THE LUNAR AND SOLAR THEORIES OF THE ORIGIN OF AEROLITES.
Besides the _cosmical_ theory of aerolites which has been adopted in
this work, and which is now accepted by a great majority of scientific
men, at least four others have been proposed: (1) the _atmospheric_,
according to which they are formed, like hail, in the earth's
atmosphere; (2) the _volcanic_, which regards them as matter ejected
with great force from terrestrial volcanoes; (3) the _lunar_, which
supposes them to have been thrown from craters in the moon; and (4)
the _solar_ hypothesis, according to which they are projected by some
tremendous explosive force from the great central orb of our system.
The first and second have been universally abandoned as untenable. The
third and fourth, however, are entitled to consideration.
THE LUNAR THEORY.
The theory which regards meteoric stones as products of eruption in
lunar volcanoes was received with favor by the celebrated Laplace: "As
the gravity at the surface of the moon," he remarks, "is much less
than at the surface of the earth, and as this body has no atmosphere
which can oppose a sensible resistance to the motion of projectiles,
we may conceive that a body projected with a great force, by the
explosion of a lunar volcano, may attain and pass the limit, where
the attraction of the earth commences to predominate over that of the
moon. For this purpose it is sufficient that its initial velocity in
the direction of the vertical may be 2500 meters in a second; then in
place of falling back on the moon, it becomes a satellite of the earth,
and describes about it an orbit more or less elongated. The direction
of its primitive impulsion may be such as to make it move directly
toward the atmosphere of the earth; or it may not attain it, till after
several and even a great number of revolutions; for it is evident that
the action of the sun, which changes in a sensible manner the distances
of the moon from the earth, ought to produce in the radius vector of a
satellite which moves in a very eccentric orbit, much more considerable
variations, and thus at length so diminish the perigean distance of the
satellite, as to make it penetrate our atmosphere. This body traversing
it with a very great velocity, and experiencing a very sensible
resistance, might at length precipitate itself on the earth; the
friction of the air against its surface would be sufficient to inflame
it, and make it detonate, provided that it contained ingredients proper
to produce these effects, and then it would present to us all those
phenomena which meteoric stones exhibit. If it was satisfactorily
proved that they are not produced by volcanoes, or generated in our
atmosphere, and that their cause must be sought beyond it, in the
regions of the heavens, the preceding hypothesis, which likewise
explains the identity of composition observed in meteoric stones, by an
identity of origin, will not be devoid of probability."--_Système du
Monde_, t. ii. cap. v.
Knowing the masses and volumes of the earth and moon, it is easy to
estimate the force of gravity at their surfaces, the distance from each
to the point of equal attraction, and the force with which a projectile
must be thrown from the lunar surface to pass within the sphere of the
earth's influence. It has been calculated that an initial velocity
of about a mile and a half in a second would be sufficient for this
purpose--a force not greater than that known to have been exerted by
terrestrial volcanoes. The _possibility_, therefore, that volcanic
matter from our satellite may reach the earth's surface seems fairly
admissible.
Since the time of Laplace, several distinguished European astronomers
have regarded the lunar hypothesis as more or less probable. It was
advocated as recently as 1851 by the late Prof. J. P. Nichol, of
Glasgow. This popular and interesting writer, after describing Tycho, a
large and well-known lunar crater, from which luminous rays or stripes
radiate over a considerable part of the moon's surface, expresses the
opinion that that immense cavity was formed by a single tremendous
explosion. "Reflecting," he remarks, "on the probable suddenness and
magnitude of that force, or rather of that _explosive_ energy one of
whose acts we have traced, as well as on the immense mass of matter
which seems to have been thus violently dispersed, is not the inquiry
a natural one, _where is that matter now_? It is a mass indeed which
cannot well have wholly disappeared. It filled a cavern 55 miles in
breadth, and 17,000 feet deep--a cavern into which even now one might
cast Chimborazo and Mont Blanc, and room be left for Teneriffe behind!
Like rocks flung aloft by our volcanoes, did this immense mass fall
back in fragments to the surface of the moon, or was the expulsive
force strong enough to give it an outward velocity sufficient to resist
the attractive power of its parent globe? The moon, be it recollected,
is very small in _mass_ compared with the earth, and her attractive
energy greatly inferior accordingly. Laplace has even calculated that
the force urging a cannon-ball, increased to a degree quite within
the limits of what is conceivable, could effect a final separation
between our satellite and any of its component parts. It is _possible_
then, and, although not demonstrable, very far from a chimera, that
the disrupted and expelled masses were, in the case of which we are
speaking, driven conclusively into space; but if so, where are they
now? where their new residence, and what their functions? In the
emergency to which I refer, such fragments would necessarily wander
among the interplanetary spaces in most irregular orbits, and chiefly
in the neighborhood of the moon and the earth. Now, while the planetary
orbits are so nicely adjusted that neither confusion nor interference
can ever occur, it is not at all likely that the same order could be
established here; nay, it is next to certain, that in the course of its
orbital revolution our globe would ever and anon come in contact with
these lunar fragments; in other words, STONES _would fall occasionally
to its surface, and apparently from its atmosphere_."--_Planetary
System_, pp. 301, 302.
We have preferred to give the views of these eminent scientists in
their own language. Olbers, Biot, and Poisson, who adopted the same
theory, estimated the _initial_ velocity which would be necessary in
order that lunar fragments might pass the point of equal attraction,
and also the _final_, or acquired velocity on reaching the earth's
surface. The several determinations of the former were as follows:
According to Olbers 1·570 miles a second.
" Biot 1·569 " "
" Laplace 1·483 " "
" Poisson 1·437 " "
The mean being almost exactly a mile and a half. The velocity on
reaching our planet, according to Olbers, would be about six and a half
miles. At the date of these calculations, however, the true velocity
of aerolites had not been in any case satisfactorily determined. Since
that time it has been found in numerous instances to exceed _twenty
miles a second_--a velocity greater than that of the earth's orbital
motion. This fact of itself would seem fatal to the theory of a lunar
origin.
At the meeting of the American Association for the Advancement of
Science, in 1859, Dr. B. A. Gould read a paper on the supposed lunar
origin of aerolites, in which the hypothesis was subjected to the test
of a rigid mathematical analysis. We will not attempt even an abstract
of this interesting memoir. It amounts, however, to a virtual disproof
of the lunar hypothesis.
THE SOLAR THEORY.
The theory which ascribes a solar origin to meteorites is not of recent
date, having been held by Diogenes Laertius and other ancient Greeks.
Among the moderns its advocates have been much less numerous than those
of the lunar hypothesis. The late Professor Charles W. Hackley, of New
York, regarded shooting-stars, aerolites, and even comets, as matter
projected with enormous force from the solar surface. The corona seen
during total eclipses of the sun he supposed to be the emanations of
this matter through the intervals of the luculi.--(See the Proceedings
of the American Association for the Advancement of Science, Fourteenth
Meeting, 1860.) An ingenious theory, differing in its details from that
of Professor Hackley, though somewhat similar in its general features,
has lately been advocated by Alexander Wilcocks, M.D., of Philadelphia,
in a memoir read before the American Philosophical Society, May 20th,
1864, and published in their Proceedings. In regard to this hypothesis
it seems sufficient to remark that it fails to give a satisfactory
account of the annual periodicity of meteoric phenomena.