solar system connected with the sun and moon and those great planets
than which no others were known in the days of Laplace. The
significant fact is that all these thirty movements take place in the
same direction. That this should be the case without some physical
reason would be just as unlikely as that in tossing a coin thirty
times it should turn up all heads or all tails every time without
exception.

We can express the argument numerically. Calculation proves that
such an event would not generally happen oftener than once out of
five hundred millions of trials. To a philosopher of Laplace's
penetration, who had made a special study of the theory of
probabilities, it seemed well-nigh inconceivable that there should
have been such unanimity in the celestial movements, unless there had
been some adequate reason to account for it. We might, indeed, add
that if we were to include all the objects which are now known to
belong to the solar system, the argument from probability might be
enormously increased in strength. To Laplace the argument appeared
so conclusive that he sought for some physical cause of the
remarkable phenomenon which the solar system presented. Thus it was
that the famous Nebular Hypothesis took its rise. Laplace devised a
scheme for the origin of the sun and the planetary system, in which
it would be a necessary consequence that all the movements should
take place in the same direction as they are actually observed to do.

Let us suppose that in the beginning there was a gigantic mass of
nebulous material, so highly heated that the iron and other
substances which now enter into the composition of the earth and
planets were then suspended in a state of vapour. There is nothing
unreasonable in such a supposition indeed, we know as a matter of