t explain all the phenomena, and that
there must be no other way of explaining all. When it is proved
that light travels with a finite velocity, we are confined to two
alternative ways of conceiving its transmission, a projection of
matter from the luminous body and the transference of vibrations
through an intervening medium. Either hypothesis would explain many of
the facts: our choice must rest with that which best explains all.
But supposing that all the phenomena of light were explained by
attributing certain properties to this intervening medium, it would
probably be held that the hypothesis of an ether had not been fully
verified till other phenomena than those of light had been shown to
be incapable of explanation on any other hypothesis. If the properties
ascribed to it to explain the phenomena of light sufficed at the same
time to explain otherwise inexplicable phenomena connected with Heat,
Electricity, or Gravity, the evidence of its reality would be greatly
strengthened.

Not only must the circumstances in hand be explained, but other
circumstances must be found to be such as we should expect if the
cause assigned really operated. Take, for example, the case of Erratic
blocks or boulders, huge fragments of rock found at a distance from
their parent strata. The lowlands of England, Scotland, and Ireland,
and the great central plain of Northern Europe contain many such
fragments. Their composition shows indubitably that they once formed
part of hills to the northward of their present site. They must
somehow have been detached and transported to where we now find them.
How? One old explanation is that they were carried by witches, or
that they were themselves witches accidentally dropped and turned
into stone. Any such explanation by supernatural means can neither
be proved nor disproved. Some logicians would exclude such hypotheses
altogether on the ground that they cannot be rendered either more
or less probable by subsequent examination.[5] The proper scientific
limit, however, is not to the making of hypotheses, but to the proof
of them. The more hypotheses the merrier: only if such an agency as
witchcraft is suggested, we should expect to find other evidence
of its existence in other phenomena that could not otherwise be
explained. Again, it has been suggested that the erratic boulders may
have been transported by water. Water is so far a _vera causa_ that
currents are known to be capable of washing huge