of the country, and also in numerous letters addressed either to my
scientific friends or to myself, the following appeared to be the
_leading facts_ attending the phenomenon. The shower pervaded nearly
the whole of North America, having appeared in nearly equal splendor
from the British possessions on the north to the West-India Islands and
Mexico on the south, and from sixty-one degrees of longitude east of the
American coast, quite to the Pacific Ocean on the west. Throughout this
immense region, the duration was nearly the same. The meteors began to
attract attention by their unusual frequency and brilliancy, from _nine
to twelve_ o'clock in the evening; were most striking in their
appearance from _two to five;_ arrived at their maximum, in many places,
about _four_ o'clock; and continued until rendered invisible by the
light of day. The meteors moved either in right lines, or in such
apparent curves, as, upon optical principles, can be resolved into right
lines. Their general tendency was towards the northwest, although, by
the effect of perspective, they appeared to move in various directions.
Such were the leading phenomena of the great meteoric shower of November
13, 1833. For a fuller detail of the facts, as well as of the reasonings
that were built on them, I must beg leave to refer you to some papers of
mine in the twenty-fifth and twenty-sixth volumes of the American
Journal of Science.
Soon after this wonderful occurrence, it was ascertained that a similar
meteoric shower had appeared in 1799, and, what was remarkable, almost
at exactly the same time of year, namely, on the morning of the twelfth
of November; and we were again surprised as well as delighted, at
receiving successive accounts from different parts of the world of the
phenomenon, as having occurred on the morning of the same thirteenth of
November, in 1830, 1831, and 1832. Hence this was evidently an event
independent of the casual changes of the atmosphere; for, having a
periodical return, it was undoubtedly to be referred to astronomical
causes, and its recurrence, at a certain definite period of the year,
plainly indicated _some_ relation to the revolution of the earth around
the sun. It remained, however, to develope the nature of this relation,
by investigating, if possible, the origin of the meteors. The views to
which I was led on this subject suggested the probability that the same
phenomenon would recur on the corresponding seasons of the year, for at
least several years afterwards; and such proved to be the fact, although
the appearances, at every succeeding return, were less and less
striking, until 1839, when, so far as I have heard, they ceased
altogether.
Mean-while, two other distinct periods of meteoric showers have, as
already intimated, been determined; namely, about the ninth of August,
and seventh of December. The facts relative to the history of these
periods have been collected with great industry by Mr. Edward C.
Herrick; and several of the most ingenious and most useful conclusions,
respecting the laws that regulate these singular exhibitions, have been
deduced by Professor Twining. Several of the most distinguished
astronomers of the Old World, also, have engaged in these investigations
with great zeal, as Messrs. Arago and Biot, of Paris; Doctor Olbers, of
Bremen; M. Wartmann, of Geneva; and M. Quetelet, of Brussels.
But you will be desirous to learn what are the _conclusions_ which have
been drawn respecting these new and extraordinary phenomena of the
heavens. As the inferences to which I was led, as explained in the
twenty-sixth volume of the 'American Journal of Science,' have, at least
in their most important points, been sanctioned by astronomers of the
highest respectability, I will venture to give you a brief abstract of
them, with such modifications as the progress of investigation since
that period has rendered necessary.
The principal questions involved in the inquiry were the following:--Was
the _origin_ of the meteors within the atmosphere, or beyond it? What
was the _height_ of the place above the surface of the earth? By what
_force_ were the meteors drawn or impelled towards the earth? In what
_directions_ did they move? With what _velocity_? What was the cause of
their _light_ and _heat_? Of what _size_ were the larger varieties? At
what height above the earth did they _disappear_? What was the nature of
the _luminous trains_ which sometimes remained behind? What _sort of
bodies_ were the meteors themselves; of what _kind of matter_
constituted; and in what manner did they exist _before they fell to the
earth_? Finally, what _relations_ did the source from which they
emanated sustain to our earth?
In the first place, _the meteors had their origin beyond the limits of
our atmosphere_. We know whether a given appearance in the sky is within
the atmosphere or beyond it, by this circumstance: all bodies near the
earth, including the atmosphere itself, have a common motion with the
earth around its axis from west to east. When we see a celestial object
moving regularly from west to east, at the same rate as the earth moves,
leaving the stars behind, we know it is near the earth, and partakes, in
common with the atmosphere, of its diurnal rotation: but when the earth
leaves the object behind; or, in other words, when the object moves
westward along with the stars, then we know that it is so distant as not
to participate in the diurnal revolution of the earth, and of course to
be beyond the atmosphere. The source from which the meteors emanated
thus kept pace with the stars, and hence was beyond the atmosphere.
In the second place, _the height of the place whence the meteors
proceeded was very great, but it has not yet been accurately
determined_. Regarding the body whence the meteors emanated after the
similitude of a cloud, it seemed possible to obtain its height in the
same manner as we measure the height of a cloud, or indeed the height of
the moon. Although we could not see the body itself, yet the part of the
heavens whence the meteors came would indicate its position. This point
we called the _radiant_; and the question was, whether the radiant was
projected by distant observers on different parts of the sky; that is,
whether it had any _parallax_. I took much pains to ascertain the truth
of this matter, by corresponding with various observers in different
parts of the United States, who had accurately noted the position of the
radiant among the fixed stars, and supposed I had obtained such
materials as would enable us to determine the parallax, at least
approximately; although such discordances existed in the evidence as
reasonably to create some distrust of its validity. Putting together,
however, the best materials I could obtain, I made the height of the
radiant above the surface of the earth _twenty-two hundred and
thirty-eight miles_. When, however, I afterwards obtained, as I
supposed, some insight into the celestial origin of the meteors, I at
once saw that the meteoric body must be much further off than this
distance; and my present impression is, that we have not the means of
determining what its height really is. We may safely place it at many
thousand miles.
In the third place, with respect to the _force_ by which the meteors
were _drawn_ or impelled towards the earth, my first impression was,
that they fell merely by the force of _gravity_; but the velocity which,
on careful investigation by Professor Twining and others, has been
ascribed to them, is greater than can possibly result from gravity,
since a body can never acquire, by gravity alone, a velocity greater
than about seven miles per second. Some other cause, beside gravity,
must therefore act, in order to give the meteors so great an apparent
velocity.
In the fourth place, _the meteors fell towards the earth in straight
lines, and in directions which, within considerable distances, were
nearly parallel with each other_. The courses are inferred to have been
in _straight lines_, because no others could have appeared to spectators
in different situations to have described arcs of great circles. In
order to be projected into the arc of a great circle, the line of
descent must be in a plane passing through the eye of the spectator; and
the intersection of such planes, passing through the eyes of different
spectators, must be straight lines. The lines of direction are inferred
to have been _parallel_, on account of their apparent radiation from one
point, that being the vanishing point of parallel lines. This may
appear to you a little paradoxical, to infer that lines are parallel,
because they _diverge_ from one and the same point; but it is a
well-known principle of perspective, that parallel lines, when continued
to a great distance from the eye, appear to converge towards the remoter
end. You may observe this in two long rows of trees, or of street lamps.
[Illustration Fig. 69.]
Some idea of the manner in which the meteors fell, and of the reason of
their apparent radiation from a common point, may be gathered from the
annexed diagram. Let A B C, Fig. 69, represent the vault of the sky,
the centre of which, D, being the place of the spectator. Let 1, 2, 3,
&c., represent parallel lines directed towards the earth. A luminous
body descending through 1' 1, coinciding with the line D E, coincident
with the axis of vision, (or the line drawn from the meteoric body to
the eye,) would appear stationary all the while at 1´, because distant
bodies always appear stationary when they are moving either directly
towards us or directly from us. A body descending through 2 2, would
seem to describe the short arc 2' 2', appearing to move on the concave
of the sky between the lines drawn from the eye to the two extremities
of its line of motion; and, for a similar reason, a body descending
through 3 3, would appear to describe the larger arc 3' 3'. Hence, those
meteors which fell nearer to the axis of vision, would describe shorter
arcs, and move slower, while those which were further from the axis and
nearer the horizon would appear to describe longer arcs, and to move
with greater velocity; the meteors would all seem to radiate from a
common centre, namely, the point where the axis of vision met the
celestial vault; and if any meteor chanced to move directly in the line
of vision, it would be seen as a luminous body, stationary, for a few
seconds, at the centre of radiation. To see how exactly the facts, as
observed, corresponded to these inferences, derived from the supposition
that the meteors moved in _parallel lines_, take the following
description, as given immediately after the occurrence, by Professor
Twining. "In the vicinity of the radiant point, a few star-like bodies
were observed, possessing very little motion, and leaving very little
length of trace. Further off, the motions were more rapid and the traces
longer; and most rapid of all, and longest in their traces, were those
which originated but a few degrees above the horizon, and descended down
to it."
In the fifth place, had the meteors come from a point twenty-two hundred
and thirty-eight miles from the earth, and derived their apparent
velocity from gravity alone, then it would be found, by a very easy
calculation, that their actual velocity was about four miles per second;
but, as already intimated, the velocity observed was estimated much
greater than could be accounted for on these principles; not less,
indeed, than fourteen miles per second, and, in some instances, much
greater even than this. The motion of the earth in its orbit is about
nineteen miles per second; and the most reasonable supposition we can
make, at present, to account for the great velocity of the meteors, is,
that they derived a relative motion from the earth's passing rapidly by
them,--a supposition which is countenanced by the fact that they
generally tended _westward_ contrary to the earth's motion in its orbit.
In the sixth place, _the meteors consisted of combustible matter, and
took fire, and were consumed, in traversing the atmosphere_. That these
bodies underwent combustion, we had the direct evidence of the senses,
inasmuch as we saw them burn. That they took fire in the _atmosphere_,
was inferred from the fact that they were not luminous in their original
situations in space, otherwise, we should have seen the body from which
they emanated; and had they been luminous before reaching the
atmosphere, we should have seen them for a much longer period than they
were in sight, as they must have occupied a considerable time in
descending towards the earth from so great a distance, even at the rapid
rate at which they travelled. The immediate consequence of the
prodigious velocity with which the meteors fell into the atmosphere must
be a powerful condensation of the air before them, retarding their
progress, and producing, by a sudden compression of the air, a great
evolution of heat. There is a little instrument called the _air-match_,
consisting of a piston and cylinder, like a syringe, in which we strike
a light by suddenly forcing down the piston upon the air below. As the
air cannot escape, it is suddenly compressed, and gives a spark
sufficient to light a piece of tinder at the bottom of the cylinder.
Indeed, it is a well-known fact, that, whenever air is suddenly and
forcibly compressed, heat is elicited; and, if by such a compression as
may be given by the hand in the air-match, heat is evolved sufficient to
fire tinder, what must be the heat evolved by the motion of a large body
in the atmosphere, with a velocity so immense. It is common to resort to
electricity as the agent which produces the heat and light of shooting
stars; but even were electricity competent to produce this effect, its
presence, in the case before us, is not proved; and its agency is
unnecessary, since so swift a motion of the meteors themselves, suddenly
condensing the air before them, is both a known and adequate cause of an
intense light and heat. A combustible body falling into the atmosphere,
under such circumstances, would become speedily ignited, but could not
burn freely, until it became enveloped in air of greater density; but,
on reaching the lower portions of the atmosphere, it would burn with
great rapidity.
In the seventh place, _some of the larger meteors must have been bodies
of great size_. According to the testimony of various individuals, in
different parts of the United States, a few fire-balls appeared as large
as the full moon. Dr. Smith, (then of North Carolina, but since
surgeon-general of the Texian army,) who was travelling all night on
professional business, describes one which he saw in the following
terms: "In size it appeared somewhat larger than the full moon rising. I
was startled by the splendid light in which the surrounding scene was
exhibited, rendering even small objects quite visible; but I heard no
noise, although every sense seemed to be suddenly aroused, in sympathy
with the violent impression on the sight." This description implies not
only that the body was very large, but that it was at a considerable
distance from the spectator. Its actual size will depend upon the
distance; for, as it appeared under the same angle as the moon, its
diameter will bear the same ratio to the moon's, as its distance bears
to the moon's distance. We could, therefore, easily ascertain how large
it was, provided we could find how far it was from the observer. If it
was one hundred and ten miles distant, its diameter was one mile, and in
the same proportion for a greater or less distance; and, if only at the
distance of one mile, its diameter was forty-eight feet. For a moderate
estimate, we will suppose it to have been twenty-two miles off; then its
diameter was eleven hundred and fifty-six feet. Upon every view of the
case, therefore, it must be admitted, that these were bodies of great
size, compared with other objects which traverse the atmosphere. We may
further infer the great magnitude of some of the meteors, from the
dimensions of the trains, or clouds, which resulted from their
destruction. These often extended over several degrees, and at length
were borne along in the direction of the wind, exactly in the manner of
a small cloud.
It was an interesting problem to ascertain, if possible, the height
above the earth at which these fire-balls exploded, or resolved
themselves into a cloud of smoke. This would be an easy task, provided
we could be certain that two or more distant observers could be sure
that both saw the same meteor; for as each would refer the place of
explosion, or the position of the cloud that resulted from it, to a
different point of the sky, a parallax would thus be obtained, from
which the height might be determined. The large meteor which is
mentioned in my account of the shower, (see page 348,) as having
exploded near the star Capella, was so peculiar in its appearance, and
in the form and motions of the small cloud which resulted from its
combustion, that it was noticed and distinguished by a number of
observers in distant parts of the country. All described the meteor as
exhibiting, substantially, the same peculiarities of appearance; all
agreed very nearly in the time of its occurrence; and, on drawing lines,
to represent the course and direction of the place where it exploded to
the view of each of the observers respectively, these lines met in
nearly one and the same point, and that was over the place where it was
seen in the zenith. Little doubt, therefore, could remain, that all saw
the same body; and on ascertaining, from a comparison of their
observations, the amount of parallax, and thence deducing its height,--a
task which was ably executed by Professor Twining,--the following
results were obtained: that this meteor, and probably all the meteors,
entered the atmosphere with a velocity not less, but perhaps greater,
than _fourteen miles in a second_; that they became luminous many miles
from the earth,--in this case, over _eighty miles_; and became extinct
high above the surface,--in this case, nearly _thirty miles_.
In the eighth place, _the meteors were combustible bodies, and were
constituted of light and transparent materials_. The fact that they
burned is sufficient proof that they belonged to the class of
_combustible_ bodies; and they must have been composed of very _light
materials_, otherwise their momentum would have been sufficient to
enable them to make their way through the atmosphere to the surface of
the earth. To compare great things with small, we may liken them to a
wad discharged from a piece of artillery, its velocity being supposed to
be increased (as it may be) to such a degree, that it shall take fire as
it moves through the air. Although it would force its way to a great
distance from the gun, yet, if not consumed too soon, it would at length
be stopped by the resistance of the air. Although it is supposed that
the meteors did in fact slightly disturb the atmospheric equilibrium,
yet, had they been constituted of dense matter, like meteoric stones,
they would doubtless have disturbed it vastly more. Their own momentum
would be lost only as it was imparted to the air; and had such a number
of bodies,--some of them quite large, perhaps a mile in diameter, and
entering the atmosphere with a velocity more than forty times the
greatest velocity of a cannon ball,--had they been composed of dense,
ponderous matter, we should have had appalling evidence of this fact,
not only in the violent winds which they would have produced in the
atmosphere, but in the calamities they would have occasioned on the
surface of the earth. The meteors were _transparent_ bodies; otherwise,
we cannot conceive why the body from which they emanated was not
distinctly visible, at least by reflecting the light of the sun. If only
the meteors which were known to fall towards the earth had been
collected and restored to their original connexion in space, they would
have composed a body of great extent; and we cannot imagine a body of
such dimensions, under such circumstances, which would not be visible,
unless formed of highly transparent materials. By these unavoidable
inferences respecting the kind of matter of which the meteors were
composed, we are unexpectedly led to recognise a body bearing, in its
constitution, a strong analogy to comets, which are also composed of
exceedingly light and transparent, and, as there is much reason to
believe, of combustible matter.
We now arrive at the final inquiry, _what relations did the body which
afforded the meteoric shower sustain to the earth_? Was it of the nature
of a satellite, or terrestrial comet, that revolves around the earth as
its centre of motion? Was it a collection of nebulous, or cometary
matter, which the earth encountered in its annual progress? or was it a
comet, which chanced at this time to be pursuing its path along with the
earth, around their common centre of motion? It could not have been of
the nature of a satellite to the earth, (or one of those bodies which
are held by some to afford the meteoric stones, which sometimes fall to
the earth from huge meteors that traverse the atmosphere,) because it
remained so long stationary with respect to the earth. A body so near
the earth as meteors of this class are known to be, could not remain
apparently stationary among the stars for a moment; whereas the body in
question occupied the same position, with hardly any perceptible
variation, for at least two hours. Nor can we suppose that the earth, in
its annual progress, came into the vicinity of a _nebula_, which was
either stationary, or wandering lawless through space. Such a collection
of matter could not remain stationary within the solar system, in an
insulated state, for, if not prevented by a motion of its own, or by the
attraction of some nearer body, it would have proceeded directly towards
the sun; and had it been in motion in any other direction than that in
which the earth was moving, it would soon have been separated from the
earth; since, during the eight hours, while the meteoric shower was
visible, the earth moved in its orbit through the space of nearly five
hundred and fifty thousand miles.
The foregoing considerations conduct us to the following train of
reasoning. First, if all the meteors which fell on the morning of
November 13, 1833, had been collected and restored to their original
connexion in space, they would of themselves have constituted a nebulous
body of great extent; but we have reason to suppose that they, in fact,
composed but a small part of the mass from which they emanated, since,
after the loss of so much matter as proceeded from it in the great
meteoric shower of 1799, and in the several repetitions of it that
preceded the year 1833, it was still capable of affording so copious a
shower on that year; and similar showers, more limited in extent, were
repeated for at least five years afterwards. We are therefore to regard
the part that descended only as _the extreme portions of a body or
collection of meteors, of unknown extent, existing in the planetary
spaces_.
Secondly, since the earth fell in with this body in the same part of its
orbit, for several years in succession, it must either have remained
there while the earth was performing its whole revolution around the
sun, or it must itself have had a revolution, as well as the earth. But
I have already shown that it could not have remained stationary in that
part of space; therefore, _it must have had a revolution around the
sun_.