be
supposed that all these objects were in view at the same moment. The
observer of a shower is provided with a map of that part of the heavens
in which the shooting stars appear. He then fixes his attention on one
particular shooting star, and observes carefully its track with respect
to the fixed stars in its vicinity. He then draws a line upon his map
in the direction in which the shooting star moved. Repeating the same
observation for several other shooting stars belonging to the shower,
his map will hardly fail to show that their different tracks almost all
tend from one point or region of the figure. There are, it is true, a
few erratic ones, but the majority observe this law. It certainly looks,
at first sight, as if all the shooting stars did actually dart from this
point; but a little reflection will show that this is a case in which
the real motion is different from the apparent. If there actually were a
point from which these meteors diverged, then from different parts of
the earth the point would be seen in different positions with respect to
the fixed stars; but this is not the case. The radiant, as this point is
called, is seen in the same part of the heavens from whatever station
the shower is visible.
[Illustration: Fig. 77.--The Radiant Point of Shooting Stars.]
We are, therefore, led to accept the simple explanation afforded by the
theory of perspective. Those who are acquainted with the principles of
this science know that when a number of parallel lines in an object have
to be represented in a drawing, they must all be made to pass through
the same point in the plane of the picture. When we are looking at the
shooting stars, we see the projections of their paths upon the surface
of the heavens. From the fact that those paths pass through the same
point, we are to infer that the shooting stars belonging to the same
shower are moving in parallel lines.
We are now able to ascertain the actual direction in which the shooting
stars are moving, because a line drawn from the eye of the observer to
the radiant point must be parallel to that direction. Of course, it is
not intended to convey the idea that throughout all space the shooting
stars of one shower are moving in parallel lines; all we mean is that
during the short time in which we see them the motion of each of the
shooting stars is sensibly a straight line, and that all these straight
lines are parallel.
In the year 1883 the great meteor shoal of the Leonids (for so this
shower is called) attained its greatest distance from the sun, and then
commenced to return. Each year the earth crossed the orbit of the
meteors; but the shoal was not met with, and no noteworthy shower of
stars was perceived. Every succeeding year found the meteors approaching
the critical point, and the year 1899 brought the shoal to the earth's
track. In that year a brilliant meteoric shower was expected, but the
result fell far short of expectation. The shoal of meteors is of such
enormous length that it takes more than a year for the mighty procession
to pass through the critical portion of its orbit which lies across the
track of the earth. We thus see that the meteors cannot escape the
earth. It may be that when the shoal begins to reach this neighbourhood
the earth will have just left this part of its path, and a year will
have elapsed before the earth gets round again. Those meteors that have
the good fortune to be in the front of the shoal will thus escape the
net, but some of those behind will not be so fortunate, and the earth
will again devour an incredible host. It has sometimes happened that
casts into the shoal have been obtained in two consecutive years. If the
earth happened to pass through the front part in one year, then the
shoal is so long that the earth will have moved right round its orbit
of 600,000,000 miles, and will again dash through the critical spot
before the entire number have passed. History contains records of cases
when, in two consecutive Novembers, brilliant showers of Leonids have
been seen.
As the earth consumes such myriads of Leonids each thirty-three years,
it follows that the total number must be decreasing. The splendour of
the showers in future ages will, no doubt, be affected by this
circumstance. They cannot be always so bright as they have been. It is
also of interest to notice that the shape of the shoal is gradually
changing. Each meteor of the shoal moves in its own ellipse round the
sun, and is quite independent of the rest of these bodies. Each one has
thus a special period of revolution which depends upon the length of the
ellipse in which it happens to revolve. Two meteors will move around the
sun in the same time if the lengths of their ellipses are exactly equal,
but not otherwise. The lengths of these ellipses are many hundreds of
millions of miles, and it is impossible that they can be all absolutely
equal. In this may be detected the origin of a gradual change in the
character of the shower. Suppose two meteors A and B be such that A
travels completely round in thirty-three years, while B takes
thirty-four years. If the two start together, then when A has finished
the first round B will be a year behind; the next time B will be two
years behind, and so on. The case is exactly parallel to that of a
number of boys who start for a long race, in which they have to run
several times round the course before the distance has been
accomplished. At first they all start in a cluster, and perhaps for the
first round or two they may remain in comparative proximity; gradually,
however, the faster runners get ahead and the slower ones lag behind, so
the cluster becomes elongated. As the race continues, the cluster
becomes dispersed around the entire course, and perhaps the first boy
will even overtake the last. Such seems the destiny of the November
meteors in future ages. The cluster will in time come to be spread out
around the whole of this mighty track, and no longer will a superb
display have to be recorded every thirty-three years.
It was in connection with the shower of November meteors in 1866 that a
very interesting and beautiful discovery in mathematical astronomy was
made by Professor Adams. We have seen that the Leonids must move in an
elliptic path, and that they return every thirty-three years, but the
telescope cannot follow them during their wanderings. All that we know
by observation is the date of their occurrence, the point of the heavens
from which they radiate, and the great return every thirty-three years.
Putting these various facts together, it is possible to determine the
ellipse in which the meteors move--not exactly: the facts do not go so
far--they only tell us that the ellipse must be one of five possible
orbits. These five possible orbits are--firstly, the immense ellipse in
which we now know the meteorites do revolve, and for which they require
the whole thirty-three years to complete a revolution; secondly, a
nearly circular orbit, very little larger than the earth's path, which
the meteors would traverse in a few days more than a year; another
similar orbit, in which the time would be a few days short of the year;
and two other small elliptical orbits lying inside the earth's orbit. It
was clearly demonstrated by Professor Newton, of New Haven, U.S.A., that
the observed facts would be explained if the meteors moved in any one of
these paths, but that they could not be explained by any other
hypothesis. It remained to see which of these orbits was the true one.
Professor Newton himself made the suggestion of a possible method of
solving the problem. The test he proposed was one of some difficulty,
for it involved certain intricate calculations in the theory of
perturbations. Fortunately, however, Professor Adams undertook the
inquiry, and by his successful labours the path of the Leonids has been
completely ascertained.
[Illustration: Fig. 78.--The History of the Leonids.]
When the ancient records of the appearance of great Leonid showers were
examined, it was found that the date of their occurrence undergoes a
gradual and continuous change, which Professor Newton fixed at one day
in seventy years. It follows as a necessary consequence that the point
where the path of the meteors crosses the earth's track is not fixed,
but that at each successive return they cross at a point about half a
degree further on in the direction in which the earth is travelling. It
follows that the orbit in which the meteors are revolving is undergoing
change; the path they follow in one revolution varies slightly from that
pursued in the next. As, however, these changes proceed in the same
direction, they may gradually attain considerable dimensions; and the
amount of change which is produced in the path of the meteors in the
lapse of centuries may be estimated by the two ellipses shown in Fig.
78. The continuous line represents the orbit in A.D. 126; the dotted
line represents it at present.
This unmistakable change in the orbit is one that astronomers attribute
to what we have already spoken of as perturbation. It is certain that
the elliptic motion of these bodies is due to the sun, and that if they
were only acted on by the sun the ellipse would remain absolutely
unaltered. We see, then, in this gradual change of the ellipse the
influence of the attractions of the planets. It was shown that if the
meteors moved in the large orbit, this shifting of the path must be due
to the attraction of the planets Jupiter, Saturn, Uranus, and the Earth;
while if the meteors followed one of the smaller orbits, the planets
that would be near enough and massive enough to act sensibly on them
would be the Earth, Venus, and Jupiter. Here, then, we see how the
question may be answered by calculation. It is difficult, but it is
possible, to calculate what the attraction of the planets would be
capable of producing for each of the five different suppositions as to
the orbit. This is what Adams did. He found that if the meteors moved in
the great orbit, then the attraction of Jupiter would account for
two-thirds of the observed change, while the remaining third was due to
the influence of Saturn, supplemented by a small addition on account of
Uranus. In this way the calculation showed that the large orbit was a
possible one. Professor Adams also computed the amount of displacement
in the path that could be produced if the meteors revolved in any of the
four smaller ellipses. This investigation was one of an arduous
character, but the results amply repaid the labour. It was shown that
with the smaller ellipses it would be impossible to obtain a
displacement even one-half of that which was observed. These four orbits
must, therefore, be rejected. Thus the demonstration was complete that
it is in the large path that the meteors revolve.
The movements in each revolution are guided by Kepler's laws. When at
the part of its path most distant from the sun the velocity of a meteor
is at its lowest, being then but little more than a mile a second; as it
draws in, the speed gradually increases, until, when the meteor crosses
the earth's track, its velocity is no less than twenty-six miles a
second. The earth is moving very nearly in the opposite direction at
the rate of eighteen miles a second, so that, if the meteor happen to
strike the earth's atmosphere, it does so with the enormous velocity of
nearly forty-four miles a second. If a collision is escaped, then the
meteor resumes its onward journey with gradually declining velocity, and
by the time it has completed its circuit a period of thirty-three years
and a quarter will have elapsed.
The innumerable meteors which form the Leonids are arranged in an
enormous stream, of a breadth very small in comparison with its length.
If we represent the orbit by an ellipse whose length is seven feet, then
the meteor stream will be represented by a thread of the finest
sewing-silk, about a foot and a half or two feet long, creeping along
the orbit.[34] The size of this stream may be estimated from the
consideration that even its width cannot be less than 100,000 miles. Its
length may be estimated from the circumstance that, although its
velocity is about twenty-six miles a second, yet the stream takes about
two years to pass the point where its orbit crosses the earth's track.
On the memorable night between the 13th and 14th of November, 1866, the
earth plunged into this stream near its head, and did not emerge on the
other side until five hours later. During that time it happened that the
hemisphere of the earth which was in front contained the continents of
Europe, Asia, and Africa, and consequently it was in the Old World that
the great shower was seen. On that day twelvemonth, when the earth had
regained the same spot, the shoal had not entirely passed, and the earth
made another plunge. This time the American continent was in the van,
and consequently it was there that the shower of 1867 was seen. Even in
the following year the great shoal had not entirely passed, and since
then a few stragglers along the route have been encountered at each
annual transit of the earth across this meteoric highway.
The diagram is also designed to indicate a remarkable speculation which
was put forward on the high authority of Le Verrier, with the view of
explaining how the shoal came to be introduced into the solar system.
The orbit in which the meteors revolve does not intersect the paths of
Jupiter, Saturn, or Mars, but it does intersect the orbit of Uranus. It
must sometimes happen that Uranus is passing through this point of its
path just as the shoal arrives there. Le Verrier has demonstrated that
such an event took place in the year A.D. 126, but that it has not
happened since. We thus seem to have a clue to a very wonderful history
by which the meteors are shown to have come into our system in the year
named. The expectations or a repetition of the great shower in 1899
which had been widely entertained, and on good grounds, were not
realised. Hardly more than a few meteors of the ordinary type were
observed.
Assuming that the orbit of the August meteors was a parabola,
Schiaparelli computed the dimensions and position in space of this
orbit, and when he had worked this out, he noticed that the orbit
corresponded in every particular with the orbit of a fine comet which
had appeared in the summer of 1862. This could not be a mere matter of
accident. The plane in which the comet moved coincided exactly with that
in which the meteors moved; so did the directions of the axes of their
orbits, while the direction of the motion is the same, and the shortest
distance from the sun to the orbit is also in the two cases identical.
This proved to demonstration that there must be some profound physical
connection between comets and swarms of meteors. And a further proof of
this was shortly afterwards furnished, when Le Verrier had computed the
orbit of the November meteors, for this was at once noticed to be
precisely the same as the orbit of a comet which had passed its
perihelion in January, 1866, and for which the period of revolution had
been found to be thirty-three years and two months.
Among the Leonids we see occasionally fireballs brighter than Venus, and
even half the apparent size of the moon, bursting out with
lightning-like flashes, and leaving streaks which last from a minute to
an hour or more. But the great majority are only as bright as stars of
the second, third, or fourth magnitude. As the amount of light given by
a meteor depends on its mass and velocity, we can form some idea as to
the actual weight of one of these meteors, and it appears that most of
them do not weigh nearly as much as a quarter of an ounce; indeed, it is
probable that many do not weigh a single grain. But we have seen that a
comet in all probability is nothing but a very loose swarm of small
particles surrounded by gas of very slight density, and we have also
seen that the material of a comet must by degrees be more or less
dissipated through space. We have still to tell a wonderful story of the
breaking up of a comet and what appears to have become of the particles
thereof.
A copious meteoric shower took place on the night of the 27th November,
1872. On this occasion the shooting stars diverged from a radiant point
in the constellation of Andromeda. As a spectacle, it was unquestionably
inferior to the magnificent display of 1866, but it is difficult to say
which of the two showers has been of greater scientific importance.
It surely is a remarkable coincidence that the earth should encounter
the Andromedes (for so this shower is called) at the very moment when it
is crossing the track of Biela's comet. We have observed the direction
from which the Andromedes come when they plunge into the atmosphere; we
can ascertain also the direction in which Biela's comet is moving when
it passes the earth's track, and we find that the direction in which the
comet moves and the direction in which the meteors move are identical.
This is, in itself, a strong and almost overwhelming presumption that
the comet and the shooting stars are connected; but it is not all. We
have observations of this swarm dating back to the eighteenth century,
and we find that the date of its appearance has changed from the 6th or
7th of December to the end of November in perfect accordance with the
retrograde motion of the crossing-point of the earth's orbit and the
orbit of Biela's comet. This comet was observed in 1772, and again in
1805-6, before its periodic return every seven years was discovered. It
was discovered by Biela in 1826, and was observed again in 1832. In 1846
the astronomical world was startled to find that there were now two
comets in place of one, and the two fragments were again perceived at
the return in 1852. In 1859 Biela's comet could not be seen, owing to
its unfavourable situation with regard to the earth. No trace of Biela's
comet was seen in 1865-66, when its return was also due, nor has it ever
been seen since. It therefore appears that in the autumn of 1872 the
time had arrived for the return of Biela's comet, and thus the
occurrence of the great shower of the Andromedes took place about the
time when Biela's comet was actually due. The inference is irresistible
that the shooting stars, if not actually a part of the comet itself, are
at all events most intimately connected therewith. This shower is also
memorable for the telegram sent from Professor Klinkerfues to Mr. Pogson
at Madras. The telegram ran as follows:--"Biela touched earth on 27th.
Search near Theta Centauri." Pogson did search and did find a comet,
but, unfortunately, owing to bad weather he only secured observations of
it on two nights. As we require three observations to determine the
orbit of a planet or comet, it is not possible to compute the orbit of
Pogson's, but it seems almost certain that the latter cannot be
identical with either of the two components of Biela's comet. It is,
however, likely that it really was a comet moving along the same track
as Biela and the meteors.
Another display of the Biela meteors took place in 1885, just giving
time for two complete revolutions of the swarm since 1872. The display
on the 27th November, 1885, was magnificent; Professor Newton estimated
that at the time of maximum the meteors came on at the rate of 75,000
per hour. In 1892 the comet ought again to have returned to perihelion,
but in that year no meteors were seen on the 27th November, while many
were seen on the 23rd from the same radiant. The change in the point of
intersection between the orbit of the meteors and the orbit of the
earth indicated by this difference of four days was found by Bredichin
to be due to the perturbing action of Jupiter on the motion of the
swarm.
It is a noticeable circumstance that the great meteoric showers seem
never yet to have projected a missile which has reached the earth's
surface. Out of the myriads of Leonids, of Perseids, or of Andromedes,
not one particle has ever been seized and identified.[35] Those bodies
which fall from the sky to the earth, and which we call meteorites, do
not seem to come from the great showers, so far as we know. They may,
indeed, have quite a different origin from that of the periodic meteors.
It is somewhat curious that the belief in the celestial origin of
meteorites is of modern growth. In ancient times there were, no doubt,
rumours of wonderful stones which had fallen down from the heavens to
the earth, but these reports seem to have obtained but little credit.
They were a century ago regarded as perfectly fabulous, though there was
abundant testimony on the subject. Eye-witnesses averred that they had
seen the stones fall. The bodies themselves were unlike other objects in
the neighbourhood, and cases were even authenticated where men had been
killed by these celestial visitors.
No doubt the observations were generally made by ignorant and illiterate
persons. The true parts of the record were so mixed up with imaginary
additions, that cautious men refused to credit the statements that such
objects really fell from the sky. Even at the present day it is often
extremely difficult to obtain accurate testimony on such matters. For
instance, the fall of a meteorite was observed by a Hindoo in the
jungle. The stone was there, its meteoric character was undoubted, and
the witness was duly examined as to the details of the occurrence; but
he was so frightened by the noise and by the danger he believed himself
to have narrowly escaped, that he could tell little or nothing. He felt
certain, however, that the meteorite had hunted him for two hours
through the jungle before it fell to the earth!
In the year 1794 Chladni published an account of the remarkable mass of
iron which the traveller Pallas had discovered in Siberia. It was then
for the first time recognised that this object and others similar to it
must have had a celestial origin. But even Chladni's reputation and the
arguments he brought forward failed to procure universal assent. Shortly
afterwards a stone of fifty-six pounds was exhibited in London, which
several witnesses declared they had seen fall at Wold Cottage, in
Yorkshire, in 1795. This body was subsequently deposited in our national
collection, and is now to be seen in the Natural History Museum at South
Kensington. The evidence then began to pour in from other quarters;
portions of stone from Italy and from Benares were found to be of
identical composition with the Yorkshire stone. The incredulity of those
who had doubted the celestial origin of these objects began to give way.
A careful memoir on the Benares meteorite, by Howard, was published in
the "Philosophical Transactions" for 1802, while, as if to complete the
demonstration, a great shower of stones took place in the following year
at L'Aigle, in Normandy. The French Academy deputed the physicist Biot
to visit the locality and make a detailed examination of the
circumstances attending this memorable shower. His enquiry removed every
trace of doubt, and the meteoric stones have accordingly been
transferred from the dominions of geology to those of astronomy. It may
be noted that the recognition of the celestial origin of meteorites
happens to be simultaneous with the discovery of the first of the minor
planets. In each case our knowledge of the solar system has been
extended by the addition of numerous minute bodies, which,
notwithstanding their insignificant dimensions, are pregnant with
information.
When the possibility of stone-falls has been admitted, we can turn to
the ancient records, and assign to them the credit they merit, which
was withheld for so many centuries. Perhaps the earliest of all these
stone-falls which can be said to have much pretension to historical
accuracy is that of the shower which Livy describes as having fallen,
about the year 654 B.C., on the Alban Mount, near Rome. Among the more
modern instances, we may mention one which was authenticated in a very
emphatic manner. It occurred in the year 1492 at Ensisheim, in Alsace.
The Emperor Maximilian ordered a minute narrative of the circumstances
to be drawn up and deposited with the stone in the church. The stone was
suspended in the church for three centuries, until in the French
Revolution it was carried off to Colmar, and pieces were broken from it,
one of which is now in our national collection. Fortunately, this
interesting object has been restored to its ancient position in the
church at Ensisheim, where it remains an attraction to sight-seers at
this day. The account is as follows:--"In the year of the Lord 1492, on
the Wednesday before St. Martin's Day, November 7th, a singular miracle
occurred, for between eleven o'clock and noon there was a loud clap of
thunder and a prolonged confused noise, which was heard at a great
distance, and a stone fell from the air in the jurisdiction of Ensisheim
which weighed 260 pounds, and the confused noise was at other places
much louder than here. Then a boy saw it strike on ploughed ground in
the upper field towards the Rhine and the Ill, near the district of
Gisgang, which was sown with wheat, and it did no harm, except that it
made a hole there; and then they conveyed it from the spot, and many
pieces were broken from it, which the Land Vogt forbade. They therefore
caused it to be placed in the church, with the intention of suspending
it as a miracle, and there came here many people to see this stone, so
there were many remarkable conversations about this stone; the learned
said they knew not what it was, for it was beyond the ordinary course of
nature that such a large stone should smite from the height of the air,
but that it was really a miracle from God, for before that time never
was anything heard like it, nor seen, nor written. When they found that
stone, it had entered into the earth to half the depth of a man's
stature, which everybody explained to be the will of God that it should
be found, and the noise of it was heard at Lucerne, at Villingen, and at
many other places, so loud that the people thought that the houses had
been overturned; and as the King Maximilian was here, the Monday after
St. Catherine's Day of the same year, his Royal Excellency ordered the
stone which had fallen to be brought to the castle, and after having
conversed a long time about it with the noblemen, he said that the
people of Ensisheim should take it and order it to be hung up in the
church, and not to allow anybody to take anything from it. His
Excellency, however, took two pieces of it, of which he kept one, and
sent the other to Duke Sigismund of Austria, and there was a great deal
of talk about the stone, which was suspended in the choir, where it
still is, and a great many people came to see it."
Admitting the celestial origin of the meteorites, they surely claim our
closest attention. They afford the only direct method we possess of
obtaining a knowledge of the materials of bodies exterior to our planet.
We can take a meteorite in our hands, we can analyse it, and find the
elements of which it is composed. We shall not attempt to enter into any
very detailed account of the structure of meteorites; it is rather a
matter for the consideration of chemists and mineralogists than for
astronomers. A few of the more obvious features will be all that we
require. They will serve as a preliminary to the discussion of the
probable origin of these bodies.
In the Natural History Museum at South Kensington we may examine a
superb collection of meteorites. They have been brought together from
all parts of the earth, and vary in size from bodies not much larger
than a pin's head up to vast masses weighing many hundredweights. There
are also models of celebrated meteorites, of which the originals are
dispersed through various other museums.
Many meteorites have nothing very remarkable in their external
appearance. If they were met with on the sea beach, they would be passed
by without more notice than would be given to any other stone. Yet, what
a history a meteorite might tell us if we could only manage to obtain
it! It fell; it was seen to fall from the sky; but what was its course
anterior to that movement? Where was it 100 years ago, 1,000 years ago?
Through what regions of space has it wandered? Why did it never fall
before? Why has it actually now fallen? Such are some of the questions
which crowd upon us as we ponder over these most interesting bodies.
Some of these objects are composed of very characteristic materials;
take, for example, one of the more recent arrivals, known as the Rowton
siderite. This body differs very much from the more ordinary kind of
stony meteorite. It is an object which even a casual passer-by would
hardly pass without notice. Its great weight would also attract
attention, while if it be scratched or rubbed with a file, it would
appear to be a mass of nearly pure iron. We know the circumstances in
which that piece of iron fell to the earth. It was on the 20th of April,
1876, about 3.40 p.m., that a strange rumbling noise, followed by a
startling explosion, was heard over an area of several miles in extent
among the villages in Shropshire, eight or ten miles north of the
Wrekin. About an hour after this occurrence a farmer noticed that the
ground in one of his grass-fields had been disturbed, and he probed the
hole which the meteorite had made, and found it, still warm, about
eighteen inches below the surface. Some men working at no great distance
had heard the noise made in its descent. This remarkable object, weighs
7-3/4 lbs. It is an irregular angular mass of iron, though all its edges
seem to have been rounded by fusion in its transit through the air. It
is covered with a thick black pellicle of the magnetic oxide of iron,
except at the point where it first struck the ground. The Duke of
Cleveland, on whose property it fell, afterwards presented it to our
national institution already referred to, where, as the Rowton siderite,
it attracts the attention of everyone who is interested in these
wonderful bodies.
This siderite is specially interesting on account of its distinctly
metallic character. Falls of objects of this particular type are not so
frequent as are those of the stony meteorites; in fact, there are only a
few known instances of meteoric irons having been actually seen to
fall, while the observed falls of stony meteorites are to be counted in
scores or in hundreds. The inference is that the iron meteorites are
much less frequent than the stony ones. This is, however, not the
impression that the visitor to the Museum would be likely to receive. In
that extensive collection the meteoric irons are by far the most
striking objects. The explanation is not difficult. Those gigantic
masses of iron are unquestionably meteoric: no one doubts that this is
the case. Yet the vast majority of them have never been seen to fall;
they have simply been found, in circumstances which point unmistakably
to their meteoric nature. Suppose, for instance, that a traveller on one
of the plains of Siberia or of Central America finds a mass of metallic
iron lying on the surface of the ground, what explanation can be
rendered of such an occurrence? No one has brought the iron there, and
there is no iron within hundreds of miles. Man never fashioned that
object, and the iron is found to be alloyed with nickel in a manner that
is always observed in known meteorites, and is generally regarded as a
sure indication of a meteoric origin. Observe also, that as iron
perishes by corrosion in our atmosphere, that great mass of iron cannot
have lain where it is for indefinite ages; it must have been placed
there at some finite time. Only one source for such an object is
conceivable; it must have fallen from the sky. On the same plains the
stony meteorites have also fallen in hundreds and in thousands, but they
crumble away in the course of time, and in any case would not arrest the
attention of the traveller as the irons are likely to do. Hence it
follows, that although the stony meteorites seem to fall much more
frequently, yet, unless they are actually observed at the moment of
descent, they are much more liable to be overlooked than the meteoric
irons. Hence it is that the more prominent objects of the British
collection are the meteoric irons.
We have said that a noise accompanied the descent of the Rowton
siderite, and it is on record that a loud explosion took place when the
meteorite fell at Ensisheim. In this we have a characteristic feature of
the phenomenon. Nearly all the descents of meteorites that have been
observed seem to have been ushered in by a detonation. We do not,
however, assert that this is quite an invariable feature; and it is also
the case that meteors often detonate without throwing down any solid
fragments that have been collected. The violence associated with the
phenomenon is forcibly illustrated by the Butsura meteorite. This object
fell in India in 1861. A loud explosion was heard, several fragments of
stone were collected from distances three or four miles apart; and when
brought together, they were found to fit, so as to enable the primitive
form of the meteorite to be reconstructed. A few of the pieces are
wanting (they were, no doubt, lost by falling unobserved into localities
from which they could not be recovered), but we have obtained pieces
quite numerous enough to permit us to form a good idea of the irregular
shape of the object before the explosion occurred which shattered it
into fragments. This is one of the ordinary stony meteorites, and is
thus contrasted with the Rowton siderite which we have just been
considering. There are also other types of meteorites. The Breitenbach
iron, as it is called, is a good representative of a class of these
bodies which lie intermediate between the meteoric irons and the stones.
It consists of a coarsely cellular mass of iron, the cavities being
filled with mineral substances. In the Museum, sections of intermediate
forms are shown in which this structure is exhibited.
Look first at the most obvious characteristic of these meteorites. We do
not now allude to their chemical composition, but to their external
appearance. What is the most remarkable feature in the shape of these
objects?--surely it is that they are fragments. They are evidently
pieces that are _broken_ from some larger object. This is apparent by
merely looking at their form; it is still more manifest when we examine
their mechanical structure. It is often found that meteorites are
themselves composed of smaller fragments. Such a structure may be
illustrated by a section of an aerolite found on the Sierra of Chaco,
weighing about 30 lbs. (Fig. 79).
The section here represented shows the composite structure of this
object, which belongs to the class of stony meteorites. Its shape shows
that it was really a fragment with angular edges and corners. No doubt
it may have been much more considerable when it first dashed into the
atmosphere. The angular edges now seen on the exterior may be due to an
explosion which then occurred; but this will not account for the
structure of the interior. We there see irregular pieces of varied form
and material agglomerated into a single mass. If we would seek for
analogous objects on the earth, we must look to some of the volcanic
rocks, where we have multitudes of irregular angular fragments cemented
together by a matrix in which they are imbedded. The evidence presented
by this meteorite is conclusive as to one circumstance with regard to
the origin of these objects. They must have come as fragments, from some
body of considerable, if not of vast, dimensions. In this meteorite
there are numerous small grains of iron mingled with mineral substances.
The iron in many meteorites has, indeed, characters resembling those
produced by the actual blasting of iron by dynamite. Thus, a large
meteoric iron from Brazil has been found to have been actually shivered
into fragments at some time anterior to its fall on the earth. These
fragments have been cemented together again by irregular veins of
mineral substances.
[Illustration: Fig. 79.--Section of the Chaco Meteorite.]
For an aerolite of a very different type we may refer to the
carbonaceous meteorite of Orgueil, which fell in France on the 14th May,
1864. On the occasion of its descent a splendid meteor was seen,
rivalling the full moon in size. The actual diameter of this globe of
fire must have been some hundreds of yards. Nearly a hundred fragments
of the body were found scattered over a tract of country fifteen miles
long. This object is of particular interest, inasmuch as it belongs to a
rare group of aerolites, from which metallic iron is absent. It contains
many of the same minerals which are met with in other meteorites, but in
these fragments they are _associated with carbon_, and with substances
of a white or yellowish crystallisable material, soluble in ether, and
resembling some of the hydrocarbons. Such a substance, if it had not
been seen falling to the earth, would probably be deemed a product
resulting from animal or vegetable life!
We have pointed out how a body moving with great velocity and impinging
upon the air may become red-hot and white-hot, or even be driven off
into vapour. How, then, does it happen that meteorites escape this fiery
ordeal, and fall down to the earth, with a great velocity, no doubt, but
still, with very much less than that which would have sufficed to drive
them off into vapour? Had the Rowton siderite, for instance, struck our
atmosphere with a velocity of twenty miles a second, it seems
unquestionable that it would have been dissipated by heat, though, no
doubt, the particles would ultimately coalesce so as to descend slowly
to the earth in microscopic beads of iron. How has the meteorite escaped
this fate? It must be remembered that our earth is also moving with a
velocity of about eighteen miles per second, and that the _relative_
velocity with which the meteorite plunges into the air is that which
will determine the degree to which friction is operating. If the
meteorite come into direct collision with the earth, the velocity of the
collision will be extremely great; but it may happen that though the
actual velocities of the two bodies are both enormous, yet the relative
velocity may be comparatively small. This is, at all events, one
conceivable explanation of the arrival of a meteorite on the surface of
the earth.
We have shown in the earlier parts of the chapter that the well-known
star showers are intimately connected with comets. In fact, each star
shower revolves in the path pursued by a comet, and the shooting star
particles have, in all probability, been themselves derived from the
comet. Showers of shooting stars have, therefore, an intimate connection
with comets, but it is doubtful whether meteorites have any connection
with comets. It has already been remarked that meteorites have never
been known to fall in the great star showers. No particle of a meteorite
is known to have dropped from the countless host of the Leonids or of
the Perseids; as far as we know, the Lyrids never dropped a meteorite,
nor did the Quadrantids, the Geminids, or the many other showers with
which every astronomer is familiar. There is no reason to connect
meteorites with these showers, and it is, therefore, doubtful whether we
should connect meteorites with comets.
With reference to the origin of meteorites it is difficult to speak with
any great degree of confidence. Every theory of meteorites presents
difficulties, so it seems that the only course open to us is to choose
that view of their origin which seems least improbable. It appears to me
that this condition is fulfilled in the theory entertained by the
Austrian mineralogist, Tschermak. He has made a study of the meteorites
in the rich collection at Vienna, and he has come to the conclusion that
the "meteorites have had a volcanic source on some celestial body." Let
us attempt to pursue this reasoning and discuss the problem, which may
be thus stated:--Assuming that at least some of the meteorites have been
ejected from volcanoes, on what body or bodies in the universe must
these volcanoes be situated? This is really a question for astronomers
and mathematicians. Once the mineralogists assure us that these bodies
are volcanic, the question becomes one of calculation and of the balance
of probabilities.
The first step in the enquiry is to realise distinctly the dynamical
conditions of the problem. Conceive a volcano to be located on a planet.
The volcano is supposed to be in a state of eruption, and in one of its
mighty throes projects a missile aloft: this missile will ascend, it
will stop, and fall down again. Such is the case at present in the
eruptions of terrestrial volcanoes. Cotopaxi has been known to hurl
prodigious stones to a vast height, but these stones assuredly return to
earth. The gravitation of the earth has gradually overcome the velocity
produced by the explosion, and down the body falls. But let us suppose
that the eruption is still more violent, and that the stones are
projected from the planet to a still greater height above its surface.
Suppose, for instance, that the stone should be shot up to a height
equal to the planet's radius, the attraction of gravitation will then be
reduced to one-fourth of what it was at the surface, and hence the
planet will find greater difficulty in pulling back the stone. Not only
is the distance through which the stone has to be pulled back increased
as the height increases, but the efficiency of gravitation is weakened,
so that in a twofold way the difficulty of recalling the stone is
increased. We have already more than once alluded to this subject, and
we have shown that there is a certain critical velocity appropriate to
each planet, and depending on its mass and its radius. If the missile be
projected upwards with a velocity equal to or greater than this, then it
will ascend never to return. We all recollect Jules Verne's voyage to
the moon, in which he described the Columbiad, an imaginary cannon,
capable of shooting out a projectile with a velocity of six or seven
miles a second. This is the critical velocity for the earth. If we could
imagine the air removed, then a cannon of seven-mile power would project
a body upwards which would never fall down.
The great difficulty about Tschermak's view of the volcanic origin of
the meteorites lies in the tremendous initial velocity which is
required. The Columbiad is a myth, and we know no agent, natural or
artificial, at the present time on the earth, adequate to the production
of a velocity so appalling. The thunders of Krakatoa were heard
thousands of miles away, but in its mightiest throes it discharged no
missiles with a velocity of six miles a second. We are therefore led to
enquire whether any of the other celestial bodies are entitled to the
parentage of the meteorites. We cannot see volcanoes on any other body
except the moon; all the other bodies are too remote for an inspection
so minute. Does it seem likely that volcanoes on the moon can ever
launch forth missiles which fall upon the earth?
This belief was once sustained by eminent authority. The mass of the
moon is about one-eightieth of the mass of the earth. It would not be
true to assert that the critical velocity of projection varies directly
as the mass of the planet. The correct law is, that it varies directly
as the square root of the mass, and inversely as the square root of the
radius. It is hence shown that the velocity required to project a
missile away from the moon is only about one-sixth of that which would
be required to project a missile away from the earth. If the moon had on
its surface volcanoes of one-mile power, it is quite conceivable that
these might be the source of meteorites. We have seen how the whole
surface of the moon shows traces of intense volcanic activity. A missile
thus projected from the moon could undoubtedly fall on the earth, and it
is not impossible that some of the meteorites may really have come from
this source. There is, however, one great difficulty about the volcanoes
on the moon. Suppose an object were so projected, it would, under the
attraction of the earth, in accordance with Kepler's laws, move around
the earth as a focus. If we set aside the disturbances produced by all
other bodies, as well as the disturbance produced by the moon itself, we
see that the meteorite if it once misses the earth can never fall
thereon. It would be necessary that the shortest distance of the earth's
centre from the orbit of the projectile should be less than the radius
of the earth, so that if a lunar meteorite is to fall on the earth, it
must do so the first time it goes round. The journey of a meteorite from
the moon to the earth is only a matter of days, and therefore, as
meteorites are still falling, it would follow that they must still be
constantly ejected from the moon. The volcanoes on the moon are,
however, not now active; observers have long studied its surface, and
they find no reliable traces of volcanic activity at the present day.
It is utterly out of the question, whatever the moon may once have been
able to do, that at the present date she could still continue to launch
forth meteorites. It is just possible that a meteorite expelled from the
moon in remote antiquity, when its volcanoes were active, may, under the
influence of the disturbances of the other bodies of the system, have
its orbit so altered, that at length it comes within reach of the
atmosphere and falls to the earth, but in no circumstances could the
moon send us a meteorite at present. It is therefore reasonable to look
elsewhere in our search for volcanoes fulfilling the conditions of the
problem.
Let us now direct our attention to the planets, and examine the
circumstances in which volcanoes located thereon could eject a meteorite
which should ultimately tumble on the earth. We cannot see the planets
well enough to tell whether they have or ever had any volcanoes; but the
almost universal presence of heat in the large celestial masses seems to
leave us in little doubt that some form of volcanic action might be
found in the planets. We may at once dismiss the giant planets, such as
Jupiter or Saturn: their appearance is very unlike a volcanic surface;
while their great mass would render it necessary to suppose that the
meteorites were expelled with terrific velocity if they should succeed
in escaping from the gravitation of the planet. Applying the rule
already given, a volcano on Jupiter would have to be five or six times
as powerful as the volcano on the earth. To avoid this difficulty, we
naturally turn to the smaller planets of the system; take, for instance,
one of that innumerable host of minor planets, and let us enquire how
far this body is likely to have ejected a missile which should fall upon
the earth. Some of these globes are only a few miles in diameter. There
are bodies in the solar system so small that a very moderate velocity
would be sufficient to project a missile away from them altogether. We
have, indeed, already illustrated this point in discussing the minor
planets. It has been suggested that a volcano placed on one of the minor
planets might be quite powerful enough to start the meteorites on a
long ramble through space until the chapter of accidents brought them
into collision with the earth. There is but little difficulty in
granting that there might be such volcanoes, and that they might be
sufficiently powerful to drive bodies from the surface of the planet;
but we must remember that the missiles are to fall on the earth, and
dynamical considerations are involved which merit our close attention.
To concentrate our ideas, we shall consider one of the minor planets,
and for this purpose let us take Ceres. If a meteorite is to fall upon
the earth, it must pass through the narrow ring, some 8,000 miles wide,
which marks the earth's path; it will not suffice for the missile to
pass through the ecliptic on the inside or on the outside of the ring,
it must be actually through this narrow strip, and then if the earth
happens to be there at the same moment the meteorite will fall. The
first condition to be secured is, therefore, that the path of the
meteorite shall traverse this narrow ring. This is to be effected by
projection from some point in the orbit of Ceres. But it can be shown on
purely dynamical grounds that although the volcanic energy sufficient to
remove the projectile from Ceres may be of no great account, yet if that
projectile is to cross the earth's track, the dynamical requirements of
the case demand a volcano on Ceres at the very least of three-mile
power. We have thus gained but little by the suggestion of a minor
planet, for we have not found that a moderate volcanic power would be
adequate. But there is another difficulty in the case of Ceres, inasmuch
as the ring on the ecliptic is very narrow in comparison with the other
dimensions of the problem. Ceres is a long way off, and it would require
very great accuracy in volcanic practice on Ceres to project a missile
so that it should just traverse this ring and fall neither inside nor
outside, neither above nor below. There must be a great many misses for
every hit. We have attempted to make the calculation by the aid of the
theory of probabilities, and we find that the chances against this
occurrence are about 50,000 to 1, so that out of every 50,000
projectiles hurled from a point in the orbit of Ceres only a single one
can be expected to satisfy even the first of the conditions necessary if
it is ever to tumble on our globe. It is thus evident that there are two
objections to Ceres (and the same may be said of the other minor
planets) as a possible source of the meteorites. Firstly, that
notwithstanding the small mass of the planet a very powerful volcano
would still be required; and secondly, that we are obliged to assume
that for every one which ever reached the earth at least 50,000 must
have been ejected. It is thus plain that if the meteorites have really
been driven from some planet of the solar system, large or small, the
volcano must, from one cause or another, have been a very powerful one.
We are thus led to enquire which planet possesses on other grounds the
greatest probability in its favour.
We admit of course that at the present time the volcanoes on the earth
are utterly devoid of the necessary power; but were the terrestrial
volcanoes always so feeble as they are in these later days? Grounds are
not wanting for the belief that in the very early days of geological
time the volcanic energy on the earth was much greater than at present.
We admit fully the difficulties of the view that the meteorites have
really come from the earth; but they must have some origin, and it is
reasonable to indicate the source which seems to have most probability
in its favour. Grant for a moment that in the primaeval days of volcanic
activity there were some mighty throes which hurled forth missiles with
the adequate velocity: these missiles would ascend, they would pass from
the gravitation of the earth, they would be seized by the gravitation of
the sun, and they would be compelled to revolve around the sun for ever
after. No doubt the resistance of the air would be a very great
difficulty, but this resistance would be greatly lessened were the
crater at a very high elevation above the sea level, while, if a vast
volume of ejected gases or vapours accompanied the more solid material,
the effect of the resistance of the air would be still further reduced.
Some of these objects might perhaps revolve in hyperbolic orbits, and
retreat never to return; while others would be driven into elliptic
paths. Round the sun these objects would revolve for ages, but at each
revolution--and here is the important point--they would traverse the
point from which they were originally launched. In other words, every
object so projected from the earth would at each revolution cross the
track of the earth. We have in this fact an enormous probability in
favour of the earth as contrasted with Ceres. Only one Ceres-ejected
meteorite out of every 50,000 would probably cross the earth's track,
while every earth-projected meteorite would necessarily do so.
If this view be true, then there must be hosts of meteorites traversing
space in elliptic orbits around the sun. These orbits have one feature
in common: they all intersect the track of the earth. It will sometimes
happen that the earth is found at this point at the moment the meteorite
is crossing; when this is the case the long travels of the little body
are at an end, and it tumbles back on the earth from which it parted so
many ages ago.
It is well to emphasise the contrast between the lunar theory of
meteorites (which we think improbable) and the terrestrial theory (which
appears to be probable). For the lunar theory it would, as we have seen,
be necessary that some of the lunar volcanoes should be still active. In
the terrestrial theory it is only necessary to suppose that the
volcanoes on the earth once possessed sufficient explosive power. No one
supposes that the volcanoes at present on the earth eject now the
fragments which are to form future meteorites; but it seems possible
that the earth may be now slowly gathering back, in these quiet times,
the fragments she ejected in an early stage of her history. Assuming,
therefore, with Tschermak, that many meteorites have had a volcanic
origin on some considerable celestial body, we are led to agree with
those who think that most probably that body is the earth.
It is interesting to notice a few circumstances which seem to
corroborate the view that many meteorites are of ancient terrestrial
origin. The most characteristic constituent of these bodies is the alloy
of iron and nickel, which is almost universally present. Sometimes, as
in the Rowton siderite, the whole object consists of little else;
sometimes this alloy is in grains distributed through the mass. When
Nordenskjoeld discovered in Greenland a mass of native iron containing
nickel, this was at once regarded as a celestial visitor. It was called
the Ovifak meteorite, and large pieces of the iron were conveyed to our
museums. There is, for instance, in the national collection a most
interesting exhibit of the Ovifak substance. Close examination shows
that this so-called meteorite lies in a bed of basalt which has been
vomited from the interior of the earth. Those who believe in the
meteoric origin of the Ovifak iron are constrained to admit that shortly
after the eruption of the basalt, and while it was still soft, this
stupendous iron meteorite of gigantic mass and bulk happened to fall
into this particular soft bed. The view is, however, steadily gaining
ground that this great iron mass was no celestial visitor at all, but
that it simply came forth from the interior of the earth with the basalt
itself. The beautiful specimens in the British Museum show how the iron
graduates into the basalt in such a way as to make it highly probable
that the source of the iron is really to be sought in the earth and not
external thereto. Should further research establish this, as now seems
probable, a most important step will have been taken in proving the
terrestrial origin of meteorites. If the Ovifak iron be really
associated with the basalt, we have a proof that the iron-nickel alloy
is indeed a terrestrial substance, found deep in the interior of the
earth, and associated with volcanic phenomena. This being so, it will be
no longer difficult to account for the iron in undoubted meteorites.
When the vast volcanoes were in activity they ejected masses of this
iron-alloy, which, having circulated round the sun for ages, have at
last come back again. As if to confirm this view, Professor Andrews
discovered particles of native iron in the basalt of the Giant's
Causeway, while the probability that large masses of iron are there
associated with the basaltic formation was proved by the researches on
magnetism of the late Provost Lloyd.
Besides the more solid meteorites there can be no doubt that the
_debris_ of the ordinary shooting stars must rain down upon the earth in
gentle showers of celestial dust. The snow in the Arctic regions has
often been found stained with traces of dust which contains particles of
iron. Similar particles have been found on the towers of cathedrals and
in many other situations where it could only have been deposited from
the air. There can be hardly a doubt that some of the motes in the
sunbeam, and many of the particles which good housekeepers abhor as
dust, have indeed a cosmical origin. In the famous cruise of the
_Challenger_ the dredges brought up from the depths of the Atlantic no
"wedges of gold, great anchors, heaps of pearl," but among the mud which
they raised are to be found numerous magnetic particles which there is
every reason to believe fell from the sky, and thence subsided to the
depths of the ocean. Sand from the deserts of Africa, when examined
under the microscope, yield traces of minute iron particles which bear
the marks of having experienced a high temperature.
The earth draws in this cosmic dust continuously, but the earth now
never parts with a particle of its mass. The consequence is inevitable;
the mass of the earth must be growing, and though the change may be a
small one, yet to those who have studied Darwin's treatise on
"Earth-worms," or to those who are acquainted with the modern theory of
evolution, it will be manifest that stupendous results can be achieved
by slight causes which tend in one direction. It is quite probable that
an appreciable part of the solid substance of our globe may have been
derived from meteoric matter which descends in perennial showers upon
its surface.