The hypothesis of the selenic origin of meteoric stones depends upon
a number of conditions, the accidental coincidence of which could
alone convert a possible to an actual fact. The view of the original
existence of small planetary masses in space is simpler, and at
the same time more analogous with those entertained concerning the
formation of other portions of the solar system.
Diogenes Laertius thought aerolites came from the sun; but Pliny
derides this theory. The fall of aerolites in bright sunshine, and
when the moon’s disc was invisible, probably led to the idea of
sun-stones. Moreover Anaxagoras regarded the sun as “a molten fiery
mass;” and Euripides, in Phaëton, terms the sun “a golden mass,”
that is to say, a fire-coloured, brightly-shining matter, but not
leading to the inference that aerolites are golden sun-stones.
The Greek philosophers had four hypotheses as to their origin:
telluric, from ascending exhalations; masses of stone raised by
hurricanes; a solar origin; and lastly, an origin in the regions of
space, as heavenly bodies which had long remained invisible: the
last opinion entirely according with that of the present day.
Chladni states that an Italian physicist, Paolo Maria Terzago,
on the occasion of the fall of an aerolite at Milan, in 1660, by
which a Franciscan monk was killed, was the first who surmised that
aerolites were of selenic origin. Without any previous knowledge
of this conjecture, Olbers was led, in 1795 (after the celebrated
fall at Siena, June 16th, 1794), to investigate the amount of the
initial tangential force that would be required to bring to the
earth masses projected from the moon. Olbers, Brandes, and Chaldni
thought that “the velocity of 16 to 32 miles, with which fire-balls
and shooting-stars entered our atmosphere,” furnished a refutation
to the view of their selenic origin. According to Olbers, it would
require to reach the earth, setting aside the resistance of the
air, an initial velocity of 8292 feet in the second; according to
Laplace, 7862; to Biot, 8282; and to Poisson, 7595. Laplace states
that this velocity is only five or six times as great as that of
a cannon-ball; but Olbers has shown that “with such an initial
velocity as 7500 or 8000 feet in a second, meteoric stones would
arrive at the surface of our earth with a velocity of only 35,000
feet.” But the measured velocity of meteoric stones averages
upwards of 114,000 feet to a second; consequently the original
velocity of projection from the moon must be almost 110,000 feet,
and therefore 14 times greater than Laplace asserted. It must,
however, be recollected, that the opinion then so prevalent, of the
existence of active volcanoes in the moon, where air and water are
absent, has since been abandoned.
Laplace elsewhere states, that in all probability aerolites “come
from the depths of space;” yet he in another passage inclines to
the hypothesis of their lunar origin, always, however, assuming
that the stones projected from the moon “become satellites of our
earth, describing around it more or less eccentric orbits, and thus
not reaching its atmosphere until several or even many revolutions
have been accomplished.”
In Syria there is a popular belief that aerolites chiefly fall
on clear moonlight nights. The ancients (Pliny tells us) looked
for their fall during lunar eclipses.--_Abridged from Humboldt’s
Cosmos_, vol. i. (Bohn’s edition).
Dr. Laurence Smith, U.S., accepts the “lunar theory,” and considers
meteorites to be masses thrown off from the moon, the attractive power
of which is but one-sixth that of the earth; so that bodies thrown from
the surface of the moon experience but one sixth the retarding force
they would have when thrown from the earth’s surface.
Look again (says Dr. Smith) at the constitution of the meteorite,
made up principally of _pure_ iron. It came evidently from some
place where there is little or no oxygen. Now the moon has no
atmosphere, and no water on its surface. There is no oxygen there.
Hurled from the moon, these bodies,--these masses of almost pure
iron,--would flame in the sun like polished steel, and on reaching
our atmosphere would burn in its oxygen until a black oxide cooled
it; and this we find to be the case with all meteorites,--the
black colour is only an external covering.
Sir Humphry Davy, from facts contained in his researches on flame,
in 1817, conceives that the light of meteors depends, not upon the
ignition of inflammable gases, but upon that of solid bodies; that such
is their velocity of motion, as to excite sufficient heat for their
ignition by the compression even of rare air; and that the phenomena of
falling stars may be explained by regarding them as small incombustible
bodies moving round the earth in very eccentric orbits, and becoming
ignited only when they pass with immense rapidity through the upper
regions of the atmosphere; whilst those meteors which throw down stony
bodies are, similarly circumstanced, combustible masses.
Masses of iron and nickel, having all the appearance of aerolites or
meteoric stones, have been discovered in Siberia, at a depth of ten
metres below the surface of the earth. From the fact, however, that no
meteoric stones are found in the secondary and tertiary formations, it
would seem to follow that the phenomena of falling stones did not take
place till the earth assumed its present conditions.