Well, it seems that none of the people who have posted versions of
Clarke's Laws to this group recently has yet gotten them quite right.
The three postings I saw that were close were one from Malcolm J. Harwood,
one from David Goldfarb that was partly a response to it, and the FAQ
list, whose compiler might wish to incorporate the rest of this posting.

Clarke's Law, later Clarke's First Law, can be found in the essay
"Hazards of Prophecy: The Failure of Imagination", in the collection
"Profiles of the Future", 1962, revised 1973, Harper & Row, paperback
by Popular Library, ISBN 0-445-04061-0.  It reads:

# [1]           When a distinguished but elderly scientist
#               states that something is possible, he is almost
#               certainly right.  When he states that something
#               is impossible, he is very probably wrong.

Note that the adverbs in the two sentences are different.  Clarke continues:

#       Perhaps the adjective "elderly" requires definition.  In physics,
#       mathematics, and astronautics it means over thirty; in the other
#       disciplines, senile decay is sometimes postponed to the forties.
#       There are, of course, glorious exceptions; but as every researcher
#       just out of college knows, scientists of over fifty are good for
#       nothing but board meetings, and should at all costs be kept out
#       of the laboratory!

Isaac Asimov added a further comment with Asimov's Corollary to Clarke's
Law, which he expounded in an essay logically titled "Asimov's Corollary".
This appeared in the February 1977 issue of F&SF, and can be found in the
collection "Quasar, Quasar, Burning Bright", 1978, Doubleday; no ISBN on
my copy.  Asimov's Corollary reads:

% [1AC]         When, however, the lay public rallies round an
%               idea that is denounced by distinguished but elderly
%               scientists and supports that idea with great fervor
%               and emotion -- the distinguished but elderly
%               scientists are then, after all, probably right.

So much for Clarke's First Law.  A few pages later on, in the final
paragraph of the same essay, Clarke writes:

# [2]           But the only way of discovering the limits of the
#               possible is to venture a little way past them into
#               the impossible.

To this he attaches a footnote:

#       The French edition of [presumably, the first edition of] this
#       book rather surprised me by calling this Clarke's Second Law.
#       (See page [number] for the First, which is now rather well-
#       known.)  I accept the label, and have also formulated a Third:
#
# [3]           Any sufficiently advanced technology is
#               indistinguishable from magic.
#
#       As three laws were good enough for Newton, I have modestly
#       decided to stop there.
--
Mark Brader, SoftQuad Inc., Toronto, utzoo!sq!msb, m...@sq.com
        We can design a system that's proof against accident and stupidity;
        but we CAN'T design one that's proof against deliberate malice.
        -- a spaceship designer in Arthur C. Clarke's "2001: A Space Odyssey"

Original text in this article is in the public domain.