ogy, which traces back
the course of history beyond the limits of archaeology, could tell us
nothing except for the assumption that, millions of years ago, water,
heat, gravitation, friction, animal and vegetable life, caused effects
of the same kind as they now cause. Nay, even physical astronomy, in
so far as it takes us back to the uttermost point of time which
palaetiological science can reach, is founded upon the same assumption.
If the law of gravitation ever failed to be true, even to a small
extent, for that period, the calculations of the astronomer have no
application.

The power of prediction, of prospective prophecy, is that which is
commonly regarded as the great prerogative of physical science. And
truly it is a wonderful fact that one can go into a shop and buy for
a small price a book, the "Nautical Almanac," which will foretell the
exact position to be occupied by one of Jupiter's moons six months
hence; nay, more, that, if it were worth while, the Astronomer-Royal
could furnish us with as infallible a prediction applicable to 1980 or
2980.

But astronomy is not less remarkable for its power of retrospective
prophecy.

Thales, oldest of Greek philosophers, the dates of whose birth and
death are uncertain, but who flourished about 600 B.C., is said to have
foretold an eclipse of the sun which took place in his time during a
battle between the Medes and the Lydians. Sir George Airy has written a
very learned and interesting memoir [2] in which he proves that such an
eclipse was visible in Lydia on the afternoon of the 28th of May in the
year 585 B.C.

No one doubts that, on the day and at the hour mentioned by the
Astronomer-Royal, the people of Lydia saw the face of the sun totally
obscured. But, though we implicitly believe this retrospective prophecy,
it is incapable of verification. In the total absence of historical
records, it is impossible even to conceive any means of ascertaining
directly whether the eclipse of Thales happened or not. All that can be
said is, that the prospective prophecies of the astronomer are always
verified; and that, inasmuch as his retrospective prophecies are the
result of following backwards, the very same method as that which
invariably leads to verified results, when it is worked forwards, there
is as much reason for placing full confidence in the one as in the
other. Retrospective prophecy is therefore a legitimate function of
astronomical science; and if it is l