surface we sometimes find solid bodies of a
stony or metallic nature, which appear to
have nothing in common with the soil on
which they lie. From time immemorial
the vulgar have attributed to these bodies
an extra-terrestrial origin. They were
believed to be stones fallen from the sky.
They have been designated pierres de
foudre, pierres de tonnerre, thunderbolts,
because they were regarded as matter shot
by lightning to the surface of the earth.
Many of these pretended thunderbolts have
been recognised to derive their origin from
the soil itself in which they were found.
Such are the ferrugineous pyrites, so
commonly occurring in chalky strata. But,
for a certain number of them, their extra-
terrestrial origin has been indisputably
ascertained. The name of aërolites (stones
of the air) is given to them as a reminder
that they fell to the earth from the depths
of the atmosphere which envelopes our
globe.

What relationship can possibly exist
between shooting stars, bolides, and aërolites?
A variety of opinions has been held on this
subject. What strikes us most is the vagueness
and indecision with which they have
been offered, the slight actual knowledge
possessed respecting the phenomena under
consideration, and at the same time the
incredulity with which philosophers have
received the accounts furnished to them
by the public.

First, as to their incredulity. In Kepler's
Ephemerides, we read, "7—17 November,
1623.* A fiery meteor, or globe of fire,
was seen throughout almost the whole of
Germany, flying rapidly from the west to
the east. It is affirmed that in Austria
something like a clap of thunder was heard.
Nevertheless, I do not believe it; for
nothing of the kind is to be found in the
accounts that we possess."

* 7, Julian date; 17, Gregorian.

In the Memoirs of the Académie des
Sciences for 1700, Lémery writes: "We
cannot reasonably doubt that the matter of
lightning and thunder is sulphur, set on
fire and shot out with great velocity. As
to the lightning-stones with which the
vulgar will have it that the thunder is
always accompanied, I take their existence
to be very doubtful, and am even inclined
to believe that there never have been any
real ones. None of these stones are to be
found on the spots that have been struck
by lightning; and even if we had found
one, we should sooner believe that it came
from some mineral matter melted and formed
by the burning sulphur of the thunder in
the earth itself, than that the stone had
been formed in the air or in the clouds,
and shot out together with the thunder."

Next, as to the vagueness and indecision
of their views. Halley several times
directed his attention to meteors, and the
causes by which they may be explained.
In a note, published in 1714, in the
Philosophical Transactions, No. 341, he relates
the occurrence of two remarkable meteors,
one of which was seen in Italy on the 21st
of March, 1676, the other in England, in
the neighbourhood of London, on the 31st
of July, 1708. He demonstrates that, from
the directions in which the latter meteor
was seen at different places, its height above
the earth may be estimated at from forty
to fifty miles. Then he adds, " I have
deeply reflected on these circumstances,
and I consider them the most important
facts that have come to my knowledge
relating to the phenomenon of meteors. I
am inclined to think that there must exist
a certain quantity of matter in ethereal
space formed by the fortuitous concourse
of atoms, and that the earth meets it while
travelling along her orbit, before it has
acquired a great rate of speed in the direction
of the sun." Here he "burned," as children
say; he was within a step or two of
what is now held to be the truth.

Some years afterwards, on the appearance
of an extraordinary meteor, seen in
England on the 19th of March, 1719 (whose
height above the earth Halley reckoned at
seventy-three miles), the great astronomer
put forth a different explanation, to the
effect that the matter constituting the
meteor had emanated from the earth,
through the effects of the preceding
unusually hot summer. Sulphurous vapours,
he thinks, have no need of air to sustain
them, but mount by a sort of centrifugal
force; they then form a train, like a train
of gunpowder, and, when inflamed by
spontaneous combustion, the fire runs along
it from one end to the other. And that
was the best explanation Halley could give
of meteors and bolides.

Mussenbrock, in his Course of
Experimental and Mathematical Physics
(translated into French, 1769), in like manner
attributes a terrestrial origin to the
materials of which fire-balls consist. "All
bodies," he says, "which form part of the
universe, emit different emanations, which
rise in the air, mingle with it, and are the
matter and cause of meteors." And