PLEASE NOTE:


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Date sent:        Wed, 15 Oct 1997 11:27:01 -0400 (EDT)
From:             Benny J Peiser <B.J.PEISER@livjm.ac.uk
Subject:          SCIENTIFIC CATASTROPHISM IN EARLY 19TH CENTURY BRITAIN
To:               cambridge-conference@livjm.ac.uk
Priority:         NORMAL

from: David Morrison <dmorrison@mail.arc.nasa.gov

--------------------------------------------

Bob Kobres has posted a prize-winning 1828 essay by David Milne on the
collisions of comets with the Earth.  A few interesting excerpts are given
below.  For full text see http://abob.libs.uga.edu/bobk/peocomet.html.

DAVID MILNE: ESSAY ON COMETS (1828)

PART III. INFLUENCE OF COMETS AND PLANETS UPON ONE ANOTHER.

. . . . Upon the whole, then, we may be assured, that, by proximity alone,
Comets are almost wholly incapable of affecting either the movements of the
Planets, or the system of things upon their surface. But the case is very
different, on the supposition of actual contact: for one of those
circumstances, which would be the chief means of counteracting a comet's
influence in approaching a planet, viz. the rapidity of its motion, would
serve, by the momentum, to give great effect to a collision. . . .

But though the probability of such a collision is extremely small, we see
that it is perfectly possible in itself; whilst the amount of that
probability may be greatly increased by lapse of time. Let us now,
therefore, shortly attend to the consequences which might ensue from such an
event It is evident that much will depend on the direction of the Comet's
course at the time of its encountering a Planet. If both be moving towards
the same quarter of the heavens, each will glide off from the surface of the
other, and no very material changes will be produced, either on their
movements or on their physical constitution. But should the directions of
their respective courses be exactly opposite, when the concurrence takes
place, (a case, however, which it is easy to see can happen only with
retrograde comets), the consequences would necessarily be far more serious
and permanent. . . .

Seeing, then, that the collision of a Comet and Planet is an event lying
within the verge of possibility, Have we any reason to suppose that it is
one which has ever happened? This question we can answer, only by examining
the movements and constitution of the Planets as they at present exist, and
tracing back the circumstances now characterizing both to those causes by
which they seem to have been produced. . . . .

It appears highly probable that none of the planetary bodies have sustained
any alterations in their orbits by the collision of a comet. But on this
account we are not to suppose that a contact has never taken place; because,
though it may not have been sufficiently violent to have altered the
planet's orbit, it may nevertheless have materially affected its physical
organization, by impinging on its surface; nor, least of all, are we to
conclude, from the experience of the past, that the collision of a comet
with any of the planetary bodies, will never happen in the course of time.
Even though it were demonstrated that such a catastrophe has never yet been
fulfilled, this circumstance could afford no assurance that it may not occur
at some future period; and therefore, it behoves us shortly to consider what
would be the nature and amount of the physical changes which the collision
of a comet would produce on the surface of a planet.

It is very true, as was formerly remarked, that the masses of comets are
usually small; and for this reason we might be disposed to imagine, that the
result of a collision would be trivial. But if a comet, moving with the
prodigious velocity which it acquires near its perihelion, should chance to
strike a planet, as for instance the Earth, then coming in an opposite
direction, the consequences would be truly disastrous . . . . waters of the
ocean, now attracted by the close approach and next driven from their
ancient bed by the contact of the comet, would sweep over the face of the
globe, covering even the highest mountains in their impetuous course, and
involving all things in undistinguishable ruin. Whole species of plants and
animals, existing in different quarters of the Earth, would, by this
cataclysm, be at once overwhelmed and annihilated: whilst the few among the
human race, who should happily be saved amid this shipwreck of Nature, would
soon relapse into a state of pristine ignorance and barbarism. After such an
event, by which all the monuments of art, and all the records of learning,
would be destroyed, mankind would necessarily for many centuries be occupied
with providing for their bare subsistence; and a long succession of ages
would elapse, before those stores of knowledge could be retrieved, which
their ancestors had been able to attain. When, however, posterity, in the
progress of time, had again become so far enlightened, as to observe and
speculate on the striking physical appearances, which in all parts of the
world would meet their attention, they could not fail to consider them as
the records of some great and sudden catastrophe, which at one period must
have befallen their globe. . . . .

There seems, then, to be no fact better authenticated in the physical
history of our globe, than that there have taken place the most violent and
extensive inundations from the ocean: The only question of doubt or
difficulty, is to fix upon the causes which could thus have impelled the
ocean from its natural bed; and I have been somewhat particular in detailing
the various phenomena, in order that we may possess some data for estimating
the character of the agent to which these striking physical convulsions must
be attributed. Now, it is quite evident, that there exists no agent on the
earth itself, at all capable of creating such vast effects as those which
have been here described; seeing that there are no physical causes of change
on the surface of our planet, but what are so local and so gradual in their
operation, as to be totally inconsistent with the sudden and extensive
convulsions which we seek to explain. Since, then, this deluge cannot be
referred to any agent residing in the Earth itself, the only foreign cause
to which it can be ascribed, is either the near approach, or the actual
contact, of a Comet. But it is not difficult to see which of those two
hypotheses is, in this case, the one to be adopted. For when we consider the
astonishing violence by which this deluge was characterised; huge fragments
of rocks rent asunder and transported over ridges and valleys; whole species
of animals overwhelmed, and even the highest mountains overtopped; the
surface of the globe broken into isolated or disjointed groups, and even a
large portion of the materials of the southern hemisphere driven beyond the
equator,--it is impossible to conceive that these tremendous effects could
have been occasioned by any other agency, not wholly miraculous, than the
collision of a Comet. . . . .

The same propensity which leads men to search into the history of the past,
awakens into the mind a still stronger desire of knowing the secrets of
futurity: And, accordingly, astronomers, not content with the endeavour to
learn the physical revolutions which the earth has already sustained by the
contact of a Comet, have sought to discover the period when it may be again
exposed to a similar catastrophe. This they have attempted to accomplish, by
computing for a multitude of successive revolutions the motions of those
Comets, whose orbits are exactly computed, and ascertaining the time of
their greatest proximity to the earth. But, before we detail the result of
these curious investigations, it may be proper to give some account of the
Comets, whose calculated orbits and periods of revolution have been verified
by observation. . . . .

[there follow order-of-magnitude calculations of the probability of a
cometary impact with Earth]

But such speculations, however striking the results, conduce to no
practical advantage, and contribute little to the advancement of science.
They afford astonishing proofs of the energy of man's intellectual power, by
which he extends his vision to the horizon of the most distant futurity, and
looks forward, it may be, with a feeling of complacent assurance, to those
momentous events, which, from his knowledge of nature, he is enabled to
foresee. But let him not rest too confidently on the verity of such
anticipations. Astronomers have prophesied, it is true, the collision of a
Comet with the earth; an event that will at once destroy the greater part of
the human species: but any slight attraction, which, in calculating the
movements of this comet, they have chanced to overlook, must invalidate all
their conclusions, and render the prediction at once vain and futile; while,
perhaps, some other comet, among the many thousands traversing the system,
and following an orbit to us unknown, may, in the mean while, come in
contact with our globe, and thus, without any warning of its approach,
produce the same terrible effects, long before the expected period have

arrived.

[For full text see:  http://abob.libs.uga.edu/bobk/peocomet.html ]



CCCMENU CCC for 1997

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.